The design, operations and economics of cage culture systems

aquacultural engineering ELSEVIER

Aquacultural

Engineering

I6 (1907)

167-203

The design, operations and economics of cage culture systems John E. Huguenin Massuchusrtts Maritime Academy. 101 Academy Drive, Buzzard.7 Bay* MA 025.32, USA

Received 1 October

1996:

accepted I

November

1996

Abstract The commercial culturing of fish in cages has expanded significantly in the past 20 years. While most of this expansion has been with salmonid species, there is still considerable worldwide diversity of cage culture species and culture conditions. Trends are toward larger individual cages and more exposed sites. Many interactive site, species, environmental, engineering, economic and operational factors must be considered during the cage system design process. This process is reviewed and potential problems in design and operations arc discussed. ‘Rules of good practice’ are provided as guidance in avoiding potential pitfalls. 0 1997 Elsevier Science B.V. Kepords:

Cage culture

system; Aquatic

cage; Fish culture

1. Introduction

The first task is to define an aquatic cage. While there is no unanimity about definitions used in aquaculture, what follows is accepted by many. A cage is a volume enclosed with some type of mesh forming a container for aquatic animals. A cage may or may not have mesh across the top. It must have mesh on the bottom. or it is considered a pen rather than a cage. Unfortunately, the two terms are often used interchangeably. Pens use natural bottoms, may have one or more sides bounded by natural shorelines, and are usually much larger than cages but are often used for the same purposes. Large size, reduced .water circulation and internal water quality changes in pens can lead to dramatically reduced production 0144~8609/97/$17.00 Pr~sol44-s~09(9~)0101Y-7

0

1997

Elsevier

Science

B.V.

All

rights

reserved

168

J. E. HugueninlAquacultural Engineering 16 (1997) 167-203

per unit volume compared to cages. This increased productivity of cages can be a factor of two over that of pens. Cages have been used to culture aquatic organisms for centuries and in all kinds of environments, including freshwater, estuarine and seawater. Good reviews exist for cage culturing practices in freshwater (Cache, 1978; Beveridge, 1987) and seawater (Beveridge, 1987). The vast majority of past experience has involved the monoculture of fish, although shrimp and even turtles have been kept in cages. Even in underdeveloped countries, the culture organisms have tended to be high value fish species for human consumption. Small cage culturing industries are diverse and wide spread. These include grouper in Southeast Asia, catfish in the US and elsewhere (only a small fraction of total US catfish production is from cages), tilapia and milkfish in Asia, salmonids just about everywhere with suitable water temperatures, and carp in various places. The high tonnage production cage culturing industries tend to be marine and in temperate climates and include yellowtail (Seriola quinqueradiata) and sea bream (Sparus aurata) in Japan, and salmon/trout world wide. Only a small fraction of the world’s aquaculture production, less than 1% based on FAO 1987 data (Nash, 1988), comes from cages, However, cage production is never the less sizeable, of high monetary value and growing at a very impressive rate. Most of the recent progress and interest has involved marine grow-out of Atlantic salmon (Table l), although an over-expansion may have already occurred. This table, while impressive, does not include other types of cage culturing. Japan is probably the number one cage producer if yellowtail (steady at about 150000 MT per year in the 1980s) and seabream are

Table 1 Estimated increase in world production of farmed salmon by major producing (approx. 80% Atlantic salmon production is from marine fish cage systems)

Norway Scotland Canada Ireland Japan Faeroes Chile Iceland Australia United States New Zealand Totals

country

(metric

tonnes)

1987

1990

% Increase

47000 12600 1500 2500 6000 4000 1600 1000 0 2500 1500 80 200

160000 25 000 16000 10000 9000 6000 3500 3000 7000 4000 3000 246500

240% 98% 967% 300% 50% 50% 119% 200% _ 60% 100% 207%

Data: Ministry of Fisheries, Oslo, modified from Eidem, 1989. The author has been told that the projected 1990 values above are in a number of cases substantially below actual 1990 production, particularly for Chile (factor of about 6.9) and Japan (factor of about 2.3). Both Canada and Scotland were about 25% greater than indicated and world production was about 284000 mt. The 1991 world production was about 301000 mt.

J. E. H~rg~lminiAquuculturcrl Engineering

I60

16 (1997) 167-203

Means of classifying cage systems

Where

operated

Surface Submerged Marine.

Estuarine.

Freshwater

Means of support

Fixed to bottom (usually via pilings)

Type of structure

‘Rigid’ (usually structure and mesh)

Access for servicing

Catwalked

Floating (bouyancy) Flexible (usually mesh only) No catwalks (usually boat/barge Operating

parameters

serviced)

Biomass loading (intensive-extensive) Species Feeding practices (fed/unfed)

Environmental

severity

Sheltered/exposed/open

(handiauto)

water

included. More information is available about these salmonid industries in Northeastern North America (Aiken, 1989; Bettencourt and Anderson. 1990). Norway (Eidem, 1989) and worldwide (Beveridge, 1987). There arc a number of ways to classify types of cages (Table 2). Cages have been and are used from full strength seawater to freshwater. Most have their tops at or slightly above the surface (to keep fish from jumping out). Some with mesh over the top operate submerged, often for only short periods of time such as during major storms. If they can be raised to the surface for servicing and relowered. they are submersible rather than submerged cages. The advantages of being below the surface are avoidance of most wave forces, independence from boat traffic and floating objects, better thermal stability, reduced biofouling and some security from harassment. The disadvantages are greater system complexity if submersible and major increased problems in servicing and operating the system underwater (see operations section). However, there is a long-term trend in this direction due to the possibility of using more exposed sites than possible with conventional systems (Willinsky et al., 1994; Willinsky and Huguenin, 1497). Cages can be tied to pilings to provide vertical support or can be floated. If pilings are used, they also provide horizontal support and eliminate the need for a mooring system as required for floating cages. The cage itself can be a flexible bag usually made of synthetic netting. If floating, the bag needs some structure for it to maintain some semblance of shape. This is usually provided by a rigid or semi-rigid collar at the surfaces and sometimes weights or a rigid shape at the bottom. While not yet common, maintaining net bag shape can also be done through the mooring system. The mesh itself can also be ‘rigid’. The term ‘rigid’ is only relative to synthetic netting, as many of the rigid meshes can easily be deflected hy hand. There are a number of trade-offs between rigid meshes and synthetic netting, including ease of cleaning, service lifetimes, stock security, and costs (see Hugucnin and Ansuini, 1978). Both approaches are used in commercial systems, although

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J. E. HugueninlAquacultural Engineering 16 (1997) 167-203

the majority are of the flexible net bag variety, due to the importance of reduced initial cost. In additions rigid cages are harder to scale up to the larger cage sizes.

2. Concepts and problem definition Aquatic cage systems have been used for three basic objectives. They are for research purposes, for the culturing or grow-out of aquatic organisms and for the short-term holding of organisms (no growth required). The specific requirements for each objective can be very different, and a system that is very adequate for one purpose may be unacceptable for another. One has to be very clear about the basic objectives. Site selection, systems design and operating procedures can all be profoundly impacted by statements of objectives. Research cages tend to be rather small and numerous and the output is expected to be good quality data. The water quality seen by the test organisms is often required to be excellent, eliminating (hopefully) many of the water parameters as unintentional factors in test results. Biomass loadings, as a consequences, are often very low, operating costs per unit biomass very high and required labor quality and quantity also very high. To query the cost per pound of production in a research project is irrelevant and absurd. However, the same question is usually critical in a culturing system. Even in non-monetary situations, the quality and quantity of output or production relative to the required resource inputs is important. The bigger the individual cages and the higher the biomass loading, the better the production/resource input ratio until loading starts seriously impacting growth rates and unit size decreases management abilities. The factors determining the optimum scale for both individual cages and the total cage system are diverse and complicated (see design section). For culturing systems the sizes of cages and the overall systems are likely to be much larger and the water quality as seen by the culture organisms much less pristine than for research systems. For rapid growth, the culture organisms cannot be highly stressed but some stress can be tolerated. For short-term holding, where growth is not required, biomass loading can be further increased and water quality diminished, reducing life support requirements and cost to the point where mortality rates become unacceptable. One can sometimes get away with a lot for short periods of time. This often happens inadvertently, even in research or culturing applications, due to storms, unexpected weather or various mishaps. It may then take some time for the organism to readjust after the removal of the source of stress. The life support criteria in decreasing order of severity are Research-Production-Holding. If one inadvertently slips down a level in criteria, the stated objectives will not be accomplished. If one goes up a level, the input resources required for the same output may increase dramatically. The costs of going too high are somewhat compensated for by increased operating margins and a decreased risk of unexpected mass mortalities, but economics often dominate. The problems of establishing requirements for a new culture system and defining them quantitatively is a very difficult task (Huguenin and Colt, 1989). It is the

J. E. Hug~l~ninlAyuacultural Enginewing 16 (1997) 167-203

171

source of the biggest and costliest mistakes. The basic statements of needs, which launch the projects are often quite subjective and not quantified. These statements of need must go through many iterations of progressively greater detail, refinements and evaluation before anything can be designed, built and operated. The problems of translating basic objectives into quantitative terms necessary for design and construction are usually not given the attention and scrutiny they deserve. One common and frequently encountered basic mistake is that the user makes a number of major system’s decisions without knowing the system consequences. without any sort of systematic evaluation and based solely on a priori judgement. These types of decisions are listed in Table 3. Such decisions, once the project is initiated, may be irreversible. These decisions and the specifics of any given situation are so interactive that the project may be doomed before it starts if they have to be accepted as ‘givens’. This list of decisions should not be considered sequential, as they are all interdependent and have to be considered together. The trade-offs involved with these decisions can be long and complex. Species selection may be fixed by research or user needs. For commercial culturing the decision is often made on marketing considerations only, but is in fact much more complex and should include many other potentially critical considerations (Webber and Riordan, 1976). Site selection is equally complex with many important considerations (see next section). While there is no such thing as a perfect site, a bad site can easily doom a project. Bad choices with respect to the other decisions in Table 3 can, depending on circumstances, be equally serious. The probability of irreconcilable conflicts between the ‘fixed’ system’s decisions and the project objectives and criteria is greatly increased if there exists firm economic constraints. It is not

