Aquueu~#ure, 29 (1982) 177---184 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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Brief Technical. Note

A SUBMERS~LE FISH CAGE THAT CAN BE ROTATED ON THE SURFACE TO REMOVE BIOFOULING AND FOR OTHER PURPOSES

ALBERT BLAIR, ROBERT CAMPBELL* and PATRICK T. GRANT

fnsfitt4te of MariPre 3~oc~e~is~y (matures ~nv~ro~~ent Research Council), Aberdeen AB 1 3RA {Great ~ritai~~

~Scott~s~ curiae ~io~ogicu~ Assoe~atio~, ~u~st~~fnage ~u~orato~y, Oban, Argy~Z, PA34 4AD (Great Britain)

(Accepted 20 October 1981)

ABSTRACT

Blair, A., Campbell, R. and Grant, P.T., 1982. A submersible fish cage that can be rotated on the surface to remove biofouling and for other purposes. Aquaculture, 29: 177-184

The design, construction and operation of a new type of fish cage is described. The rectangular structure is based on a rigid polypropylene frame that can be readily assembled from a kit of prefabricated parts. Buoyancy of the cage can be regulated by inflation and deflation of neoprene cylinders enclosed within the rigid frame. A submerged or floating mode of operation can be selected. In the floating mode the neoprene cylinders can be used to effect the rotation of cage about the long axis in steps of 45 or 90”. In this way, mesh panels can be exposed on the surface for removal of biofouiing, for harvesting of fish and for controlled exposure of the captive population to therapeutic agents.

INTRODUCTION

A high stocking density combined with a rapid rate of growth of a captive fish population is a prime objective for the fish farmer. Critical factors in the attainment of the objective include both the frequency and rate of flow of water through the cage st~cture, These should be sufficient to sustain the population by providing dissolved oxygen and for removal of excess food and waste products of metabolism.

Conventions fish cages are usually suspended from a floating platform which also served as a walk-way, The cage is usually a rigid or flexible net. The sites for these moored cages are usually a compromise between a shel- tered site where cages will not break up with the force of the current, wave and wind and an exposed site where the flow of water is more suited to main- tain the captive fish population. A further limiting factor is the seasonal bio- fouling of mesh surfaces that can seriously affect the flow of water through the cage. The removal of biofouling or replacement of the net at frequent in- tervals is essential work that requires specialized staff and is highly labour- intensive.

0044.8486~82~~~00-000~~$02.75 0 3.982 Blsevier Scientific Pubiishing ~ornpa~y

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In the development of techniques for fish farming there is a need for the design and testing of cage structures that can minimize the problems posed both by biofouling and the torsional stresses that are inherent in the use of conventional fish cages. The construction and operation of one possible alternative design is described here. The design concept (United Kingdom Patent Number 8005399, dated 18 February 1980 and vested in the National Research Development Corporation, London) is, however, capable of wide variation in size, shape and detail to suit particular operational requirements.

MATERIALS AND COMPONENTS OF CAGE

Two end plates were made by cutting out a central hole (500 mm diameter) and four peripheral holes (169 mm diameter) with centres located 597 mm from the intersection point of diagonal lines drawn on square polypropylene plates (1.22 X 1.22 m X 25 mm thick; Paragon Plastics, Doncaster). Two rectangular slots (64 X 39 mm) were cut out on the chord of each peripheral hole as shown in Fig. 1. Sleeved stub-flanges of polypropylene (length of stub, 80 mm; diameter of flanges moulded at each end of the stub, 280 mm; Paragon Plastics, Doncaster) were positioned on one face of the end-plate so that the sleeve of each stub-flange (169 mm internal diameter) was aligned with a peripheral hole in the end-plate. The flanges were locked to the end- plate by stainless steel nuts and bolts (Figs. 2 and 3).

