Development of an intensive culture system for sea bass (Dicentrarchus labrax) larvae in sea enclosures

AqUaCultUre ELSEVIER

Aquaculture

142 (1996) 43-58

Development of an intensive culture system for sea bass ( Dicentrarchus labrax) larvae in sea enclosures Odile Nehr a, Jean-Paul Blancheton a PROVENCE-AQUACULTURE, b Station Ed-pPrimrnt&

d’Ayuuculture

Archipel

de I’IFREMER.

clu Frioul.

b, Elisabeth Alliot ‘.* 13001

Murseille.

Fruttce

Chemitl de Mo,qu~lor~~w. 34250 Poloutr.~ Ir.\ Flats.

FiYllKY!

Accepted 5

December 1995

Abstract A system for the intensive production of sea bass larvae (L>icentrarchus /abra.v) in sea structures was developed between 1987 and 1994. In 1994, this system was used successfully for two larval rearing cycles (with 400000 and 420000 2-day-old larvae) at a fish farm. One hundred days after hatching the survival rate was II%, and the mean weight of fish was 2.5 g. The water temperature varied from 13.9 to 25.4”C in the cylindro-conical enclosures (40 m’) which were set in the sea. The density of fish larvae at starting time was about 10 I- ‘. Larvae were fed initially with Artemia nauplii (hatched out on land). Digestive content observations showed that they also fed on natural plankton in the enclosure. Weaning onto microparticles started when the mean length of the larvae reached I2 mm. Temperature variations were largely dependent on the water surrounding the enclosure but the chemical parameters inside were quite different from those of the sea. Many problems solved during the preliminary experiments are discussed, including the control of fouling, but contamination by predators or competitors is still a risk. The natural food web which develops in the enclosure, with the fish larvae as the main predators. seems to be an advantage for larval rearing. The use of such an enclosure system. which is not expensive and is easy to set up at a farm, is discussed. Kryxwrdst

Culture method; Fish larvae; Sea enclosures;

Dicwrtrurc~hu.~ Iuhrcr.~

* Corresponding author. Tel.: (33) 91 04 16 42; fax: (33) 91 04 I6 .3S; e-mail: [email protected]~mrs.fr 0044.X486/96/$15.00

0

SSDl 0044.8486(95)01248-6

1996 Elsevier Science B.V. All rights reserved

44

0. Nehr et al./Ayuaculture

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1. Introduction

There is great potential for the expansion of aquaculture, since natural fish stocks are diminishing and demand for marine products is increasing. In some countries, aquaculture production has been developed in an extensive way, such as in the Italian ‘valli’. Techniques aimed at ensuring mass production of fish have been developed. Fish cultivation is now becoming an economic activity in developed countries since many of the main problems have been overcome. A better knowledge of fish biology has led to progress in rearing technology and health control of cultivated species. As a result, for some species, fish farming is now based on well established models. A good review can be found in the book ‘1’Aquaculture du Bar et des Sparides’ (Barnabe and Billard, 1984). To meet the demand from fish farms, fish juveniles are produced intensively on an industrial scale in hatcheries. However, the cost of fry production is still high as hatcheries are land-based and circulation and recycling of water and temperature control are expensive. Moreover, the need for live food makes the mass cultivation of organisms (such as rotifers, Arremiu) in special production units necessary. An alternative strategy for mass production of fry is being developed, especially in Norway; larvae are reared in large ponds (from 1000 to more than 10000 m3> and fed on natural plankton (Kvenseth and IZliestad, 1984; Oiestad et al., 1985; Van der Meeren, 1991a). This is an attempt to combine the advantages of extensive cultivation, and the availability of diversified prey organisms, for the production of larvae (Oiestad et al., 1985; Naas, 1990). Oiestad et al. (1976) showed that turbot produced in such conditions have better pigmentation than fish produced in intensive systems. Large scale production of fry is expected to result in a significant reduction in production costs (0iestad et al., 1985). In this context, the transfer of intensive production systems to coastal waters could be of interest as it would lower the initial investment and the operating cost of hatcheries. Floating fish farms could then produce their own fry at lower cost and with better quality control. Furthermore, it would permit them to diversify fry production. To achieve low operating costs, the cultivation of live prey and food sequence would have to be simplified. Although progress has been made (‘enriched’ prey, new strains, microparticles for larvae), start feeding is still a delicate stage in intensive production systems and requires large production facilities. Feeding larvae on natural and diversified planktonic organisms has been demonstrated to be an advantage (Watanabe et al., 1983; Witt et al., 1984; Van der Meeren, 1991a). Naas (1990) showed that start feeding of larvae using diversified prey organisms contributed to the successful cultivation of species such as cod and turbot. Blaxter (1976) suggested that, if cultivation is undertaken with the aim of restocking, fry raised in a natural milieu and with natural plankton would have a better ability to survive than fish produced in intensive systems. For the intensive floating hatchery system, a study is required to identify to what extent natural plankton can be utilised, without introducing competitors or predators of larvae in the enclosures. Moreover, larvae feeding in farms cannot rely only on natural production. Our aim was then to have diversified food sources at least as a complement to commercial feed.