Table 3 Major cage system decisions

Site Monoculture or polyculture Species Fixed, floating. submersible or submerged cages Types of flotation. moorings and connections Source of seed stocks Harvesting parameters and marketing arrangements Approaches to servicing and maintenance Schedule for design, construction and deployment Capacity or scale (initial and future) Operational schedule (seasonal or all year) Biomass loading (normal and max as function of the season) Design storm Locations for support functions System lifetime Redundancy and reliability Operator skill levels Future options

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J. E. HugueninlAquacultural Engineering 16 (1997) 167-203

uncommon for there to be ‘no acceptable solution’ without fundamental changes in the project. Determining the optimum scale for the overall system is a major decision. This decision can be time dependent, with both initial and future values. Greater size can result in substantial economies of scale in bulk purchases of seed stocks, feed, equipment, supplies and services. In addition, larger system scale allows for the economic benefits and increased management control inherent in vertical integration. Control over required inputs can also substantially reduce risks. There are minimum scale thresholds where owning and operating your own becomes economically attractive for such things as processing plants, hatcheries, marketing structure, and feed mills. This also reduces the risks of sudden unavailability of critical services. Due to inefficiencies inherent in small-scale operations, many small operators find that much of the potential profit is made by processing plants and marketeers, who shoulder little of the risks or efforts involved with culturing. However, many of these advantages of larger scale do not require co-location. A number of similar small cage sites acting together to get the benefits of greater scale can, in fact, be a ‘big’ system. Such cooperative arrangements among a number of small ‘farms’ is very common world wide. The minimum size for a given location is then determined by labor availability, logistical considerations and the size of individual cages. The considerations and trade-offs involving many of the other decisions of Table 3 will be considered later.

3. Site selection Ideally, the site should be selected by means of a thorough site reconnaissance and site selection process. Because cages are immersed in the ambient environment, favorable physical, environmental and water quality conditions are imperative for success. However, it is important to remember that there may be several different locations involved with the overall system. Obviously, the conditions at the cage location are critical but the conditions involving a hatchery’s water systems may be even more critical, due to the greater sensitivity of younger life forms. The hatchery, if there is one, may be co-located in immediate proximity to the cage site, but this is very unlikely. In addition, there will be a shore site near the cages to provide direct logistical support, servicing and maintenance to the cages and possibly a fourth location for administration, processing, and vehicle storage/maintenance. Some of the more important site selection considerations for both cage and hatchery locations are summarized in Table 4. Gathering such site environmental data specifically for cage deployment has been reviewed by Landless and Edwards (1976) and Beveridge (1987, Chapter 4) and for hatchery sites by Huguenin and Colt (1989). Using these data in computer models to evaluate cage sites has also been reviewed (Bell and Barr, 1990). However, many other factors, including social, political, legal and economic aspects can be critical to success (Webber, 1971, 1973; New, 1975).

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Table 4 Important

site factors. This listing includes only those factors most important

the system. An overall

system may involve multiple

facility and cage deployment.

Meteorological

shore side support

This list does not include the many social, political,

aspects that can be equally important 1971. 1973; Huguenin

to the technical aspects of

sites, such as for a hatchery,

legal. and economic

or even critical in the selection of sites (modified

from Wehber.

and Colt, 1989)

factors Winds -

prevailing directions. velocities, seasonal variations.

storm intensity and frequency Light -

total annual solar energy impingement,

quality. photoperiod Air temperature Relative

-

intensity.

diurnal cycle

and variations

humidity or dew point and variations

Precipitation

-

amount. annual distribution.

storm maximums

and frequency Locational

factors Watershed

characteristics

(elevations

-

area gradients

and distances). ground cover. runoff.

up-gradient

activities

Groundwater

supply -

aquifers. water table depth.

quality Tides -

ranges, rates, seasonal and storm variations.

oscillations Waves -

amplitude,

wave length, direction,

and storm variations, Hydrography Water

-

storm frequency,

seasonal

fetch lengths

depths and bottom types

quality -

normal variations.

short- and long-term

threats Coastal currents -

magnitude.

direction

and variations,

exchange rates Existing facilities and characteristics Accessibility of site History of site -

prior uses and experiences

Soil factors Soil type, profile, subsoil characteristics Percolation

rate -coefficient

Topography

and distribution

of hydraulic permeability of soil types

Particle size and shape Angle of repose -

wet, dry

Fertility Microbiological Leachable

population

toxins -

pesticides. heavy metals. other

chemicals Biological environment Primary productivity Local ecology -

-

photosynthetic

activity

numher of trophic levels, dominant

species Wild populations

of desired species -

adults. sources

of seed stocks Presence and concentrations

of predators

-

land. water.

airborne Endemic

diseases,

parasites

and toxic algal blooms

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J. E. HugueniniAquacul!uraI Engineering 16 (1997) 167-203

In many projects the site selection process is very abbreviated because the site options are few, the resources too limiting or, not uncommonly, the site is already predetermined at the start. This site might be already owned by the project principals or dictated by legal, political or economic reasons. Trying to design and successfully operate a cage unit or hatchery at an ‘unfavorable’ site is probably the most common cage system problem. These risks are particularly severe for commercial systems due to their economic constraints, as site problems often result in dramatically reduced production and increased cost. In many cases, a completely fatal problem may be more merciful than slow ultimate failure due to the continuous expenditure of scarce resources fighting site limitations. Some of these strangulation type site problems that have been encountered with cage systems are summarized in Table 5. The site decisions cannot be made independently of other major decisions. Since the site fixes the environment for the culture organism, the site’s hydrology and water quality must be very well matched to the organism’s physiological requirements under all likely conditions. In addition, the biological environment (microorganisms, parasites, predators, etc.) must be favorable. It is clear that a site ideal for one culture organism may be unsuitable for another. The site-species interactions are critical but there are others. The site and the feasibility of various technical approaches to design and operations are also closely connected. The difficulty in acquiring suitable well-sheltered sites for cage deployment has been partially responsible for forcing the development of submersible cages and is now encouraging design and development work on systems for more exposed environments. The new more severe environments require changes in operating and servicing procedures (more later). Site considerations as important as the already discussed water quality and biological environment are factors of physical environment that are critical from an engineering and operations aspect. While the normal daily environment may be near optimum, the system must be designed to survive and function under unusual or rare conditions. While most cage sites are somewhat sheltered, there are very few sites so well sheltered that they do not have at least one vulnerable, relatively open water, direction. It should be remembered that support facilities on the shoreline might also be quite vulnerable. There is a specific storm whose wind velocities, associated waves and tidal currents, if combined with unfavorable conditions such as direction and timing, will just avoid destroying the system. Defining this ‘Design Storm’ is a critical decision with engineering, management and economic aspects. This storm is stated as the ‘X Year Storm’, such as 5, 20 or 100 year storm, where the X is the average recurring period for a storm of this severity in this location. Obviously the larger the X, the fiercer the storm, and the more expensive the system. This is a complex subjective judgement involving trade-offs of risk, cost, very specific site conditions and regional meteorological historical data. These problems are usually compounded if the available historical storm data for a specific site are less than complete or ambiguous. There can be strong penalties for over-specification of design requirements, for under-specification, and for just being unlucky.

J. E. HlcgurniniAquaculrrtrui Engineering 16 (1997) 167-203 Table 5 Severe site limitations

for cage systems

Excessive distance or limited accessibility between cage site and support facilities. This leads to excessive time. effort and resources being devoted to all support activities. It also results in poorer monitoring

and control capability leading to higher

risks and reduced production Underestimating

the effects of indigenous predators

scavengers. The consequences by predators

and

from damaged fish and escapes caused

may be much greater than direct losses. Some scavengers

can create holes in netting while attempting on the bottom.

population

and the problems they cause will grow with time unless

successfully countered.

In addition.

to get at

mortalities

It is important

the local predator to consider all potential

predators Nearhy

marina,

drainage

industrial or municipal

discharges or bypass. Under

may he very acceptahle,

plants or seasonal/storm normal conditions water quality

but under unusual. extreme or infrequent

conditions water quality may drop dramatically

and unexpectedly

result

in mass mortality High biofouling biofouling

site. The composition

community

and quantity of the

can vary considerably

from site to site and

seasonally, even over relatively short distances. Fighting net biofouling can consume as much as 4096 of total labor. It is important to anticipate hiofouling

and plan for the type and quantity of

to be encountered

The last five conditions sheltered

are common to shallow water

sites. which often offer considerable

protection

against direct storm wave and wind damage but respond rapidly to above or below ‘average’ meteorological

conditions.