The long-axis of the cage structure consists of four rigid polypropylene tubes (140 mm internal bore; wall thickness, 12 mm; 2.72 m long, Paragon Plastics, Doncaster). Holes (25 mm diameter) were bored through both sides of the tube, 57 mm from each end and in the same plane. Two half-moon

(a) t (b)

Fig, 1. (a) Location of cutouts made in square polypropylene endplate. (b) Outer face of complete endplate showing air-lines for inflation and deflation of neoprene tubes con- tained in rigid polypropylene tubes. H, valve; - water level in normal floating mode of operation.

c-

c

w

c/

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Fig. 2. Cross-section of stub-flange (a) bolted to endplate (b); tube (c) with retaining nuts locks endplate to long rigid tube(d); inflatable neoprene tube (e) contained in rigid tube (d) and attached by nylon rope to half-moon shaped discs (f) welded to rigid tube (d); free-rotating swivel (g) with mooring ropes passing through tube (c).

shaped polypropylene disks (12.5 cm chord, 12 mm thick with a central hole 15 mm diameter) were welded at each end as shown in Fig. 2. The four inflatable tubes were 2.64 m long and 150 mm diameter when inflated at 3 lbs/sq. inch (Leyland Rubber Co., Birmingham). Each tube was moulded in one piece to contain an attachment loop at each end and air inlet and outlet tubes (8 mm internal diameter; 1.27 m long) were fitted at one end. The in- flatable neoprene tubes were drawn inside the rigid polypropylene tubes and located under light tension with nylon ropes (15 mm thick) attached through the loop at each end to the half-moon shaped discs welded at the end of the rigid tubes (see, Figs. 1 and 2). The terminal end of the air-inlet tubes were fitted with manually operated plastic valves with PTFE seatings (George Fisher Ltd., London).

The hard-wood frames (2.44 m X 927 mm) to hold the mesh panels (plastic or metal) were made from strips (64 mm X 28 mm) using mortice and tenon joints with wooden dowels. The long members of the frame project as stubs (20 mm long). Plastic mesh (12 mm mesh; Netlon Limited, Mill Hill, Blackburn) was cut to size and sandwiched between the frame and strips of hardwood (64 mm X 10 mm) using nylon bolts and nuts (Fig. 3.).

The same mesh, strips of hardwood, nylon nuts and bolts were used to fit a mesh panel over the central hole of each panel (see Fig. 1).

RESULTS AND DISCUSSION

Assembly of cage from prefabricated components

The ends of the long rigid polypropylene tubes, fitted with the internal in-

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flatable tubes, are eased into the corresponding sockets (sleeved stub-flange) on the end-plates and located approximately in position by lining up the holes in the rigid tubes with the corresponding holes bored in the stub-flange on the end frame (Fig. 2). Each side panel containing the mesh is now fitted where the stubs projecting from each end of the side panel frame fits into the corresponding slot cut in the end-panels (Fig. 1). The holes in the stub- flanges and the corresponding hole in the tubes are now finally lined up. The structure is locked by the insertion of a heavy duty polypropylene tube (19 mm internal diameter, 4 mm wall thickness, 230 mm long, screwed at both ends), that is held securely in place with polypropylene nuts at each screwed end (Fig. 2). These heavy duty tubes serve the dual purpose of locking the cage structure in position as well as serving as an anchor point for mooring ropes that pass through each tube (Fig. 2). Finally, the air-inlet lines and valves to each of the four enclosed inflatable tubes are attached to one or other of the spare pre-bored holes on the outer flange of the end-plate (Fig. 1).

As the cage can be built from the prefabricated component parts in about 20 min, this permits assembly at the water edge when the tide is half-way out. After inflation of the neoprene tubes, the cage can be floated off on the incoming tide.

The mass of the cage (240 kg in air) is mainly made up of polypropylene (density 0.98) and wood (density 0.96). For this reason, the cage structure floats in seawater (salinity, 32°/oo, density 1.025 at 10°C) with about 7 cm freeboard showing above the water surface when all four flotation tubes are deflated, The addition of 20 kg (2 X 10 kg of lead weights) disposed symmet- rically on the upper surface was required to make the whole structure float just below the water surface.

In the normal floating mode of operation (flotation tubes A and B inflated, tubes C and D deflated, Fig. l), the cage floated with about 17 cm of free board above the surface. The addition of 90 kg (9 X 10 kg lead weights dis- posed symmetrically on the upper surface) was required to reduce the buoy- ancy so that the whole cage structure floated just below the water surface. Thus the net upthrust of the cage in normal floating mode is equivalent to 70 kg of lead.