0. Nehr et nl./Aquaculture Table I Fry production

and survival rates during preliminary

10000 (I cage)

2

Waste accumulation,

46000 (2 cages)

6

Hydrozoa

27400 (2 cages)

3

Lack of oxygen in the enclosure

5500 (1 cage)

2

dystrophic crisis in the pond Shortage of artificial food supply

Fry production

1987

IFREMER Palavas les flots IFREMER Pakavas les flots IFREMER Palavas les flots

1990

Pinarello Bay Corsica

1987- 1991 Specific difficulties

Location

1989

experiments:

45

Survival rate (%I

Year

1988

142 (1996) 43-58

a

water stratification

fouling in the cages due to a

’ Number of 2 g fish.

Preliminary studies were carried out between 1987 and 1990. The first larval rearing trials took place in a structure at IFREMER (Palavas, France) in 1987. The enclosure was a cage in the shape of a cube made of canvas, with a volume of 40 m3. Maximum water renewal (using a water pusher, of the same construction as a plankton collector; Barnabe, 1985a), was about 30% of cage volume per hour. Rearing methodology was based on intensive hatchery techniques. Larvae were transferred to the enclosure after the eggs had hatched in land based facilities. Several problems were encountered: (a> poor water quality in the cage (no waste removal, stratification of the water column, insufficient water renewal, fouling, oily film on the water surface); (b) a complicated methodology, involving incubation of eggs on land, and complicated food sequence. During the following trials, improvements were made to overcome these difficulties. Cage bases became conical and had a device for waste removal. More powerful water pushers ensured a higher rate of water renewal in the enclosure. Cage walls were covered with antifouling paint and oily film on the water surface was removed. At the same time, attempts were made to simplify larval rearing, especially with incubation of eggs in the enclosure and reduction of the food sequence to one stage on live prey (Artemia nauplii) followed by a gradual switch to artificial food. The results are summarised in Table 1. These results have allowed, since 1992, the development of the project at Provence Aquaculture, a farm located on islands close to Marseille, where a hatchery has been set up at sea. Here, experiments were carried out in order to test the structures and to establish a valid and standard protocol. In spring 1994, this new method was used successfully for the commercial production of sea bass larvae and the results are presented here. During the course of these experiments, various technical as well as biological problems arose. Some of the problems concerned the sea enclosure method, others were similar to those encountered in intensive hatcheries. The methods used to overcome these problems are discussed.

46

0. Nehr et al./Aquaculrure

2. Materials

142 11996) 43-58

and methods

2.1. Infrastructure The enclosure (Fig. l(a)) was a ‘cage’ made of canvas with a total volume of 40 mm3 (diameter 5 m, maximum height about 4 m>. Its cylindro-conic shape gave it good stability at sea and satisfactory water circulation inside (because there are no dead angles) The material was blue sail-cloth which was strong and flexible and almost waterproof. The fabric was covered with non toxic antifouling silicon based paint. The

1

3

4 5

6

7

(4

04

1;a7

Fig. 1. A sea enclosure for the culture of fish larvae: (a) general features; (b) schema of water circulation and waste removal: I, water inlet; 2, windows (4); 3, shutter; 4, detachable mesh; 5, glass fibre rod; 6, chain; 7, draining bucket; 8, manual pump; 9, water renewal (outboard engine); 10, air diffuser; broken line, water circulation; solid line, waste removal.