They may also have relatively low flushing rates Winter

chill (mass mortality

temperatures

due to lower than expected water

during a cold winter)

High temperature

stress (dramatic

increase in mortalities

higher than expected summer water temperatures)

due to

with corresponding

low dissolved oxygen Rapid salinity drop (excessive freshwater weather

inflow from extreme

event)

Unexpectedly

high tidal currents leading to cage structural

mooring system failure, current forces are proportional current velocity squared Possibility over a long period of operations fertilizing

of excessively

the bottom or enriching the water column

or

to the water

175

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J. E. HugueninlAquacuNural

4. Design considerations

Engineering I6 (IYW) 167-203

and trade-offs

Before it is possible to proceed with design, it is necessary to have at least tentative answers and values for the decisions in Table 3. If there is particular uncertainty about some decision, such as site locations, pan-size fish versus mature, or technical approach, it is possible, at least initially, to carry out design efforts in parallel using the alternative decisions. As design work gets more and more detailed, the resources in time and effort required for parallel efforts will increase dramatically, at some point forcing a decision between alternatives. The decisions that have already been made can and should be continually reevaluated during the design process to surface any ‘Red Flag Items’ or critical problems. The earlier critical problems can be defined during design, the more likely a satisfactory solution can be found. System design involves the complete system, including hardware, possibly multiple culture phases, support functions, management methods and auxiliary activities. However, we will concentrate our efforts on the cage culture phase and related direct support activities. There are a number of cage unit design decisions that have to be made during the early stages of the design effort (Table 6). Unfortunately, they cannot be made arbitrarily. They will be heavily influenced by site-specific factors, species culturing requirements and planned operating, servicing and maintenance approaches. Even within Table 6, the decisions are not independent of each other, such as cage size influencing type of structure, materi-

Table 6 Cage unit design decisions. These decisions have to be made with respect to the stated project objectives and only after careful evaluation of alternatives. They are not independent of each other or other system factors and are not made sequentially. The answers, if they together meet the project objectives and constraints, form a set of cage unit specifications which can proceed to detail design and construction

Floated, fixed, submersible or submerged cages? Size and shape of individual cages? Catwalked or not? Partial platforms? If floating, type of floatation, freeboard (if any) and water plane area? Are cages nested or moored separately? What is the configuration? Mesh type and size? Expected system lifetime? Materials of construction for various components? Solid or articulated cage collars, if any? Connections - nets to collars/frames, between cages, collars to servicing platforms and collars/frames/platforms to mooring lines? Means to control cage shape and volume in presence of water currents, if any? Underwater predator netting, if any’? Bird netting, if any? Spray ice shielding, if any’? Anticipated biofouling composition and rates on system components? Planned operating and servicing procedures?

.I. E. H~r~lf~nbllAyltnclrltlrul

Engirwering 16 (1997) 167-20.3

177

als and access requirements. In addition, anticipated servicing and operating procedures will influence most of the hardware decisions. The determination of the optimum sizes for both individual cages and for the total cage system involve complicated and partially subjective trade-off processes. In determining the optimum individual cage size, the dominant trade-off is usually cheaper initial cost per unit volume for larger cages versus cheaper and more efficient operations, marketing considerations and reduced losses per unpleasant event favoring smaller cages. In addition, under conditions where water circulation inside the cages is already limiting, larger cages will dictate lower stocking densities to keep water quality within guidelines. Hence, larger cages may have lower productivity per unit volume. The cheaper initial volumetric cost advantage for larger cages is often substantial (Huguenin and Ansuini, 1978) but not inevitable. as smaller cages may be of simpler construction, such as not having reinforcement along each seam or thinner mesh strands. In at least one case (Lindbergh, 197Y), determining optimum cage size over three generations of system design resulted in a 90% reduction in individual cage volume without an increase in volumetric cost. a 50% reduction in required labor and a substantial increase in overall system size. In short, the optimum cage size is determined by balancing the initial economic advantages of larger units against the increased risks and management difficulties of monitoring and operations. Decisions made for a variety of sheltered locations and conditions indicate optimal individual cage sizes, with considerable variations. in the order of 500-2000 cubic meters for salmonid species, up from 200-500 about 20 years ago, and around 1500 cubic meters for yellowtail. Cages in exposed sites tend to bc much larger, typically 3000-12000 cubic meters each, and indepcndent rather than nested, due to the greater importance of engineering and cost considerations (Carson, 198X). The cost advantage for larger sizes in exposed locations is real but not as clearcut. due to the increased engineering design and support costs associated with more severe environments. Individual cages as large as SO000 cubic meters have been used to hold blue tin tuna. Research systems often have a requirement for many individual cages to allow replications of test conditions. The tendency is to make these cages as small as possible. Unfortunately, there are minimum cage sizes that depend on species, size and water conditions. Cages that are too small can result in physical damage to the animals and dramatically altered behavior that raises questions about the applicability of any derived research data. This has been a common problem in the past. Adequate access to the cages is essential for carrying out the required servicing and operating functions for the system. Many of these involve need for frequent access for monitoring the system. Access also includes logistical requirements, as substantial weights of feed, fouled netting and culture organisms can be involved. One way of providing this essential access is by having a catwalk on two or more sides of each cage. The alternative is access from a boat or preferably a more stable platform such as a barge or raft. Since most cages have flotation collars, some authors have categorized them into narrow collars (no catwalks) and wide collars (catwalked). Catwalks add appreciably to the initial cost of the cages

178

J. E. HugurninlAquaculturaI

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(Huguenin and Ansuini, 1978) but, due to their common usage, they must be well worth the cost under many conditions. Decisions on flotation will strongly influence current and wave forces on floats and collar components. Slabs of flotation foam placed inside catwalks is a cheap and easy solution but places the collars in the water, which can result in high current and wave forces on the catwalks themselves. The increased in-water biofouling on the collars can also substantially increase forces both vertically and horizontally. Increased wave and current forces lead to the system’s motion being much more responsive to even small waves due to the high water plane area. Float cans that allow the collars to be above water level with some freeboard will reduce environmental forces on the collars and will reduce collar motion but generally have higher initial cost. Selection of construction materials has several trade-offs. These include availability and initial cost, which are often counterbalanced with reduced lifetime, potential for corrosion or aging, increased required maintenance and increased risks. Since low initial cost and availability usually dominate, some cage components, such as synthetic mesh bags, often have relatively short service lives. Due to the value of contained stocks and risks of loss, many operators buy new nets every production cycle. Numerous disasters have occurred due to ‘old’ netting. The specification of the ‘Design Storm’ when combined with site-specific conditions provides most of the engineering design criteria. However, before this process can be discussed, it is important to look at sources of environmental forces on cage systems. Most of the extreme forces that the system will be designed to survive are associated with the Design Storm (specified waves, currents, winds and minor debris impacts). Other sources include impacts under normal conditions (boats, drifting objects) and ice. For the few sites where ice, either growing in place or drifting, can occur it is often a very serious survival problem. If cages become frozen into ice and the ice then moves, the cage facility is likely to be destroyed. Small pieces of drift ice at low current velocities may not be a serious problem, but collisions with large pieces, even at very low speeds, may not be survivable. The best way to avoid ice problems is through proper site selection. Impacts of large objects or debris under storm conditions are usually not survivable without major damage and can be partially avoided by good site selection or strong floating booms. If the booms are not strong enough or are not adequately moored, they themselves become a survival threat. Figure 1 shows the sources, locations and transmission of storm associated forces through a nested cage system. Forces can increase rapidly in a nested configuration and all structural components have to be designed with this in mind. In addition, even minor structural failures in a nested system during a storm can rapidly lead to a cascading failure due to self-generated debris and the redistribution of loads leading to over design stresses in some components. The figure also lists the more common causes of environmentally induced structural failures. It must also be remembered that many of the forces are cyclic and that fatigue is often a major factor in cage systems’ failures. It is clear that poor system servicing and main-

17’)

J. E. Hlc~L~~leninlAqLlaculturulEnginrmring 16 (1997) 167-20.3

tenancc can lead to failures even for a storm much less severe than the Design Storm. Figure 2 defines the engineering structural design process. Storm wind data are usually tabulated as the speed of a gust 1 mile long (fastest mile of wind) at an

CAGE MESH+-CAGE COLLAR,& STRUCTURE & FLOATATION EQUIPMENT

NESTED 4 EONF~GURATION r

WAVES CURRENTS

ADDITION OF SUM TOTALS OF FORCES FROM ALL FORCESON MULTIPLE CAGES NESTED CAGES

WAVES CURRENTS IMPACTS WINDS

IMPACTS

MOORING SYSTEM

FAILURE DUE TO FAILURE DUE TO FAILURE DUE TO FAILURE DUE TO * over design conditions * floats loss bouyancy 81 sink

* over design conditions * mesh old & worn

* biofouling * sinks due to leads to overweight of loads biofouling * stress * collisions/ concentrationsrepeated seams & corners abrasion * fretting of * fails at joints mesh between partscorrosion/ dynamic factors * Mesh chewed * slack between on 81 weakened parts 81 high by fish / seals impact forces

Fig.

I

Accumulation

* over design conditions *fails at joints between cages due to corrosion /dynamicfactors *collision with drifting objects, debris or boats * override of one cage over anot her * slack between cages & high impact forces

* over design conditions *fails at fittings due to corrosion /dynamic factors * fretting lines

insufficient holding capacity of “anchors” * redundant load paths undersized - cascading failure * redundant load * slack in lines paths undersized & high impact leading to forces cascading failure

and transmission of environmental

*

of

forces.

180

J. E. HugueninlAquaculturaI Engineering 16 (1997) 167-203

Input SystemDesiiing Data&WC.MU.WaVe F~=SOnFklatS.C4bS& Mesb

input Factors for EntrahmentdRain6Spray

Calc.Max.waveForcesal

Cak.Max.MohgFortxs

1

EVerylhirQAgree6iS AaepBble?tfNot. Reexaminebcwns& Redesign.Modity& Reiterate Fig. 2.

