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An economical floating cage for marine culture
Aquaculture, 4(1974)323-328 o Elsevier Scientific Publishing
AN ECONOMICAL
PETER
Company,
FLOATING
Amsterdam
--Printed
in The Netherlands
CAGE FOR MARINE CULTURE
Dunstafifnage (Received
Marine
March
Research
Laboratory.
Oban, Argyll
(Great
Britain)
3rd, 1974)
ABSTRACT Landless, P.J., 323-328.
1974.
An economical
floating
cage for marine
culture.
Aquaculture,
4:
A floating cage suitable for holding up to 1 tonne of fish is described. Construction is of galvanised scaffolding tubingsupporting a nylon net, 4 m x 4 m x 4 m and the flotation is of expanded polystyrene encased in glass reinforced concrete. The cage costs about &200 and takes 4 man h to build.
INTRODUCTION
There is an increase in the interest of both researchers and entrepreneurs in the culture of salmonids and marine fish in floating sea cages. In the case of rainbow trout this arises from the relative scarcity of adequate flowing water. A 100 tonne production unit of rainbow trout requires up to 0.75 m3 /set (10 000 g/min) of water, and this must be satisfied by the minimum summer flow of a river or pumping system. Since rainbow trout are anadromous, however, m.dch of the rearing can be done in sea water, reducing the need for a large fresh-water supply. For salmon and marine fish sea water is essential and floating cages provide an economical way of enclosing it. The basic requirements of a floating cage for commercial use are that it should be inexpensive, sturdy and workable. Although large cages (Hunter and Farr, 1970) and exotic ones (Caillouet, 1972) have been described there is a requirement for a smaller cheaper cage. Five of the cages described in the present paper are in use for experiments on marine culture of rainbow trout, particularly of feeding regimes, in Dunstaffnage Bay near Oban. The choice of a cage design lies between rigid wire mesh enclosures or flexible netting usually of nylon. Rigid mesh requires rigid support and this adds greatly to the cost of rafts, particularly since interchangeable panels or mechanical lifting gear are necessary if the cage is going to be any larger than about a 2 m cube, since this is the maximum size that four men could hope to lift out of the water for cleaning, fish catching and so on. Rigid mesh is, however, easier to clean in situ and Milne (1969) has shown that galvanised weld-
324
mesh has a resistance to fouling. Nylon netting on the other hand can be hung as a bag net and large nylon nets can be handled by two men. CAGE
STRUCTURE
The cage consists of three basic components, the framework, the flotation/ walkway and the net. The framework is of standard (48.3 mm outside diameter x 4 mm thick) galvanised scaffolding tubing and is constructed using standard scaffolding fittings. The framework consists of a basic 4.3 m square shape with two sides of the square being extended to hold the flotation/walkway. A low railing 0.6 m high is built onto the square with the vertical railing posts leaning inwards slightly so the top rail forms a smaller square. The net is tied to this top rail and because the posts lean inwards it does not foul the framework below it. The net is a product of a fishing net manufacturers and is a 4 m cube with netting on five sides, the top being covered with a coarser net to keep out birds. All edges are roped, the ropes being extended at the bottom four corners for about half a metre so that a light framework can be attached outside the net to give it shape. This framework is a 4.2 m square of 25.4 mm outside diameter galvanised tubing using Kee Clamp 90” elbows at each corner. These clamps are welded on for added security, and as they are streamlined, they do not therefore catch the net when the framework is raised or lowered. It is possible just to drop a 4 m square frame into the bottom of the net but this can wear holes. The flotation is expanded polystyrene which is the cheapest closed cell foam,
j-j
i ;/ i ii i
ii
ii 0 /I II jl II ii ii
;, 11 11 II 11 Ii ii jj
ii !!
t-
r-
~
-+.JS,,
6.0 m
----!T_
Fig.1. Plan of a cage. The flotation/walkway is shaded, the existing rafts use floats as shown on the left, hut the modification shown on the right float would be advantageous in that it would allow some flexing of the float without causing damage.