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47

Table 2 Mesh size of window nets as a function of the mean size of larvae Size of larvae (mm)

Maximum mesh size


8-12

12-20

> 20

300 p,rn

500 wrn

I

2mm

mm

enclosure was fixed to a floating deck (jet float type) and shaded from the sun by a cover (decreasing the light intensity by 90%) in order to reduce photosynthetic activity and direct sunlight that might be harmful to the larvae. A chain was used as a ballast (7 m long, 14 mm diameter) and was fixed around the top of the cone. At the junction between the cylinder and the cone, a fibre glass rod in a sheath maintained the round shape of the enclosure. At the bottom of the cone there was a draining bucket which was connected by a pipe to a manual pump screwed onto the deck. This device allowed the removal of a large part of the material that fell to the bottom of the cage (dead eggs and larvae, faeces, algae, etc.>. To ensure water renewal in the cage, external sea water was pumped through a sleeve that was equipped with a filter net attached to the side of the enclosure (2 mm mesh). The propeller of an outboard engine (12 V, Truster Mercury type, Eagle 20001, set in a PVC nozzle which was connected to the cage sleeve, pushed the water inside the enclosure. A plankton net surrounding the propeller, the mesh size of which was increased (from 125 to 500 p,m) as the larvae grew, prevented competitors or predators (such as large copepods, microjellyfish, other larvae) from getting inside the cage. This engine ensured water renewal of up to 100% h _ ’ . On the opposite side of the cage, four windows equipped with filter nets allowed the water to flow out. The windows were framed with velcro strips: on the outside strips, shutters could be attached and on the internal strips, the filter nets were attached. The use of velcro strips permitted the mesh size of the filter nets to be changed as the larvae grew (Table 2) and ensured maximum water outflow. A central air diffuser prevented stratification of the water in the cage (Fig. l(b)). Two types of surface cleaners were used to eliminate the oily surface film that appeared as soon as hatching occurred. The first type was very simple and was based on sucking up and removing the surface layer. It was made of a floating Plexiglas rod which had a hole at each end and water was removed through tubing connected to a vacuum pump (12 V) (Fig. 2). The second type of surface cleaner was based on surface foam concentration by air pulse in a limited area. It was similar to the device used in an intensive hatchery (Foscarini, 1988). In addition, absorbent material such as that used for oil spills (rubberiser type) was placed in the cage during the first weeks. Incubation of eggs was carried out in situ in a cylindro-conical incubator (about 350 1 volume) (Fig. 3(a)). The cone was made of polyester and the cylinder was made of plankton net (300 km mesh). Water circulation inside the cylinder was obtained by an air lift system and central air diffusion (Fig. 3(b)). The energy requirements for one larval cycle were about 80 kW per enclosure. Maximum power, when everything was working, was about 342 W h-’ (water push (12

0. Nehr et al./Aquuculture

48

142 (1996) 43-58

Fig. 2. Water surface cleaner (type I): 1, float; 2, holes for suckin g up surface device; broken line, water circulation; solid line, waste elimination.

V, 23 A): 276 W h-’ ; air compressor A): 42 W h-‘). 2.2. Environmental

film; 3, pump; 4, Venturi

(12 V, 2 A): 24 W h-l; vacuum

pump (12 V, 3.5

control and water quality management

The cage was set up and filled with filtered (80 pm> sea water 2 or 3 days before starting, to check the water quality and ensure the absence of competitors and predators. Environmental parameters in the enclosure were checked as follows: oxygen was measured with IP 65 YSI (58) sensor; pH with an IP 67 pH meter: salinity with an ATAGO S-10 refractometer; nutrients (NO,, NO,, NH,) with Sea Tests (Aquarium Systems). The measurements were made in a similar manner to those in intensive hatchery systems (Anonymous, 1983). Water renewal had to be adjusted constantly depending on the changes in the physico-chemical parameters inside and outside the cage. Replacing window nets with larger mesh sizes was done as soon as possible to ensure good water circulation and outflow (Table 2). Daily cleaning of the walls of the enclosure was carried out using a broom and a ‘vacuum cleaner’ as soon as the larvae were large enough (about 8 mm). During cleaning, water renewal was stopped. The two surface cleaners, the central air diffuser and the cover were put in place at the beginning and stayed there until the end of larval rearing. 2.3. Rearing sequence

and larval stages

The usual criteria (ethological, morphometrical, morphoanatomical) (Chatain, 1994) were used to check the development of the larvae. A sanitary survey (presence of bacteria, parasites, etc.) was carried out by the regular sanitary office.