Cage system structural

design procedure.

lnwtSmemCesignoataa cak.Max.cwNntDrag Foroeson Fbats. Cdlars, a Mesh

Cab. Max. Curent Forces onTotatSyste#n

J. E. HlrgurninlAquaculturai

Engineering

16 (1997)

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1x1

elevation of 30 feet. Shorter gusts will have higher velocities and the wind speed near the surface will be about 25% less than at 30 feet. The maximum wind velocity to be used in calculations is that for a gust length which is the same as the length of the nested cage system. The significant wave height of the waves generated by the design storm can be calculated given the average wind speed of the design storm, the fetch length to the site in the most exposed direction, the water depths along this fetch length, and an estimate of the duration of constant storm wind velocity from this direction (US Army, 1984). Reviews of the analytic methods in use are provided by Wei et al. (1990) and Bell and Barr (1990). The significant wave height is defined as the average wave height of the highest onethird of the waves in the wave spectrum. For the given storm, this wave is far from being the biggest wave. Determining a ‘Design Wave’ is not very straightforward, as higher and higher waves are possible at lower and lower probabilities of occurrcnce. There is a subjective component in this decision and the multiplication factor to the design storm’s significant wave height is in the order of 2. With a design wave height, it is necessary to find some other design wave parameters, specifically wave length and period. Unfortunately, for a given wave height there are no unique values of wave length and period, so most probable values should be used. It is also necessary to compensate for shadowing or shielding effects of successive cages on winds and currents on downstream cages. Studies indicate that this shadowing effect can be substantial, especially for fouled netting (Rudi et al., 1988; Aarsnes et al., 1990). Shadowing can also become a very important factor in water circulation in downstream cages under normal conditions. Wave forces include both drag and inertia components, which are not in phase. Fortunately, wave forces decay hyperbolically with depth. At a water depth of onehalf wavelength wave forces are negligible. Even at a quarter wavelength of water depth, wave forces are small. With a major storm nearby, the water surface conditions may be very nasty, but neither fetch nor duration are likely to be sufficient to produce anything but relatively short wavelengths. Thus, very little depth of submergence is needed to avoid massive wave forces. This is one of the major advantages of submersible cages for relatively exposed sites. Major storms also can produce unusually high and low water elevations (Taylor, 1980). Unusually low water can be very dangerous if combined with storm waves, since breaking waves may occur in places impossible under normal circumstances. Since forces from breaking waves can be as much as 100 times higher than from the same non-breaking wave, cage systems caught in a storm’s surf zone are very unlikely to survive, It is important to assure that this cannot happen by calculating breaking water depths (US Army, 1984) for the design wave under storm conditions. Major storms can also produce large increases in tidal currents, especially at inshore sites. Since current forces on structures are proportional to the current velocity squared. it is very important to determine the maximum current associated with the design storm. High currents can also substantially collapse synthetic mesh cages even with corner weights, reducing volume as much as 90% (Aarsnes et al., 1990). Such volume reductions in high stocking density cages usually will produce

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fatal physical damage to the fish. Determining the maximum design current can be difficult, as the values can be very specific to a precise location. Local storm tide data and local knowledge will be helpful in estimating the value associated with the Design Storm, assuming unfavorable timing of storm arrival relative to the tides. The procedural details and needed input information for calculating wind, wave and current forces on floating and submerged objects are best covered by the US Army (1984), and discussed with specific regards to cage systems by Milne (1970), Beveridge (1987) Carson (1988) Rudi et al. (1988), Oltedal et al. (1988), Manuzza and Riley (1989) Cairns and Linfoot (1990) and Aarsnes et al. (1990). In addition, the discussion of several structural analyses of specific cage designs have been published. These include the PolarCirkel cage (Slaattelid, 1990), the 20 m Wavemaster cage system (Whittaker et al., 1990), the Dunlap Tempest cage (Fearn, 1990), and the Trident cage (Willinsky et al., 1991). Considerable caution is advised in estimating maximum forces, especially for wave forces on large nested systems. The classic quasi-static methods often assume rigid structures, small deflections, solid surfaces and no alteration of wave or current properties through the structure. The effects of biofouling and large quantities of mesh on entrained water, inertial effects, variability of drag coefficients, and dynamic responses of complete systems are largely unknown. The deformations, deflections and rotations of a large nested cage system from major storm waves can be substantial, visually impressive and unlike those of any other floating structure. The computational tools available for ships and offshore structures are not well suited for fish farms (Oltedal et al., 1988). The basic problem is that some cage system components tend to move while others do not, creating tremendous forces between them. In addition, wave forces in one part of a large system at a given moment in time may be quite different in both magnitude and direction than in another, as a wave passes through, so time. inertia and damping factors can be very important. With motion included, there is also the possibility of natural frequency resonance effects on some components. There is some hint in the literature on the importance and complexity of dynamic effects to determination of wave forces in nested systems. The many inherent problems are currently under serious study through use of computer models and test tank data. Engineering capabilities are improving rapidly but remain to be proven under prolonged field conditions. Due to the engineering uncertainties associated with nested systems in severe environments, there is a strong tendency to either make the total collar/support system ‘rigid’ like a barge or to separate and decouple cages in exposed locations. An additional advantage from separation is the greatly reduced shielding or shadowing effects on water circulation in downdrift cages. This leads to much better water quality, increased stocking density and better, healthier fish. It also leads to much higher but more predictable storm loads on downstream cages, generally greater cost and more difficult access for servicing. Before the total environmental forces can be calculated or even estimated, all dimensions and detailed system parameters have to be known. In short, a completely specified design must be carried out. After the loads have been calculated, the structural analysis has to be redone to determine if the specified structure can

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survive the Design Storm with adequate factors of safety. If not, it is necessary to redesign and reiterate. Every time the design is changed the environmental loads and structural analysis should be recalculated. 5. Operational

and management

considerations

The successful grow-out of an aquatic organism, assumed but not necessarily restricted to being a fish, requires a number of activities to be carried out. For a project to be a success, these operating and servicing functions must be considered early in the project before design decisions are fixed. Failure to do this results in more inefficient operations and is one of the common problems with cage projects. Some of these activities may be needed only on some culture cycles, on an ifrequired or seasonal basis, some only on stocking or harvest and others daily. These functions are listed in Table 7. This is a neglected area in the literature, which is finally getting some of the attention it deserves (Kuo and Beveridge, 1990) and some of the large new cage designs are considering these factors during the design phase (Willinsky et al., 1991). It is critical that these activities be accomplished efficiently at the required scale and with minimum stress on the culture organisms. An assured supply of good quality seed stocks on a timely basis is an essential requirement for success. Dependence on outside sources can be risky and the quality is often questionable. Sufficient system size to justify an integral hatchery capability can provide considerably more control over seed stock availability and quality but does not completely remove the risks. Another seed stock issue is stocking size. The larger and older the stocked organisms the shorter the grow-out period and generally the lower the mortality rate and overall risk. However, larger seed is usually more expensive, sometimes considerably more costly. Other factors Tahlc 7 Cage unit operating

and servicing

functions

Stocking of organisms Counting organisms Measuring/weighing organisms Grading organisms Feed preparation and/or storage Feeding of organisms Prophylactic treatment of organisms Monitoring water quality and flowrate Monitoring and control of status and health of organisms Harvesting and processing of organisms Cleaning of system (biofouling control and good hygiene) Logistical support for organisms and personnel (trucks. boats. etc.) Mechanical maintenance (connections, moorings, equipment) Support facilities and services for personnel (including shelter) Storage for equipment and supplies

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can be site limitations, such as short growing periods and possible weather winsuch as holiday periods with high dows, and market timing requirements, consumption of fresh product. An integral hatchery capability also provides considerably greater control over stocking parameters than external purchase. In addition, the specific procedures used in stocking the cages can have major impacts on subsequent mortality rates (Flagg and Harrell, 1990). Another important issue is whether the culture organism has to be graded and if so when and how often. Grading is often required to assure uniformity in size and condition at harvest. A harvest with a wide spread in size and condition may be a serious marketing and economic disadvantage. Starting the fish all at the same size at cage stocking is very important. Non-uniformity in subsequent growth can be caused by many things, but mixing stocks from different sources or even from different hatchery tanks, where conditions may have differed, is one way to get such diversity. Some mixing of stocks may be unavoidable, especially for very large cages. Japanese yellowtail, if carefully graded at stocking, generally do not have to be graded again during the cage grow-out phase. This may be one reason that yellowtail cages are generally larger. On the other hand, salmonid species usually require several gradings over the grow-out period. Some species are particularly sensitive to handling. Grading, if carelessly done, can be a significant source of mortality. Efficiently grading large quantities of fish in a floating cage with minimum stress on the culture organisms is not a trivial matter. The size of the cage and its biomass are obviously major considerations. Considerable time and effort has gone into developing grading methods as well as other handling procedures but little quantitative information for cage culture has ever been published. It is clear that in-water handling is much less stressful than out of water handling (Flagg and Harrell, 1990). Most of the established grading methods appear very simple and are effective but not particularly efficient. Problems involve high labor requirements, ‘leakage’ and, inevitably, some degree of stress on the culture organisms with resulting mortalities and growth reduction. For mesh bags, procedures often involve crowding the fish by collapsing the bag and forcing the fish through an inwater bar grader or dip netting them and then putting them through an above-water grader. ‘Rigid’ cages may use a panel with an integral bar screen, which is towed through the cage. In both in-water cases, fish from one side of the grader, usually the ‘large’ fish, have to be removed and the cycle repeated several times. Stocking density (biomass loading) and maximum carrying capacity are critical issues in cage culturing. They are interrelated and involve several important tradeoffs. There is, at any given time during the cage grow-out phase, a maximum stocking density or carrying capacity, usually expressed in weight per unit volume or numbers of a given size fish per unit volume. This maximum density is the value that produces the maximum overall growth, including losses from mortalities. This involves ‘culturing criteria’ as previously defined and not conditions for maximum individual growth or zero stress. The economic optimum is to have the cage at maximum capacity at all times, but there are several problems. If one stocks at the maximum value, the cage volume must grow with the fish, which is unlikely, and