325
but it h.as little mechanical strength. With suitable joinery this could be enclosed :m wood, but the floats used in the present cages were made commercially by Concrete Utilities Ltd, and consist of expanded polystyrene encased in glass reinforced concrete. There are two lengths of scaffolding tubing enclosed along the length of the flotation unit and projecting for 0.3 m at either end, the flotation/walkway itself being 4 m x 0.6 m x 0.3 m. Two such units are needed per cage and these provide a total of about 1.25 tonnes of buoyancy. Since Ehe raft weighs about 0.3 tonnes it floats in about 8 cm of water with no net. There is, therefore, an ample reserve of buoyancy. Experience has shown that these long floatswill bend slightly and this causes cracking of the glass reinforced concrete skin. A suitable modification would be to replace each float with two shorter ones, 1.85 m x 0.6 m x 0.3 m joined end to end as shown in Fig.1. This would allow a small degree of flexing without causing any damage. A strong connection between the flotation and the cage structure is essential since the cage literally hangs from the flotation, and considerable forces are exerted for long periods in a choppy sea. Separate cages are held together by a 0.5 m lengths of scaffolding tubing at the water level and 1 m lengths of tubing at the level of the upper rail. These are held by rigid scaffolding couplers so adjacent rafts are fastened rigidly together. Flexible couplings were tried but these broke during storms. The rigid arrangement is satisfactory in Dunstaffm +e Bay which can be very choppy but it may be less successful in areas subject to a swell or to larger waves. The five cages in Dunstaffnage are moored individually to a heavy ground chain which is fastened to three concrete blocks and in addition the total string of five cages is held by four anchors. The cages should be in at least 8 m of water at low tide so that any fouling of the bottom which occurs below the cages does not affect the water quality within them. STOCKING
DENSITY
The cages are designed to hold up to 1 tonne of rainbow trout at a density of 20 kg/m3. The densities which can be achieved are dependent on the amount of water interchange which is provided by the lateral movement of the tide. Hisoaka et al. (1966) found tidal flow inside a net enclosure to be 70% of that outside it and Milne (1972) describes the physics of the system in some T
0.6
water
rr
_I
1
3.
1_
Fig.2. S Ide view of cage and net.
rr
326
Fig.3. Five cages in Dunstaffnage Bay near Oban. Three of the cages are fitted automatic feeders and trout can feed on demand by operating a light sensitive
with trigger.
detail. He also points out that the degree of fouling can considerably affect the interchange. Even with these considerations in mind the interchange can be considerable since the entire area of the side of the cage is available to receive incoming water. With the present cage in a relatively low tidal flow of 0.3 m/min (approx 0.01 knot) as measured inside the cage, there will be an influx of 4.0 m3/min (880 g/min), while at the maximum tidal flow recorded inside the cages in Dunstaffnage Bay of 6 m/min there is a tidal water interchange within the net in 40 sec. As with all considerations of water flow it is the minimum, slack water, flow which is important, but during these periods the movement of the trout themselves cause appreciable interchange of water through the net sides. Work is at present in progress to determine the minimum water flows and oxygen levels at various points in the tidal cycle. DISCUSSION
Because the rafts are constructed
with scaffolding
tubing it is possible to
327
build a cage in about 2 h after the flotation units are provided and the tubing cut to length. There are therefore no fabrication costs and rafts can be built interti(dally and floated off at high tide. It is quite feasible to tow a raft with a rowing boat, the net being added after it is moored. The design could be scaled up by 1 m all round allowing the use of a 5 m cubic net and this would double the enclosed volume at little extra cost. It is not easy to enlarge this design beyond these limits since it would be necessary to use tubing longer than 7 m between the floats, and 7 m is the nominal delivered length. One would also be creating quite a large unsupported “bridge” between the floats with a consequent reduction in strength. Floats along all four sides would become necessary and this would tend to negate any economic advantage of increasing the size. The number of fish which can be held is not necessarily proportional to the volume of the cage, but to the water flow through it, which is in turn proportional to the side area of the net. This is increased by a factor of 1.6 in a 5 m cubic net compared to a 4 m cube, but these considerations must be viewed in relation to local