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49

2.3.1. Incubation Incubation took place in situ, in the incubator floating in the cage. When the eggs arrived (if coming from another farm), temperature and pH were checked, both in the enclosure and in the water used to transport the eggs. To equalise the temperature, the egg containers were placed in the enclosure and cage water was gradually mixed with water inside the containers. Eggs were then poured into the incubator at a density of up to 1500 l- ’ . The airlift system prevented the eggs from sinking to the bottom or sticking to the net. Incubation duration depended on temperature (Barnabe, 1985b) and on the development stage of the eggs at the start of the experiment. When hatching was

Fig. 3. Incubator for fish eggs: (a) general view; (b) schema of air lift device and water circulation. weight; 3, plankton mesh (300 pm); 4, air diffusers; broken line, water circulation.

1, float; 2,

50

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completed, the hatching rate was estimated. Air diffusion was increased to achieve a good distribution of the larvae and to resuspend dead eggs. Live and dead larvae together with unhatched eggs were counted in 15 samples (200 ml each). The hatching rate was estimated from the number of live larvae. If it was above 50%, the larvae were released into the enclosure by gently immersing the incubator. The stocking density of larvae in the cage was between 8 and 12 1.‘. 2.3.2. Yolksac larval stage During this period, the larvae did not exhibit any movement (Barnabe, 1978; Kentouri, 1985). The central air diffuser kept the water circulating and prevented aggregation of the larvae. 2.3.3. From mouth opening to weaning The larvae were fed on newly hatched Artemia nauplii. These small crustacea are commonly used as prey (nauplii, metanauplii) for sea bass larvae at the start of feeding. Artemia cysts of various origins are available; at hatching, nauplii size may vary according to the strain from 428 to 517 brn (Vanhaecke and Sorgeloos, 1980). The nutritional value of nauplii may vary as well (LCger et al., 1986). The choice of Artemia was based on size at hatching, nutritional value, availability and price. The Brasil strain, with nauplii of about 450 km at hatching, suits the requirements of the larvae from mouth opening to later stages when they have reached a size of 10 mm. From then until weaning, bigger and less expensive Artemia nauplii can be used, such as the Utah type. Artemia nauplii were distributed twice a day, at sunrise and around solar midday. Artemia cysts were incubated (in sea water, 28°C) for 24 h before each meal, in the usual way. The amount of food required was difficult to estimate. Observations of larval behaviour (hunting, cannibalism) and of digestive tract contents were used to estimate the number of nauplii to be given. In some experiments, natural plankton was collected using a device described by Barnabe (1985a) and sieved in order to obtain a range of organisms appropriate to the size of the larvae (Iizawa, 1983). However, owing to low plankton production in the area and to the risk of introducing harmful species, natural plankton was not systematically added in the enclosure. 2.3.4. Weaning From 1987 to 1992 the artificial food used for weaning consisted of KYOWA pellets calibrated to 80-250 km for A250, and 250-400 pm for B400. In 1994, we used Marin Start extruded pellets (Le Gouessant Aquaculture diffusor) calibrated to 100-300 pm for ALO, and 300-500 km for ALl. When weanin g was completed, larvae were fed with less expensive pellets (Sevbar, Aqualim). The amount of food increased gradually and pellet sizes were chosen according to the size of the larvae. However, biomass estimation was difficult and pellets were fed in excess to prevent cannibalism. Weaning started as soon as the larvae were 12 mm long. The pellets were distributed continuously from sunrise to twilight using automatic belt feeders. The duration of the weaning period depended on the response of the larvae to the change in food. When the

0. Nehr et al./Aquuculture

142 (1996) 43-58

larvae had difficulty adapting to pellets, the weaning to be distributed in more places around the cage.

51

period lasted longer and food had

2.3.5. Transfer Fish were transferred to net cages (2 mm mesh) as soon as mean length was about 20-22 mm. As the transfer is a long and stressful operation, counting was not carried out until after the fish had recovered.