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the risks of something going wrong are high. Alternatively, one can stock at a very low density and have the fish near optimum density at harvest. Other options include thinning them during grow-out or transferring them in block to larger cases as they grow. Grading, if required, and mortalities both provide for reductions in density during grow-out. Before stocking, one must know the maximum carrying capacity of the cage, the anticipated mortalities, and anticipated grading/thinning procedures. Unfortunately, the value for the maximum carrying capacity is very difficult to determine with any precision and is not constant but varies with time and conditions. In the end, actual biomass loadings in cages, when they are realistically amenable to management decisions, are usually subjective judgements based on experience. Due to the uncertainties, it is best to be conservative with biomass loadings in cages. Increasing the biomass loading will improve the economics up to the point that individual growth rate starts to seriously degrade and mortalities increase. Increasing the biomass loading to high values will also increase the risk of sudden catastrophic losses due to changing conditions or misjudgments. The maximum carrying capacity of a case is a function of the incoming water quality, incoming water quantity (velocity), physiology of the organism at that particular stage of development, and cage system design parameters (mesh parameters, cage size, etc.). Figure 3 shows the basic process. Incoming water quality and quantity for a specific cage are limited by minimum water velocities and these are determined by site conditions, biofouling of meshes, and position of a given cage in a nested configuration. The biofouling status of the meshes on any individual cage is particularly variable with time. This can be further complicated by animal behavior. Studies have shown that schooling fish can generate appreciable circulation under low current conditions, at least for small cages (Chacon-Torres et al., 1988). Determinations of water exchange for marine systems is somewhat easier than for freshwater systems due to generally more predictable circulation effects of waves and currents. There are many factors to consider, some very difficult to quantify, and considerable caution is advised in setting the maximum biomass density for actual held use. However, some guidelines can be provided. The maximum density, usually at harvest, for sites with good water quality and circulation is in the range of 1-l ..5 lb ft 3 (16-24 kg m -‘) for most New England cage systems, 20 kg mP3 in Norwegian operations and about 30 kg m ’ in more intensive Scottish cages (Bettencourt and Anderson, 1990). The high value for the Scottish cages is believed to have been typical of operations more than a decade ago. The maximum stocking density in Scotland at the present time is believed to be closer to lo-15 kg m-j. Norwegian Atlantic salmon have also been stated to be stocked at an average value of 20-25 kg m ~~’and chinooks in British Columbia, Canada at 8 kg mm ’ (Bjorndal, 1990). Cage systems tucked away in corners with low circulation can have maximum densities that are only about 50% of these values. These numbers might also have to be adjusted downward for very large cages and large nested systems, due to internal changes in water quality and shadowing effects on downstream organisms. Both mathematical models and confirming tests have shown that the velocity in the second cage in line can be in the order of 60% of free stream velocity and only 45% in the third (Aarsnes et al.,

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186

1990). Not only is the flow in the third cage reduced by 55% but this water has already been degraded by use in two previous cages. This work also showed how biofouling of nets can easily reduce flow in the third cage to the range of lo-20% of the outside velocity. The severity and consequences of shadowing may be a major under-appreciated factor in today’s nested culture systems. Feed and feed-related practices are the dominant economic activity in commercial cage culture, representing 30-60% of the total costs (see economics section) and utilizing 15-35% of the total labor (Huguenin and Ansuini, 1978). Fish feeding practices contain many complex considerations and both short- and long-term informational feedback loops (Webber and Huguenin, 1979). Feeding rates are based on the species’ physiological requirements, water quality, animal size and total numbers. Sampling can be used to estimate average size but the estimate of total numbers of fish or biomass per cage at any given time can be quite a guess. Fish can be damaged and die, be taken by predators or escape through holes in the mesh or by jumping out the top. Daily checks and recovery of mortalities from the cage bottoms, while labor consuming, enables adjustment of the estimated number of animals and is good hygiene. Unexplained ‘shrinkage’ can still be very high (in

Minimum Site Water Velocity

t MeshParameMs& Anticipated Biofouling

v Estimated Miiimum WaterFkwRale

4

Maximum Allowabfe

Fig. 3.

Determination

of maximum

cage stocking

density under

normal

conditions.

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187

some cases 50% or more). There are a large number of potential causes for unexpected ‘shrinkage’ at harvest and, even after the fact, it may not be possible to assess blame among several possible factors with any confidence. A small salmonid mortality at the bottom of a cage under summer conditions can completely disappear in about 3 days. Mark and recapture methods, with the appropriate statistics, can be used to estimate population size but are rarely used. In general, the larger the individual cage size, the harder it is to maintain a good estimate of numbers. Because of its economic importance, technical approaches for efficiently counting and, to a lesser extent, measuring fish in cages in situ are currently a very high research priority. An assessment study of alternative technical approaches has been done (Huguenin, 1993). Even if the correct contained biomass can be determined for feeding purposes, for the feed expense to be justified the feed must actually be eaten by the culture organisms. Wasted feed can be a major economic loss. In Norway with Atlantic salmon, it was estimated that one-third of the feed supplied to the cages was wasted based on feed conversion rates under more controlled conditions (Seymour and Bergheim, 1991). Another source (Dahle and Oltedal, 1990) states that the actual feed used was 35% above optimum. Again the larger the cage, the harder it is to assure that the feed is actually eaten. While vigorous feeding behavior in a small cage is probably generally indicative, in a large cage only a fraction of the population may be feeding and it is much easier to be misled. There are a number of opinions on automated versus hand feeding in cage culturing, but for commercial operations of any size, due to the quantities involved, some automation is very desirable. The quick solution was to slightly underfeed. Another is to somehow acquire feedback information on feed utilization. Feed utilization information can be acquired by, at least partially, hand feeding and observing behavior. Another way is to collect and monitor the uneaten food that drops through. Some fish species will feed off a horizontal surface, such as a feeding tray which can be monitored, while others, such as salmonids, will not generally eat off any surface that might be the bottom. A promising acoustic system to detect uneaten food dropping through salmon cages has been tested in Norway (Juell, 1991; Juell and Westerberg, 1993; Juell et al., 1993). Direct feed ingestion monitoring may also be possible. especially for large cages. Research results have indicated that very small quantities of microencapsulated fluorescent tracers in feeds can be very efficient in monitoring feed ingestion through resulting water properties and appear attractive and economical (Walsh et al., 1987). A major problem with cage systems, which can be especially severe in the marine environment, is biofouling of cages meshes and other in-water components. Biofouling of meshes can quickly and dramatically reduce water circulation in cages, thereby also reducing the cage’s carrying capacity. This obviously can be life threatening to the culture organisms and is especially important for commercial systems where the biomass densities in the cages must be relatively high. Biofouling rates on meshes are highly variable and are seasonal. There can be high variability in both quantity and composition between two sites relatively close together. Mesh hole size, mesh material and other mesh dimensions and prior

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mesh fouling can also be important. Biofouling may be relatively constant over a specific seasonal period, sometimes supplemented by sudden and intense biofouling from specific fouling organisms which have ‘bloomed’. While highly variable in quantity and composition, some idea of values specifically for meshes is available in existing literature (Huguenin and Ansuini, 1981). Additional potentially serious problems from biofouling are due to its weight, even in water, which can require substantial excess buoyancy from float components. Biofouling can also add appreciable underwater area and inertia to calculations for current and wave forces and further add to these forces by increasing the drag coefficients of the cage components. Biofouling of cage systems must be controlled. This can be so consuming of labor and other resources that a number of cages have been designed and built with features to reduce biofouling problems, especially those of the meshes. Biofouling control can require as much as 20-38% of the total labor required (Huguenin and Ansuini, 1978). Generally, ‘rigid’ cages are much easier to clean and can use biofouling resistant materials such as Cu-Ni expanded metal, which works well but inevitably results in higher initial cage cost. Some rigid cage designs are rotatable, always having one side exposed to air and sun to kill biofouling organisms, and both cylindrical and spherical shapes have been used. At least some of these are large and capable of withstanding the rigors of exposed sites (Willinsky et al., 1991). Cages using mesh bags often have the synthetic meshes painted with antifouling paints. This can and has created problems for the culture organisms and broader environmental problems. These problems can be especially severe for large nested systems due to the quantities of antifouling paint involved and cumulative effects on culture organisms in the downstream parts of the system. The specific net treatment procedures used can also be important. Problems are more likely to occur if netting is painted directly on the catwalks rather than offsite and not preleached for a short period before use. Biofouled meshes must either have the fouling removed or the mesh replaced. ‘Rigid’ meshes are usually cleaned in place using water jets, mechanical means or air drying. Synthetic mesh bags are usually partially collapsed, a clean bag placed around the dirty one, the fish dumped into the clean bag, and the dirty one removed for cleaning elsewhere. In all cases, the operational problems involved, in terms of weights and logistics, increase appreciably with cage size. Biofouled nets, due primarily to trapped water, can increase in weight more than lOO-fold in air (Milne, 1970) representing substantial tonnage that might have to be man-handled. Biofouling control doesn’t appear to have any major economies of scale for larger overall systems but smaller cages are obviously easier to handle. Cleaning or net replacement can be required as often as weekly to about four times per year with antifouling treatment and a low fouling site. Biofouling-resistant mesh doesn’t protect against the accumulation of drifting debris, which is a separate problem. Although not in common practice, there is another biofouling control option of using herbivorous or foraging animals in the cages with the culture species. Spot prawn have successfully controlled biofouling on coho cages with no adverse interactions with the salmon (Rensil and Prentice, 1979). Herbivorous rabbit fish