tidal conditions. It is unwise to make the cage excessively deep since this makes observation and removal of dead fish very difficult, and it is a necessary part of good husbandry to have the stock under continual observation. This is particularly important if the water is turbid for long periods, although observation can be much improved by looking through a well shaded glass bottomed box. Predation is often quoted as an argument against nylon netting. There is an accumulating body of evidence that seals will not break into a net since they are rehmtant to become entangled with the mesh before reaching the fish. Seals will of course damage salmon stake nets but here they grasp the trapped fish before becoming entangled with the net. Otters may be a problem but there is no information on this in Scotland. Both cormorants and herons have been observed to attack fish inside a net. Herons fish through the top covering and can be dissuaded by a suitably fine mesh while cormorants dive towards fish near the side walls of the net and injure them with their breaks, they do not brteak the net, however, and do not therefore actually eat any fish. If this problem becomes serious panels of coarse wire netting on a light framework can be placed outside the net and attached to the scaffolding tubing at the water l.evel. COST
OF CAGES
(1973
prices)
The costs for a cage with a 4 m cubic net are detailed Scaffolding ScaflYolding Floats Net Moo rings
tubing fittings
50 m @ g0.60/m 40 right angle couplers @ 60.50 2 @ 640 1@ $40 say 630
= %30 = = = =
620 L80 $40 230
6200
below:
328
This compares with a capital cost of about El 000 per tonne of fish for raceways and $300 per tonne for glassfibre tanks, these costs being exclusive of plumbing which is not of course required for sea cages. It is necessary to add to sea cages the cost of boat access and net cleaning. The latter may be required at monthly intervals during the summer, though this depends on the site. One may anticipate a life of 10 years for the rafts and 3 to 4 years for a net. Nets were supplied by W & J Knoxe LM, Kilbirnie, Ayrshire, Scotland and floats by Concrete Utilities Ltd, Great Amwell, Ware, Herts, Great Britain; Kee Clamps were from H.L. Goodman and Sons Ltd, Deacon Trading Estate, Earls Street, Newton-Le-Willows, Lanes, Great Britain. ACKNOWLEDGEMENT
This work is supported by a Nuffield Foundation demand feeding and marine culture of salmonids.
Fellowship
to investigate
REFERENCES Caillouet, C.W., 1972. Rotatable cage for high density aquaculture. Progve Fish Cult., 34(l): 8 Hisaoka, M., Nogami, K., Takeuchi, O., Suzuki, M. and Sugimoto, H., 1966. Studies on sea water exchange in fish farm - II Exchange of sea water in floating net. Bull. Naikai Reg. Fish. Res. Lab., Contribution 115: 21-43 Hunter, C.J. and Farr, W.E., 1970. Large floating structure for holding pacific salmon. J. Fish. Res. Board Can., 27(5): 947-950 Milne, P.H., 1969. Fish farm enclosures. I Marine growths on netting. Wld Fishg., 18(12): 26-28 Milne, P.H., 1972. Fish and Shellfish Farming in Coastal Waters. Fishing News (Books), London, 208 pp.
AN ECONOMICAL
PETER
Company,
FLOATING
Amsterdam
--Printed
in The Netherlands
CAGE FOR MARINE CULTURE
Dunstafifnage (Received
Marine
March
Research
Laboratory.
Oban, Argyll
(Great
Britain)
3rd, 1974)
ABSTRACT Landless, P.J., 323-328.
1974.
An economical
floating
cage for marine
culture.
Aquaculture,
4:
A floating cage suitable for holding up to 1 tonne of fish is described. Construction is of galvanised scaffolding tubingsupporting a nylon net, 4 m x 4 m x 4 m and the flotation is of expanded polystyrene encased in glass reinforced concrete. The cage costs about &200 and takes 4 man h to build.
INTRODUCTION
There is an increase in the interest of both researchers and entrepreneurs in the culture of salmonids and marine fish in floating sea cages. In the case of rainbow trout this arises from the relative scarcity of adequate flowing water. A 100 tonne production unit of rainbow trout requires up to 0.75 m3 /set (10 000 g/min) of water, and this must be satisfied by the minimum summer flow of a river or pumping system. Since rainbow trout are anadromous, however, m.dch of the rearing can be done in sea water, reducing the need for a large fresh-water supply. For salmon and marine fish sea water is essential and floating cages provide an economical way of enclosing it. The basic requirements of a floating cage for commercial use are that it should be inexpensive, sturdy and workable. Although large cages (Hunter and Farr, 1970) and exotic ones (Caillouet, 1972) have been described there is a requirement for a smaller cheaper cage. Five of the cages described in the present paper are in use for experiments on marine culture of rainbow trout, particularly of feeding regimes, in Dunstaffnage Bay near Oban. The choice of a cage design lies between rigid wire mesh enclosures or flexible netting usually of nylon. Rigid mesh requires rigid support and this adds greatly to the cost of rafts, particularly since interchangeable panels or mechanical lifting gear are necessary if the cage is going to be any larger than about a 2 m cube, since this is the maximum size that four men could hope to lift out of the water for cleaning, fish catching and so on. Rigid mesh is, however, easier to clean in situ and Milne (1969) has shown that galvanised weld-