3. Results 3.1. 1992 experiments One million sea bass eggs were divided between two cages and incubation started on 22 May. Hatching (Day 0) occurred on 24 May and hatching rate was about 60%. During the first 60 days, mean temperature was 19.2”C (23.9-16.4”C) (Fig. 4). Chemical parameters remained relatively stable and were always below the critical thresholds: dissolved oxygen saturation rates were between 49 and 108% (mean 95%), pH between 8 and 8.3 (mean 8.1), ammonia content between 0 and 0.3 (mean NH, content 0.1 mg 1-l). Artemia were distributed on Day 5, weaning started on Day 12 with KYOWA as artificial food and from Day 20, Artemia were given every third day until Day 35. Table 3 shows the amount of food distributed. On Day 20, 40% of larvae had a functional swim bladder. On Day 60, the mean size of larvae was 22.0 mm and they were transferred to net cages without counting or weighing. On Day 13 1, the mean weight of the fry was 7.8 g and at this stage the survival rate was close to 4% (i.e. 24000 fry) and the malformation rate was about 20%. Results in both cages were quite similar. Linear growth is shown in Fig. 4, together with the feeding sequence. The main difficulties were as follows. 1. Inefficiency of the surface cleaners in the cages when the wind was blowing, allowing the oily film to remain on the surface during the critical stage of swim bladder inflation. 2. From Day 10, jellyfish invaded the cage and this was followed by high mortality of the larvae. Abnormal swimming behaviour of larvae was observed (twists and turns) and they did not feed. Healthy fish tended to remain below the layer where jellyfish were abundant. 3. Between Days 22 and 32, fish had digestive problems and some exhibited a hyperinflated swim bladder. High mortality was observed (100000 larvae) and the growth curve was flat (Fig. 4). 4. Early weaning, due to Artemia production shortage and unavailability of natural plankton, was not a complete success and may explain the digestive problems in larvae. The surviving larvae fed on the fouling attached to the tank walls were, when transferred to net cages, unable to adapt to artificial food. They exhibited lethargic behaviour, remained close to the wall and schooling behaviour disappeared. At this time, cannibalism and high mortality (about 50%) were observed.

0. Nehr el al./Aquuculrure

52

142 (1996) 43-58

26

* 24 -

inside outside -

22 g 0 202 .

E & 18$ + 1614 12 1 5 10 IS 20 25 24 June May

2

0

10

-z 8 nauplii W p Kyowa A 250 _ B &by B 400 2

20

1 5 10 15 20 25 July

40 30 age (days)

’

+-L.-L;.

50

60

70

.___._

Fig. 4. Water temperature,mean ( + SD) total length (mm) of sea bass and feeding schedule during the 1992 larval rearing cycle in a sea enclosure. 1, mouth openin g; 2, swim bladder inflation; 3, transfer to net cage (2 mm mesh)

5. Automatic feeders did not work properly with KYOWA pellets that got wet and sticky. Feeding had to be done by hand, which was time consuming and could not be done continuously. As a result, weaning was more difficult. 3.2. 1993 experiments Finding solutions to the above problems was the main concern. Attempts were made to improve water quality at all levels. Fouling on the enclosure walls was removed by

0. Nehr et al./Aquaculture Table 3 Total dietary amount used in an enclosure Year

1994

53

to feed larvae until transfer into net cages

Feed Artemia nauplii (million)

I992

142 (1996) 43-58

Microparticles

Pellets (kg)

(kg)

350

Kyowa ( < 250 pm)

Kyowa

3.7 Marin Start (I 50/300 pm) 9

670

24

inside outside

12m 1620 25 May

Sevbar (500/900 10

(250/400 pm) 2.2 Marin Start (300/500 pm) 8

pm)

-

10 15 20 25 June

1 5

0 10 20 30 40 2 age (days) Gnauplii I_ SMarine Start 150/3OOpm 7 r 3 Marine Start 300/5OO~m P

50

I S

JO IS 20 July

60

70

Fig. 5. Water temperature, mean ( f SD) total length (mm) of sea bass and feeding schedule during the 1994 larval rearing cycle in a sea enclosure. I, mouth opening; 2, swim bladder inflation

54

0. Nehr et al./Aquaculture

142 (1996) 43-58

suction and the water surface was cleaned using different devices. Microparticulate were fed using automatic feeders, and other types of pellets were tested.