J. E. Hllg~~minlAquuculrltral

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1x0

(S@rnus sp.), an attractive culture species in their own right, have successfully controlled biofouling on tropical cages in high biofouling sites but may damage some meshes (Ben-Yami, 1974). Lastly, crabs are occasionally thrown into salmonid cages primarily to consume mortalities but they also somewhat reduce biofouling. Treatment of culture organisms for disease or parasites during cage grow-out, if required, can present some serious problems. The problems usually do not involve knowing what vaccines or chemical to use but rather how to efficiently carry out the treatment with a minimum of cost, environmental complications and stress on the culture organisms. The treatment materials arc often rather costly and have potentially objectionable environmental consequences, especially if released in quantity. These problems increase with scale. Both the European Atlantic salmon and Japanese yellowtail industries have in the past been seriously limited by parasite problems. The current procedures are adequate but could definitely be improved. There are essentially two approaches to in situ treatment (Dobson and Tack. 1991) but unfortunately very little published information. One procedure is to surround a cage to be treated with an impermeable skirt, treating the contained cage with a moderate concentration of chemicals for time periods in the order of an hour and then to remove the skirt and release the solution. The other is to use a completely closed, impermeable cage or ‘box’ with a high concentration of chemicals and a mesh bag inside. One side of the ‘box’ can be lowered below the water line. The cage containing the animals to be treated is brought along side and one of its sides lowered, forming a sill between the two cages. The fish are crowded into the treatment ‘box’ by collapsing the mesh cage. The fish are treated for a few minutes and transferred back to the original cage by collapsing the mesh bag inside the treatment ‘box’. This approach can substantially reduce the amount of chemicals released to the environment. Predators are a problem that by itself is not usually fatal, but they can contribute significantly to ‘a death of a thousand cuts’. The damage that is done by predators can be much more substantial than the number of fish they actually consume. Long billed birds can mortally wound many more fish than they eat, even through carelessly applied bird netting. Dogfish and seals can make holes in meshes through which the entire cage population can quickly escape: on the other hand, taut mesh appears to dramatically reduce these losses. One advantage of smaller individual cages is that the consequences of such an event is reduced. Decisions on protective bird netting and large mesh barrier around the cage system are a tradeoff of reduced losses from predators against additional costs and complications associated with these barriers. Other considerations involve possible long-term effects of the cage system developing and supporting local expanding predator populations. The predators could include species that might not be immediately obvious, such as raccoons and snakes. Some of the more obnoxious predators may be protected by law, setting up a conflict situation. The damage potential of twolegged predators should also not be underestimated, especially for small systems where a continuous presence may not be possible. Some of these predators arc equipped with refrigerated trucks.

190

.I. E. HugueninlAquaculturai Engineering 16 (1997) 167-203

There is little published information on labor requirements and productivity of labor for cage systems. Bettencourt and Anderson (1990) indicates that in the summer of 1989 the Northeastern US employed about 250 full-time people in 13 firms (2-12.5 people each), while total cage production was about 1550 metric tonnes (92% Atlantics). However, five of the 13 firms were not in the production figure, expecting their first crop the following year. Compensating for this with a factor of two, results in an estimated productivity of labor of about 13000 kg per man-year. This also happens to be the value that can be deduced from Aiken (1989) for the discussed nominal 24 cage system at full production. Edwards (1978) in an analysis of several 20 mt year ’ cage systems using one full-time ‘farmer’ with additional help for changing nets and harvesting, comes up with a labor productivity of about 17000 kg per man-year. Productivities in the order of 25000 kg per man-year, for the cage culturing phase only, are believed to be obtainable for operations above 50 metric tonnes of production per year (Huguenin and Ansuini, 1978). Recent Norwegian data indicate that a 250 mt year ’ Atlantic salmon system can typically be operated by three workers and one manager for a productivity of 63000 kg per man-year (Bjorndal, 1990). From the variations in labor productivity, it is clear that there are important project-specific variables. These include scale factors, species differences, site biofouling rates and management experience and maturity. The composition of labor for cage systems appears to be consistent with the ‘Aquacultural l-3-6 Rule’, i.e. one high level professional, three middle-level people with relevant education or experience and six conscientious and intelligent, but not necessarily educated or experienced workers. Operating cage systems is a risky proposition. Many things can happen unexpectedly. These include major storms, atypical seasonal water quality, a nearby pollution incident, red tides, biofouling blooms and outbreaks of diseases or parasites. The time to plan for these events is before they happen. Contingency plans along with essential equipment and supplies should be available for the more probable and most damaging situations. Routine servicing and maintenance should not be neglected. This includes all fittings, connections, joints and mooring components. Aged equipment, such as nets, should be replaced and biofouling control carried out. The price for neglect is very likely to be paid all at once when one of the unexpected events occurs.

6. Systems evaluation and economics The economic value and viability of a cage culture system is dependent on many factors, many unique to a specific situation. Assuming efficient management and good compatibility between species, site and basic design, the major variables are individual cage size, system size and yearly production. A consistent theme of this paper is that individual cage size is inversely related to management efficiency and that the scale of overall operations is an important economic variable. Thus, what are the economics of cage grow-out under different circumstances? Lots of bits and pieces of data exist but little of them are in a useable form. A

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I0I

previous publication which addressed this problem presented eight cases in a percentage of total operating cost format (Table 5, Huguenin and Ansuini, 1978). While dated, this information, in terms of relative costs and distributions, is still generally valid. Table 8 presents additional cases in the same format for comparison. The two tables are in general agreement, but the new data explain the considerable interest in Atlantic salmon in recent years. However, it should be noted that culturing Atlantics to respectable sizes presents considerable risks to the inexperienced culturist, due to the relative sensitivity of the fish to handling and other stresses, and to the comparatively long culture period. For example. with Norwegian Atlantics over a range of possible operating parameters, total costs arc dominated by the variable cost component (74-79%), while fixed costs are small (1 l-15%) and depreciation and interest on fixed investments are even smaller (l&12%) (Bjorndal, 1990). This is believed to be generally the case with other cage culture species, places and conditions. The economics of cage culturing are very sensitive to market price and survival rate, and moderately sensitive to feed price (Aikcn, 1989. Huguenin and Rothwell, 1979). Feed price is only part of the feed cost, which is the single biggest cost item. Feed conversion and feed price together are economically very important. There is considerable opportunity for reduced feed wastage and better diet formulations to significantly impact system economics. Cultured salmon prices have dropped considerably in the very recent past and the presented returns in Table 8 may now be a bit optimistic. Insurance costs can run as high as 20% of total operating costs for new farms, dropping to about 5% for established farms with a good record (Bettencourt and Anderson, 1990). Mortality rates contained in the numbers in Table 8 run in the 9-27% range for the cage grow-out phase. Obviously. much higher mortality rates can occur when things go wrong. The economic importance of system size is obscured in Table 8, due to wide differences in conditions and circumstances. However, three very similar operations, which differed mainly in size (90, 500 and 2100 mt year ‘), had estimated production costs that were about 8% less for the largest and about 6%) less for the 500 mt year ’ operation than was the case for the smallest (Bettencourt and Anderson, 1990). A Norwegian study for two-cage systems with the same smolt costs, mortality, feed conversion and environment but differing in total farm size (8000 vs. 12000 cubic meters), indicated 637% lower total production costs for the larger (Bjorndal, 1990). There are definitely economies of scale for increasing farm size, but these advantages are not huge and will decrease with increasing size. The optimum farm size will depend heavily on site characteristics but is generally towards the larger end of current practices. On the other side, a minimum size for even a chance of viability of about 50 mt year ’ has been suggested, unless cooperative arrangements for bulk purchases and marketing have been made (Huguenin and Ansuini, 1978). At small scales of operation, unacceptable amounts of time and effort can be required for arranging acquisition of seed stocks. feeds and equipment. Marketing efforts can also be prohibitive. Capital costs are important because they are up front. However. overall economics arc not very sensitive to the initial costs of the system. An indication of rcccnt

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192

Table 8 Operating costs of cage culture systems. The operating costs are shown as a percentage and in a format enabling direct comparison with previously published data on eight other cage culturing situations (Huguenin and Ansuini, 1978). These estimates are based on many assumptions, not all explicitly stated here, and should be used with some caution. In addition, there has been some interpretation of data required to present them in a consistent format Salmonids” NE USA (Bettencourt and Anderson, 1990)

Atlantic$ Bay of Fundy (Aiken, 1989)

Atlantics’ Norway (Bjorndal, 1990)

Chinooks” B.C. Canada (Bjorndal, 1990)

Rainbows’ Norway (Edwards, 1978)

Seed stock Feed Wages/labor Repair/maintenance Depreciation Interest Medicine&hem. Boat supplies Sea rent Insurance Process/marketing Miscellaneous

lo-25% 32-50% IO-13% _

15% 31% 31% _

15.6% 40.2% 9.0% 3.0% 11.0% 5.0% _ _

9.3% 52.0% 8.3% _

17.0% 53.5% 5.7%

19.5% 49.3% 5.0% _

11.0% 4.9%

8.4% 4.8%

9.3% 6.3%

5% 7-20% 9%

4% 12% 7%

5.6% _

2.4%

10.6%

12.1%

10.0%

10.0%

Total Cost per kg

100% $2.25, $2.70’ -, $8.50

100% $5.17

100% Kr 26.6

100% $4.10

$10.55

100% Kr 12.0 (15.9) Kr 35.0

-, 215

104

Approx. 40 50

100% Kr 10.5 (14.4) Kr 15.0

_

42.5

191

Price per kg Return

(%)

Culture period (cage phase only)

10%

Atlantics’ Norway (Edwards, 1978)

_

6-18, 15-20

18-20 months

15-24 months

Avg. 18 months

(4) 1 year

(120) 2 years

“Includes both Atlantic salmon and Rainbow trout systems, 16 sites sampled, yearly productions of 90 to over 300 mt year - 1, median operation with nine cages, about 8000 cubic m, 1988 data, debt service not included. h1987 data, US dollars, 24 cage nominal system, about 92 mt year-‘, depreciation of capital not included, would be lo-15% of new total and add about $0.62 per kg to production costs. ‘8000 cubic m farm, 250 mt year-‘, 3.7 kg fish, four full time people. 15% mortality, feed conversion 1.3, interest on tixed investment in depreciation, 1989 data. “Approx. 16000 cubic m farm, 400 mt year-‘. 3.1 kg fish, five people, 27% mortality, feed conversion 1.7, interest on hxed investment in with depreciation, 1989 data, $ Canadian. ‘Both cage systems produce 20 mt year- ‘, rainbows weigh I.5 kg, salmon 4.0 kg, 1976 data, labor shown does not include full-time ‘farmer’, productivity of labor both 17 mt per man-year, production costs and returns in parentheses include labor at given labor rate. “First number for trout, second for salmon.