These buoyancy characteristics in the normal floating mode confer the following advantages on the cage structure:

(1) the cage remains floating even after accidental deflation or leaks in flotation tubes; (2) the cage can tolerate marked changes on the density (salinity) of the seawater without sinking; (3) the density of the mesh struc- tue can be changed without significantly affecting performance. For example replacement of the plastic mesh used in these experiments (104 sq. ft.) by the more dense metal mesh (copper-nickel alloy) added only 17.9 kg of high- density material to the st~cture,

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Anchor system for cage in surface mode of operation

The ropes from the four stub-flanges at each end of the cage (Fig. 2) were joined and attached to one end of a freely-rotating shackle. The other end of each shackle was attached to a single rope passed through a ring bolt embedded in a concrete base block and upwards to a buoy on the water sur- face. In practice, the cage was usually moored so that the long axis was at a right angle to the prevailing current. This serves to maximize the water flow through the cage structure.

Step-wise (90”) rotation of the cage about the long axis

In the normal floating mode of operation, the cage could be rapidly ro- tated in steps about the long axis so that each mesh panel can be brought to the surface in turn for cleaning or replacement of the panel. For the first step, flotation tube C is inflated and tube A is deflated using valves located above the water surface (Fig. 1). The cage rotates smoothly in about 30 s so that the inflated flotation tubes B and C are now on the surface. The whole operation takes about 2 min.

The maintenance time required to minimize biofouling of the mesh on the present cage was compared with that required for a conventional cage con- sisting of a flexible polypropylene net suspended from a floating platform. This comparison was made when both cages were moored in Dunstaffnage Bay, Oban, during an &month period, March to October 1980, It proved necessary to effect three changes and replacement of the net in the conven- tional cage due to impedance of water flow to the rapid growth of adhering marine organisms. The work involved was equivalent to 48 man-h and this included time spent in cleaning the fouled nets on the shore. In the same ex- perimental period of time the present cage was kept essentially free of bio- fouling by rotating the structure once per week through a single 90” step. As the weekly rotation was done during one of the routine visits to feed the fish additional work involved was equivalent to 2 man-h during the &month period. Brushing or cleaning of the panel of mesh newly exposed above the water surface proved unnecessary as the fouling organisms did not survive exposure to the air for 7 days.

Partial (45”) rotation of the cage about the long axis

The harvesting of captive fish was simplified by rotating the cage through an angle of 45” so that only half of the internal volume of the cage is below the water surface. Inflation of tube C (Fig. 1) causes the structure to rotate through 45” so that tubes A and C float on the water surface and tube B projects well clear of the surface (Fig. 3).

A further advantage of this procedure is that a captive fish population can be treated in the cage with defined concentrations of drugs or dyes for a pre-

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Fig. 3. (a) Assembled cage structure. (b) Cage floating at 45” for harvesting of fish. (c) Cage floating in normal mode of operation.

determined period of time. A light canvas or plastic trough of triangular cross-section and closed at both ends was prefabricated so that it fitted close, ly over both the exposed panel surface and ends of the cage floating in the 45” position (Fig. 3). The cage was then rotated through 180” so that the fish are contained both in the cage structure and in the plastic trough. After exposure to the drug for a predete~ined time, the cage is again rotated through 45” so that the drug concentration is almost immediately reduced by dilution and rapid mixing with the surrounding seawater.

Sub-surface mode of operation

It has proved practicable to completely submerge the present cage from a normal floating mode of operation down to a predetermined depth and re- turn it to the surface when required. In practice, four lead weights (each 12 kg) were attached by ropes of a suitable length to each of the four stub- flanges exposed on the surface when the cage was in the normal floating mode. The air-lines above the water surface to flotation tubes A and B (Fig. 1) were fitted with extension lines terminating in additional valves. The cage remained positively buoyant (see, Float characteristics of cage structure) until air was released from tubes A and B so that the structure, with the weights suspended underneath became just negatively buoyant and sank

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slowly until the attached weights rested in the sea bottom. As the cage itself remains positively buoyant, it was thus anchored by the weights at a distance from the sea bottom determined by the length of the ropes attaching the cage to the weights. When required, the cage was returned to the surface by inflating tubes A and B from the surface via the extended air-line and re- moving the weights.

Used in the submerged mode, the primary anchor points of the cage are the lead weights where the normal anchoring system is additional but of secondary importance. In this mode, torsional stresses on the whole cage structure are imposed only by the sub-surface movement of water. Moreover, it has proved feasible to submerge the cage into a layer or mass of water that is at a more favourable temperature or other condition than surface water for the sustained growth of the captive fish population.