diets

3.3. 1994 experiments The system was tested with two batches of 2-day-old larvae (400000 and 420000). The experiments started on 18 May in two cages simultaneously. During the first 65 days, mean water temperature was 19.2”C (25.4-13.9”C) (Fig. 5). Chemical parameters were stable and below the critical thresholds. Dissolved oxygen saturation rate varied between 45 and 110% (mean 87%), pH between 7.9 and 8.3 (mean 8.1) and ammonia content between 0 and 0.3 (mean NH, content 0.1 mg l- ’ 1. Larvae were fed on Artemia from Day 4 until Day 54 and no natural plankton was added. Weaning started on Day 28 and the artificial food used was Marin Start. Table 3 shows the amount of food distributed. On Day 20, 75% and 95% of larvae had functional swim bladders in the two cages. On Day 65, the mean size of juveniles was 21.7 mm, and mean weight was 0.85 g. They were not transferred to net cages (2 mm mesh) before Day 75 because the cages were not ready. On Day 100, the mean weight of fry was 2.5 g, the survival rate was 11% (i.e. 91 000 fry) and the malformation rate was about 15%. No significant differences could be observed between the two cages. Linear growth is shown in Fig. 5, together with the feeding sequence. Microscopic analysis of digestive tract contents showed that larvae started feeding on live prey present in the cage (rotifers, Prorocentrum, etc.) as soon as their mouths opened (Day 5) and they began to feed efficiently on Artemia nauplii from about Day 6-7. The feeding pattern was then more or less opportunistic, including Artemia nauplii larvae and natural zooplankton from the cage (copepod nauplii; crustaceans, mollusc larvae, and for larger larvae: harpacticoids, pellets, etc.).

4. Discussion 4.1. Rearing environment In the model developed so far for sea enclosures, some technical points need further improvement. Although energy requirements have been reduced, there is still a need for an energy source nearby. Using solar energy and a small turbine for water renewal instead of the energy-consuming outboard engines, may make it possible for the farm to operate without other sources of electricity. Some species require access to the water surface to inflate their swim bladders by gulping air (Von Ledebur and Wunder, 1938; Doroshev and Comacchia, 1979; Kitajima et al., 1981; Foscarini, 1988; Chapman et al., 1988; Oumiis-Guschemann, 1989; Battaglene and Talbot, 1990) and in intensive hatchery, low inflation rates have been mainly linked to the presence of oily surface films (Foscarini, 1988; Chatain and Ounais-Guschemann, 1990). In 1992, the low percentage of larvae with a functional swim bladder on Day 20 may have been due to incomplete removal of the oily film. The two types of surface cleaner

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I42 (1996) 43-58

55

tested in 1992 and 1993 seem to be complementary: the first type does not require much energy but is not very efficient when the sea is choppy. The second type is more efficient even if the surface is not smooth, but is a heavy energy consumer. Problems related to variable water quality (pH changes and ammonia content changes) that occurred in the first trials, were eliminated by improving water renewal. The use of more air diffusers helped to eliminate stratification of the water column but could not compensate for decreased levels of dissolved oxygen inside the cage. To solve this problem, water renewal was increased as the surrounding sea water had a high content of dissolved oxygen. However, the oxygen content in the sea water in a farm area may become depleted and to cope with such an emergency it is necessary to have pure oxygen available. It appears to be impossible to totally eliminate the impact of certain features of the cultivation site on the performance of the cultivation system. For instance, the ammonia content of the sea water increased greatly after feeding the large fish on the farm. Therefore, we had to adapt water renewal in order to avoid large variations in the larvae enclosures. As pointed out by many authors, it is not just the intensity of parameter variations, but the lack of stability which is harmful for larvae. The experiments showed that in the case of unavoidable temperature changes, the enclosures are large enough to smooth and reduce variations, especially dial variations. For other environmental conditions, the enclosures seemed to have an evolution of their own to a certain extent. Abundant fouling as well as the growth of hydrozoa and jellyfish in the enclosure were real problems (clogging up the outflow mesh, presence of competitors and predators, etc.), causing high mortality. In the first experiments, this fouling became resuspended when the fry were harvested and transferred into net cages, creating very bad water conditions at the time of transfer. To overcome this problem, the walls of the cage were covered with a non-toxic antifouling paint and brushed as often as possible and the incoming water was carefully filtered to prevent plankton getting into the enclosure. However, the risk of predators and competitors entering the enclosure can only be reduced, and sea-spray for instance cannot be prevented from doing so. 4.2. Production The latest results were satisfactory