J. E. H~1~~i~ninlAyuaculrItrcrl Etginrering 16 (lY97) 167--703

193

cage costs is given in Table 9. The considerable variation in cost per cubic meter can be partially explained by the presence or absence of catwalks or servicing area and by the severity of the intended operating environment. Engineering systems

Table 9 Floating

fish cage costs. These numbers generally

used with caution general

since the circumstances.

include costs for cages and floats only and should be

materials

and included

services vary considerably.

do not include shipping costs, taxes, custom duties or auxiliary equipment,

bird nets. spare nets. mooring

equipment

or feeders.

Catwalks

Prices in

such as predator

may or may not he included.

or

Cages arc

listed in increasing size. Costa are in US dollars

Volume

Cost

Cost per

(cubic m)

($)

cubic m

450

4800

II

Comments

Source

12 m dia. octagnal by 3 m deep. wood Mallock

S-IS

10-15000

IX-28

Aiken,

198Y

Aiken.

I9XY

cage

Square 2 x I2 x 3.X m deep, galv. steel. Skretting or Hercules

675

so00

7.5

I? m dia. octagonal hy

Bettencourt

6 m deep, wood Mallock

and

Anderson.

1090

cage 720

1l-14000

15-20

Square

12x 12x5

m

Bettencourt

and

Anderson,

deep. galv. steel.

1990

Skretting x00

27 750

34.7

Square 5 x IS m catwalked

Bjorndal,

19Y0

Hoat collar, Norway. same cage in B.C. Canada

$24583

breakdowns IO00

20000

20

Spherical

US. cost

in Reference

(12 m D.) Lamellar

structure of aluminium. and submersible. operated

rotatable

usually

I13 volume above

Willinsky Allen.

and 1903:

Willinsky ct al., lYY7

water, Trident

I 150

15-20000

13-17~5

Square

15 x IS x5 m

Bettencourt

deep, galv. steel, Hercules

and

Anderson,

1990

or Viking 13.50

I 350 000

7.50

4 cage SEACON

as

B,jcrke, 1YYlI

built for Spain. concrete

(four cages)

and steel platform.

cages

I5 m dia. x Y m deep, estimated

cost for Norway

for ‘raw’ structure '600

7000 +

3+

Circular

Aiken,

22 m dia. by

IYXY

6.X m deep, plastic tube float. no catwalk, PolarCirkel 0500

I 19 000

IX

Hexagonal

inflatable

collar.

approx. dia. 30 m. depth IO m. no catwalk, predator and bird net incl.

Gunnarsson.

IYXX

194

J. E. HugurninlAquaculturaI Enginwing

16 (1997) 167-203

for exposed environments does appear to have major cost consequences. It is also not clear what is included or excluded in some of the numbers. In addition, initial costs, such as those of Table 9, even when completely defined, are not a good indication of true cage costs. A comparative evaluation of cage costs should also consider major possible differences in service life, operating/maintenance costs, auxiliary equipment requirements, sea state limitations, culture system performance and a number of other factors. Cage system cost data are usually contained within broader economic studies on specific cases, but some comparative data exist (Willinsky and Allen, 1993). Generally, about half the cage facility cost is the cages and float assemblies, while spare nets, mooring system, predator/bird nets and auxiliary equipment (feeders, trucks, boats, etc.) are approximately another half. Land facilities can again double the investment cost, but this is highly variable depending mostly on what is already on site. System capital costs for each case can be roughly estimated from Table 8, by using the given production costs and an average 5 year depreciation schedule. This is the value used in many of the cases shown but it is a little simplistic. More representative lifetimes for different types of cage culturing equipment are shown in Table 10. With the investment costs about lo%+ (in some cases it is not clear if interest is on the investment or on working capital), this works out to initial systems cost of about 60% of total yearly operating costs. As reference points, the 8000 cubic meter Norwegian Atlantic system (Table 9) has a total cage system cost including outfitting of Kr 4250000 ($637500 US) or $2.55 per kg yearly production while the Canadian Chinook cage system total cost is $980000 C ($816667 US) or $2.04 US per kg yearly production (Bjorndal, 1990). The very substantial differences in stocking density (20-25 kg per cubic meter vs. about 8) and mortality rate (15% vs. 27%) are included. A word of caution is in order. The returns on Table 8 are generally very attractive but many are based on a number of important assumptions. These returns are the expected values when everything occurs as expected which, unfortu-

Table 10 Estimated cage culture equipment service life and depreciation rates. These nominal values are from a number of sources and should be used with caution as individual circumstances and differences in maintenance/servicing policies can be important. Capital depreciation will run in the order of S-20% of total operating costs

Floating cages (wood) Floating cages (plastic or Metallic) Synthetic netting Mooring components and lines (inspection and maintenance Feeders Oxygen meters and other instruments Feed/maintenance sheds Conventional buildings (offices, warehouse, garage) Trucks Boats Mechanical equipment (pumps, motors, compressors, etc.)

particularly

important)

5 years 10 years 3 years 5 years 5 years 5 years 10 years 20 years 3 years 7 years 5 years

.I. E. H~c~~~llc~ninlAyl~uculrltral Engineering 16 (1997) 167-203

195

nately, often does not happen. The values are also representative of relatively efficient management of established operations. The start-up of new operations, even with very experienced operators, cannot initially be expected to experience this kind of performance. It may take several production cycles to solve unique problems and get production up to planned capacity levels with somewhat predictable and consistent system performance. Cage culturing is a high risk proposition and things are always going wrong, even in the best systems. The differences are not in whether there arc or are not recurring crises but whether or not they impact production or goals.

7. ‘h-ends and future possibilities Well-sheltered aquatic sites are limited and are usually surrounded by densely populated shorelines which have many social uses. Multi-use conflicts are common and critical. Many of these other uses, such as boating, societal waste water disposal, industrial plants and maritime commercial facilities, can seriously threaten the water quality needed for culturing. In short, many of the potential sheltered sites are unavailable for water quality and/or political reasons. As commercial cage culture systems increase in scale and duration at specific locations, self-pollution can, and has, become a major problem (Aure and Stigebrandt, 1990; Seymour and Bergheim, 1991). A large biomass of fish is an aquatic feed lot, with all of the implied potential environmental problems. With increasing scale, conflicts with other users of sheltered waters can be expected to increase. Factors such as flushing times, water depth under cages, depth versus water circulation and seasonal meteorological variations become much more important. The expansion of cage culturing requires going to more exposed sites, either larger lakes or offshore. This is definitely the trend in Japan and Northern Europe and it generates a whole set of new problems, but it also greatly reduces some of the old ones (Table 11). Inshore and offshore environments have many differences that have consequences, many not being obvious, that must be considered in system design and operations (Gowen and Edwards, 1990; Linfoot et al., 1990). In well-sheltered sites the circulation can be high due to tidal currents but the same body of water may be coming back on the next tide. In short, circulation dots not always equate with flushing, and at some scale of operation this can become important. The water quality in well-sheltered sites also tends to respond faster to seasonal meteorological variations such as a cold winter, hot summer, high sudden rainfall, etc.. than offshore, where water quality is usually more stable. There are cvcn some indications of reduced mortality and better quality of fish in offshore cages (Svealv. 1988; Anonymous, 1991). in sheltered areas, the worst environmental forces are likely to be from storm-heightened tidal currents, while offshore storm waves arc likely to dominate. The offshore environment is very likely to have the toughest engineering challenges. Another offshore challenge involves development of satisfactory procedures for accomplishing all the activities in Table 7 in a more hostile environment and with generally much larger cages.

There are essentially three approaches to dealing with the tougher storm environment at exposed sites. Storm-generated wave forces can either be fought or avoided. Placing the cages at a water depth of one-quarter to one-half wavelength greatly reduces or eliminates wave forces. This can be done by placing cages on the bottom. In this case, too little depth does not give adequate protection from storm waves while too great a water depth can complicate access for divers, which may be critical. This creates a narrow useable water depth range, which may limit the number of suitable sites. While this approach has a number of advantages, it also considerably complicates the accomplishment of the required servicing and operational activities of Table 7. This approach has been evaluated and tested on a small scale (Huguenin and Rothwell. 1979) but apparently has not yet been widely used for commercial culturing. The submersible cage approach has a substantial advantage over a bottom mounted system in that it can be raised to the surface and serviced in a manner similar to that of floating cages and the depth to the bottom

Table 11 Advantages

and disadvantages

of sheltered

vs. exposed

cage sites

Advantages

Protected from storm waves and winds from many (but not necessarily all) directions Easier access for monitoring, servicing, operating and maintaining the cage system Known procedures for carrying out all required activities Fewer unknowns, more experience, easier to evaluate risks

Disadvantages Sheltered sites Probability currents,

of very high tidal especially during storms

Variable and somewhat unpredictable seasonal water quality parameters Potential pollution threats from other activities in area Possibility of inadequate flushing High probability of conflicts other water users Possibility of self-pollution

More predictable, more stable and usually better water quality Lower tidal currents and better, more consistent circulation and flushing Lower probabilty of pollution from other activities

with

Exposed sites Very vulnerable to storm waves and winds, engineering problems Difficulties in access and logistics

Fewer interactions with other activities, possibility of less opposition fndications of reduced mortality and better quality fish Reduced risks of disease or parasite transmission into system

Need to develop new methods for monitoring, servicing, operating, and maintaining the cage system Higher probability of impacts from large vessels Possibility of major crop losses due to waves washing fish out top