324
mesh has a resistance to fouling. Nylon netting on the other hand can be hung as a bag net and large nylon nets can be handled by two men. CAGE
STRUCTURE
The cage consists of three basic components, the framework, the flotation/ walkway and the net. The framework is of standard (48.3 mm outside diameter x 4 mm thick) galvanised scaffolding tubing and is constructed using standard scaffolding fittings. The framework consists of a basic 4.3 m square shape with two sides of the square being extended to hold the flotation/walkway. A low railing 0.6 m high is built onto the square with the vertical railing posts leaning inwards slightly so the top rail forms a smaller square. The net is tied to this top rail and because the posts lean inwards it does not foul the framework below it. The net is a product of a fishing net manufacturers and is a 4 m cube with netting on five sides, the top being covered with a coarser net to keep out birds. All edges are roped, the ropes being extended at the bottom four corners for about half a metre so that a light framework can be attached outside the net to give it shape. This framework is a 4.2 m square of 25.4 mm outside diameter galvanised tubing using Kee Clamp 90” elbows at each corner. These clamps are welded on for added security, and as they are streamlined, they do not therefore catch the net when the framework is raised or lowered. It is possible just to drop a 4 m square frame into the bottom of the net but this can wear holes. The flotation is expanded polystyrene which is the cheapest closed cell foam,
j-j
i ;/ i ii i
ii
ii 0 /I II jl II ii ii
;, 11 11 II 11 Ii ii jj
ii !!
t-
r-
~
-+.JS,,
6.0 m
----!T_
Fig.1. Plan of a cage. The flotation/walkway is shaded, the existing rafts use floats as shown on the left, hut the modification shown on the right float would be advantageous in that it would allow some flexing of the float without causing damage.
325
but it h.as little mechanical strength. With suitable joinery this could be enclosed :m wood, but the floats used in the present cages were made commercially by Concrete Utilities Ltd, and consist of expanded polystyrene encased in glass reinforced concrete. There are two lengths of scaffolding tubing enclosed along the length of the flotation unit and projecting for 0.3 m at either end, the flotation/walkway itself being 4 m x 0.6 m x 0.3 m. Two such units are needed per cage and these provide a total of about 1.25 tonnes of buoyancy. Since Ehe raft weighs about 0.3 tonnes it floats in about 8 cm of water with no net. There is, therefore, an ample reserve of buoyancy. Experience has shown that these long floatswill bend slightly and this causes cracking of the glass reinforced concrete skin. A suitable modification would be to replace each float with two shorter ones, 1.85 m x 0.6 m x 0.3 m joined end to end as shown in Fig.1. This would allow a small degree of flexing without causing any damage. A strong connection between the flotation and the cage structure is essential since the cage literally hangs from the flotation, and considerable forces are exerted for long periods in a choppy sea. Separate cages are held together by a 0.5 m lengths of scaffolding tubing at the water level and 1 m lengths of tubing at the level of the upper rail. These are held by rigid scaffolding couplers so adjacent rafts are fastened rigidly together. Flexible couplings were tried but these broke during storms. The rigid arrangement is satisfactory in Dunstaffm +e Bay which can be very choppy but it may be less successful in areas subject to a swell or to larger waves. The five cages in Dunstaffnage are moored individually to a heavy ground chain which is fastened to three concrete blocks and in addition the total string of five cages is held by four anchors. The cages should be in at least 8 m of water at low tide so that any fouling of the bottom which occurs below the cages does not affect the water quality within them. STOCKING
DENSITY
The cages are designed to hold up to 1 tonne of rainbow trout at a density of 20 kg/m3. The densities which can be achieved are dependent on the amount of water interchange which is provided by the lateral movement of the tide. Hisoaka et al. (1966) found tidal flow inside a net enclosure to be 70% of that outside it and Milne (1972) describes the physics of the system in some T
0.6
water
rr
_I
1
3.