(at Day 100 after hatching

the survival rate was by Lucet et al. (1984) with weaning starting around Day 40 (2.3 g at Day 100 at 25°C). Person-I-e Ruyet et al. (1991) showed that, with good food supply and weaning starting on about Day 30 (as we did), the mean weight of juveniles on days 70-90 was between 1 and 1.7 g for fry that had been raised in an intensive hatchery, at temperatures between 19 and 25°C. In any case, the fry produced in the sea enclosures in 1994 showed a better growth rate. The food sequence was the same as in an intensive hatchery. However, there were three obstacles: difficulty in estimating accurately the biomass in the enclosure, the inability to control water temperature and the presence of natural zooplankton in the enclosure. In an intensive hatchery, weaning starts around day 40, i.e. after a long period of feeding with live prey. At sea, earlier weaning appears to be more suitable. Some 1 l%, and the mean weight was 2.5 g>. Our results are close to data obtained

56

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research has been carried out on the relationship between the size of the larvae and weaning success (Barahona-Femandez and Girin, 1976; Person-Le Ruyet et al., 1989; Coves et al., 1991). These authors showed that weaning can start when the larvae are 12 mm long, although complete development of the digestive tract is not achieved until metamorphosis (post-flexion stage, larvae size 18 mm) and before that, their ability to digest artificial food is limited (Vu, 1983). Since Artemia were fed to the larvae as newly hatched nauplii there was no need for supplementary feed and, furthermore, the oily surface film was avoided. The larvae were fed Artemia nauplii until metamorphosis. Van der Meeren (1991a) observed that, even if live prey of larger size were available, turbot larvae fed on copepod nauplii. This suggests that larvae gradually learn how to hunt larger prey and during this ‘apprenticeship’, they feed on what they are used to. Natural plankton may be used to complement Artemia nauplii and simple devices for plankton collection have been described (Barnabe, 1985a). The amount and characteristics (species, size, shape) of the natural plankton need to be monitored to ensure that the supply of prey is suitable for the larvae (Iizawa, 1983; Kentouri, 1985). In the present floating hatchery system experiments, the distribution of natural plankton was limited because of the associated fouling problems and lack of plankton in the sea during this period. However, observations of the contents of the digestive tract showed that the larvae fed on plankton living in the enclosure. Soon after the mouth opened, microplankton could be found in the digestive tract. Van der Meeren (199 1b) showed the same phenomenon in codfish (Gadus morhua) larvae and suggested that algae which are known to contain large amounts of free amino acids can provide the young larvae with essential nutrients. Gamble and Houde (1984) observed that, even at very low concentrations, natural plankton was a more efficient food than monospecific cultivated prey. In our environmental conditions, it was not possible to feed the larvae on collected plankton because of the quantitative and qualitative variability of the production in the farm area. However, some types of plankton (microplankton, harpacticoids, copepods) may be used in sea enclosures without any input of external nutrients. This natural production seems, if kept under control (by water filtration, cleaning, use of antifouling, etc.), to improve the ability of the larvae to start feeding on Artemia nauplii. Based on the present results the production of larvae in sea enclosures appears possible and profitable. The fry produced in 1994 were grown on the Provence Aquaculture fish farm. The production cost as estimated by the farm, including all expenses, was 1.65 FF per fry (2.5 g). During the following autumn and winter, these fish grew faster than fry bought from hatcheries which were reared under the same conditions as on the farm. Sea ‘cages’ are not expensive and are easy to adapt to any site, have a low energy requirement and might be of great interest for isolated or small farms. During the larval rearing period, a full-time technician may be able to manage two or three production cycles, using four cages (40000-50000 fry per cage). It should be pointed out that the economic outlook has changed since 1987 owing to an increasing number of hatcheries and lower prices for fry. Nevertheless, this system for production of larvae in the open sea has many advantages: farms can produce their own fry, have better control of fish quality, and facilities in which to develop rearing techniques for other species.

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142 (1996) 43-58

57

Larval survival and growth depend on food that meets the nutritional requirements of larvae and the small trophic web which develops naturally in the cage is not important quantitatively but qualitatively. Furthermore, since it is an inexpensive system, it may be useful for the intensive production of fry in coastal waters of undeveloped countries. In the future, larval production at sea may be of interest for the restocking of areas where fish stocks are depleted without the need for very large investment in facilities.

Acknowledgements This study was supported by a CIFRE grant from the Agence Nationale pour la Recherche Technique and Provence Aquaculture and funds from ANVAR. We thank IFREMER and the University of Aix-Marseille II for giving help and technical assistance. We wish to thank the anonymous reviewers for their helpful critics and advices.

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