J. E. H~rgurninlAquaculturctl Enginrrring 16 (1997) 167-203

1w

is not as critical. Submersible cages are widely used at exposed sites in Japan and there is considerable interest in their use with Atlantic salmon. However, tests have proven that Atlantic salmon as well as Rainbow trout require access to air for survival. Submersible cages for these fish require a submerged captive air bubble and a light source inside the cage (Dahle and Oltedal, 1990). One commercially available submersible cage of 1000 cubic meters has a Lameliar (geodesic type) spherical structure and was first deployed in 1991 (Willinsky et al., 1991). This structural approach provides the highest strength combined with low weight and has good wave response characteristics. The last approach is to make the floating cage system storm survivable at the surface and there are several ways to do this. There are a lot of development activities in a number of countries, but relatively little published data on system parameters and performance specifications. There are large (3500 cubic meters) cages for exposed areas being commercially built in Scandinavia (Svealv, 1988) and many similar, but relatively unpublicized, cages in Japan. There are new mooring approaches, using low wave response spar buoys (Loverich et al., 1989) and the use of modified merchant ships as long as 116 m (Anonymous, 1989). There is a claim that over 12.5 cages (each about 6500 cubic meters) designed for exposed sites (storm wave, current and wind specifications not given), are in commercial service, mostly in Northern Europe (Gunnarsson, 1988). The Norwegians have designed a number of floating cage systems based on offshore oil semisubmersible technology and have tested small-scale systems of 1560 tons with four cages each of 1350 cubic meters off the coast of Spain (Bjerke, 1990). The Norwegians have apparently also designed and are building in series a system that is a cross between a 12 cage rigid floating cage collar and a barge. The cage collar has a length of 126.5 m, beam of 32.2 m, depth of 3.2 m, and design draft of 1.8 m, while the cages are 14 x 14 x 15 rn3 deep having a total net volume of about 35000 cubic meters (Oltedal et al., 1988; Wray, 1989). It is intended for fixed deployment at relatively exposed coastal sites (design conditions not specified) and the first unit was deployed on the lee side of some offshore islands off Ireland with Atlantic salmon in April 1989. The Irish barge proved to be at a site with a very severe wave environment and has been removed from service after having sustained structural damage. A barge of similar dimensions (126 m length, 40 m beam, 5.4 m depth, with 12-5000 cubic meter cages) but different structural design was deployed off Spanish Morocco in 1991 (Wray, 199 1). This Moroccan barge, of Norwegian design and Spanish construction, provided excellent culture system performance with gilthead seabream but sank in 4 months due to dynamic wave effects resulting from waves of 2-3 m height, when it was designed to last 8-10 years and survive 6 m breaking waves (Anonymous, 1991). In fairness, the dynamic response characteristics to waves of the one that sank were stated to bc quite different from those of the Irish barge, which also experienced a more scverc environment. It is clear that these types of structures can share some of the dynamic structural problems and engineering unknowns of nested cages in exposed sites. Another structural approach, which promises open water capability and

198

J. E. HugurninlAquacultural

Engineering 16 (1997) 167-203

improved mobility, results in a system resembling more a raceway system based on floating dry dock principles than traditional cages (Huguenin et al., 1980). In many ways, including structurally, it is a ship. This approach places the design in an area with considerable prior and relevant naval architectural design experience and greatly reduces the structural design uncertainties. All of the exposed water approaches being developed or proposed have a common serious problem. This is that the many essential servicing and operating functions of Table 7, which are already problems, become much worse in exposed locations. The number of days in any given season when the system is accessible or serviceable may be seriously limited by weather or sea state conditions. In addition, the distances for logistics and services are probably considerably greater than for sheltered sites. Because of the distances, functions that were done from land, such as providing power, feed preparation and/or storage, equipment storage and facilities for personnel, may have to be done offshore. This places additional design requirements on the floating system or requires specially designed and dedicated platforms and support vessels. While it is not inevitable that the sizes of individual cages must increase for more exposed sites, this is the consistent trend and some of the new cages are very much larger than older more established cage designs (Table 9 vs. Huguenin and Ansuini, 1978). As has already been discussed, increased individual cage size by itself complicates the monitoring and operating functions. As has also been discussed, many of the operating and servicing techniques now used in sheltered sites are only marginally satisfactory. In many cases, they are not likely to readily and efficiently scale up for use with much larger cages in much more severe environments. Some entirely new approaches may become necessary for large cage systems at exposed sites. Most of the problems of carrying out the functions of Table 7 are associated with trying to do them in each cage individually and acquiring the necessary feedback information for controlling the operations. The larger the cage the harder it is to monitor and control. At a certain scale, operating methods closer to those used in stock yards might be worth examining. Many types of fish can be induced to enter ducts or tunnels by the use of properly placed collimated light sources, mild water currents and/or crowding. Tunnels are already being used to transfer fish between cages. ‘Ducting’ of the fish can be combined with on-line feeding, counting, measuring, visual inspection, prophylactic treatments, grading and harvesting. An on-line ‘parlor’ for grading, feeding and/or treatment, including automatic gates, a rotary ‘Venetian blind’ fish crowder, and adjustable bar graders, might be reasonably small but have the capability to handle under good control large quantities of fish. Since a single unit could be used with many cages, even a very high cost might be justifiable. This approach has already been proposed and there are a number of relevant precedents for some of the aspects (Webber and Huguenin, 1979). Because of time and quantities, most of the feeding would probably have to be done in the individual cages, but ‘special’ feeds could be dispensed in such a feeding parlor due to the expected improvements in feed monitoring capability.

J. E. H~c~~rminlAq~tuculrltrul Engineeritq 16 ( 1997) 167-313

19’)

8. Summary commandments The following are a list of basic guidelines and rules for maximizing the probability of success with cage culture systems. They have been gained from experience. ( 1) Take great care in quantifying the requirements,

(2)

(3) (4) (5)

(6)

(7)

(8)

(9)

as they will greatly effect the performance, complexity and cost of the system. Iteratively specify, quantify, evaluate and re-specify until the total system is acceptable or you decide to quit. Have you considered all service functions? Have you anticipated and evaluated all potential problems? Make sure that you are objective with yourself in carrying out the above evaluations. Many cage projects are doomed by carelessly made early decisions that lock them into a ‘no win’ situation, which is either irreversible or reversible only at great cost. These decisions often involve interactive effects between species, site and culturing approach. Never try to force a given cage system and culture species into a ‘bad’ site. Consider long-term as well as short-term requirements, even though they are more difficult to quantify. Be especially careful about specifying the ‘Design Storm’. If you underspecify, the probability of failure is high, and if you overspecifjr, the initial cost may be unacceptable. Quantify, as best you can, the maximum wind speeds, tidal currents, water depths, and wave parameters associated with the chosen Design Storm under the worst set of circumstances and design accordingly. Do not underestimate the forces of nature. Remember that if you do encounter the Design Storm, your cage system is unlikely to be brand new and whistle clean. During design allow for anticipated levels of biofouling on meshes and system components, corrosion allowances for metallic parts (cage fittings and mooring components). and age and sun damage effects on non-metallics (especially netting). After this is done, add factors of safety to your calculations. Do not forget to actually carry out at least the minimum degree of biofouling control on netting that was planned. Biofouling on netting substantially reduces water circulation in cages and increases environmental loads, among other unpleasant effects. Remember that most biofouling is negatively buoyant. Allow sufficient reserve buoyancy during design to compensate for the weight of biofouling on all in water components. This extra weight can be substantial. Avoid slack between components of nested cages. If relative translations are unavoidable, assure high damping to limit shock loads. If possible, avoid abrupt structural discontinuities, such as rapid changes in section. stiffness, or load direction, by making transitions as gradual as possible to avoid stress concentrations. Structural failures almost inevitably occur at points of stress concentration.

200

J. E. HugueninlAyuaculturaI Engitmring 16 (1997) 167-203

(10) Use large factors of safety on all joints, connections and fittings, particularly between nested cages and between mooring system components. This is where many of the failures occur. (11) Make sure that the mooring system is designed for the worst case conditions and has no unintentional ‘weak’ link. Be especially conservative with anchor holding calculations as storm conditions may dramatically reduce normal anchor holding capacity. (12) If possible, design the system so that the structural failure of any single element does not result in the loss of the total system (fail-safe design). Cascading failures have not been uncommon with cage systems. (13) Major cost underestimates are more likely to result from necessary but uncounted components and services or increased requirements rather than errors in specific items. (14) Consider operating approaches and procedures before the design is fixed. Significant input from operational personnel, who will be responsible for operating and maintaining the system, is needed in the design and construction phases. Assure that you have considered all servicing functions during the design phase. (15) Remember that the key to low risk and high performance systems is large amounts of high quality water. Do not forget the shadowing effects of upstream cages on water circulation. In short, conservative biomass loading relative to the available water quality and quantity, Remember to adjust the biomass as seasonal conditions change and anticipate meteorological variations (i.e. hot summer, cold winter). If a system is working well, increasing the biomass will increase the risks. (16) As the size of the individual cage increases in a given environment, the optimum biomass loading per unit volume will decrease. (17) Anticipate probable failures and plan accordingly to minimize the consequences. In particular, there should be a plan of action to be implemented when a major storm is expected. Necessary equipment and supplies should be gathered before they are actually needed. Plan for the worst, hope for the best. (18) Responsibilities and decision-making procedures for emergencies should be decided before crises occur, remembering that they rarely occur at convenient times. (19) Do not forget routine maintenance, particularly on nets and mooring components, and inventorying necessary spare parts and equipment when operations are going well; this is twice as important when things are not going well. (20) In a field with so much misleading or incomplete information, data voids, ‘experts’ and variations in conditions, it is worth remembering when dealing with the unknown or uncertain that one test, using the actual conditions to be encountered, may be worth one thousand expert opinions.

201

J. E. H~rgtenit~lAyuuc~rrIluml Engitwering I6 (1997) 167-313

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