1_
Fig.2. S Ide view of cage and net.
rr
326
Fig.3. Five cages in Dunstaffnage Bay near Oban. Three of the cages are fitted automatic feeders and trout can feed on demand by operating a light sensitive
with trigger.
detail. He also points out that the degree of fouling can considerably affect the interchange. Even with these considerations in mind the interchange can be considerable since the entire area of the side of the cage is available to receive incoming water. With the present cage in a relatively low tidal flow of 0.3 m/min (approx 0.01 knot) as measured inside the cage, there will be an influx of 4.0 m3/min (880 g/min), while at the maximum tidal flow recorded inside the cages in Dunstaffnage Bay of 6 m/min there is a tidal water interchange within the net in 40 sec. As with all considerations of water flow it is the minimum, slack water, flow which is important, but during these periods the movement of the trout themselves cause appreciable interchange of water through the net sides. Work is at present in progress to determine the minimum water flows and oxygen levels at various points in the tidal cycle. DISCUSSION
Because the rafts are constructed
with scaffolding
tubing it is possible to
327
build a cage in about 2 h after the flotation units are provided and the tubing cut to length. There are therefore no fabrication costs and rafts can be built interti(dally and floated off at high tide. It is quite feasible to tow a raft with a rowing boat, the net being added after it is moored. The design could be scaled up by 1 m all round allowing the use of a 5 m cubic net and this would double the enclosed volume at little extra cost. It is not easy to enlarge this design beyond these limits since it would be necessary to use tubing longer than 7 m between the floats, and 7 m is the nominal delivered length. One would also be creating quite a large unsupported “bridge” between the floats with a consequent reduction in strength. Floats along all four sides would become necessary and this would tend to negate any economic advantage of increasing the size. The number of fish which can be held is not necessarily proportional to the volume of the cage, but to the water flow through it, which is in turn proportional to the side area of the net. This is increased by a factor of 1.6 in a 5 m cubic net compared to a 4 m cube, but these considerations must be viewed in relation to local tidal conditions. It is unwise to make the cage excessively deep since this makes observation and removal of dead fish very difficult, and it is a necessary part of good husbandry to have the stock under continual observation. This is particularly important if the water is turbid for long periods, although observation can be much improved by looking through a well shaded glass bottomed box. Predation is often quoted as an argument against nylon netting. There is an accumulating body of evidence that seals will not break into a net since they are rehmtant to become entangled with the mesh before reaching the fish. Seals will of course damage salmon stake nets but here they grasp the trapped fish before becoming entangled with the net. Otters may be a problem but there is no information on this in Scotland. Both cormorants and herons have been observed to attack fish inside a net. Herons fish through the top covering and can be dissuaded by a suitably fine mesh while cormorants dive towards fish near the side walls of the net and injure them with their breaks, they do not brteak the net, however, and do not therefore actually eat any fish. If this problem becomes serious panels of coarse wire netting on a light framework can be placed outside the net and attached to the scaffolding tubing at the water l.evel. COST
OF CAGES
(1973
prices)
The costs for a cage with a 4 m cubic net are detailed Scaffolding ScaflYolding Floats Net Moo rings
tubing fittings
50 m @ g0.60/m 40 right angle couplers @ 60.50 2 @ 640 1@ $40 say 630
= %30 = = = =
620 L80 $40 230
6200
below:
328
This compares with a capital cost of about El 000 per tonne of fish for raceways and $300 per tonne for glassfibre tanks, these costs being exclusive of plumbing which is not of course required for sea cages. It is necessary to add to sea cages the cost of boat access and net cleaning. The latter may be required at monthly intervals during the summer, though this depends on the site. One may anticipate a life of 10 years for the rafts and 3 to 4 years for a net. Nets were supplied by W & J Knoxe LM, Kilbirnie, Ayrshire, Scotland and floats by Concrete Utilities Ltd, Great Amwell, Ware, Herts, Great Britain; Kee Clamps were from H.L. Goodman and Sons Ltd, Deacon Trading Estate, Earls Street, Newton-Le-Willows, Lanes, Great Britain. ACKNOWLEDGEMENT
This work is supported by a Nuffield Foundation demand feeding and marine culture of salmonids.
Fellowship
to investigate
REFERENCES Caillouet, C.W., 1972. Rotatable cage for high density aquaculture. Progve Fish Cult., 34(l): 8 Hisaoka, M., Nogami, K., Takeuchi, O., Suzuki, M. and Sugimoto, H., 1966. Studies on sea water exchange in fish farm - II Exchange of sea water in floating net. Bull. Naikai Reg. Fish. Res. Lab., Contribution 115: 21-43 Hunter, C.J. and Farr, W.E., 1970. Large floating structure for holding pacific salmon. J. Fish. Res. Board Can., 27(5): 947-950 Milne, P.H., 1969. Fish farm enclosures. I Marine growths on netting. Wld Fishg., 18(12): 26-28 Milne, P.H., 1972. Fish and Shellfish Farming in Coastal Waters. Fishing News (Books), London, 208 pp.