We have referred to the three types of pens earlier (Fig. 2) -i.e. the completely enclosed pen with net/rigid structures on all four sides in the middle of a bay/open water in lake without any foreshore; a shore enclosure with a forshore extending to deepwater surrounded by a net structure (c.f. osland enclosure) and a bay or lock/fiord enclosure with an embankment or net barrier only at the entrance (of the bay).
While all the pens would come under the above 3 categories, we can deal with the design and construction of pens, in a different classification, as indicated below:
Rigid pens
Embanked pens
Net enclosures
Flexible pens (netting)
Outer barrier nets.
The embanked pens and net enclosures have already been referred to under ‘pen culture in different parts of the world’, but here we shall describe the design (structural details) and construction more particularly. Under ‘Net enclosures’ material selection, fouling and corrosion will not be dealt with here, as these topics have been covered under ‘Cage Culture’. However, structural details, and fouling and other effects will be referred to briefly as pertinent to the points in question; some mention of these have already been made under ‘Site Selection’. The “outer barrier nets” will not be discussed separately, but discussion on these is incorporated under the section on “net enclosures”.
As a section under “Rigid enclosure - net enclosures”, the milk -fish pens (nets and bamboo poles) are also discussed.
Intertidal enclosures such as those at Adoike in the Inland Sea in Japan and Ardtoe in Britain are examples of rigid enclosures which have stone-pitched or concrete walls as embankments (Fig. 5).
Since such embankments are costly, such intertidal enclosures are not being built lately.
Fig. 5. Cross section of rockfill dam at Artdoe (from Milne, 1979)
Fig. 6. Piled net barrier at Hitsuiski Japan showing cross section - restraining anchor blocks and weighted meshnet are shown (after Milne, 1979a)
All embankments have sluices to allow water circulation Embanked pens are constructed in the sublittoral area also. In some cases there are to barriers, in which case often one, the shorter wall, is usually an embankment and the other a piled barrier net, as described separately. A general view of the Ardtoe (Scotland) embankment, which we have described earlier and a cross section of the Ardtoe Rockfill Dam, are given in Fig. 3 and 5 respectively. The rockfill dam has a puddle clay core as impervious barrier, and has two sluices. The Adoike embankment has an enclosure 27 ha in area and also has two sluice gates.
We have already referred to the several rigid net enclosures (including barrier nets) in our review of pen culture in various parts of the world, especially those at Hitsuishi, Matsumigaru and Ieshima in Japan and those at Flogoykgolpo and Bjordal and the Osland enclosures in Norway. While the gross descriptions have been given earlier we shall now refer to certain specific aspects of design and construction here. The piled net barrier at Matsumigaru fish farm in Japan is anchored on the two Anchor blooks at the two ends on the banks of the narrow mouth of the bay - the extended length of the barrier as provided by the shape of the barrier ensures better circulation. A vinyl covered wire not (15mm square mesh) is stretched between steel piles and smaller concrete blocks on two sides of the barrier restrain the net barrier. The barrier also incorporated floating boom for passage of boats.
A cross section of the piled net barrier at Hitsuishi (Japan), showing restraining anchor blocks and weighted mesh net and other details, is given in Fig. 6.
For the net enclosures of the yellow-tail fish farm at Megishima 3 piled - net barriers are built more or less in concentric positions - overlapping one another. Perhaps this has advantages in that there is saving in net materials when more area is brought within the enclosure. Different culture operations where segregation is needed can also be attended to in this kind of concentric pens (in contiguous enclosures also - but here the level of overlap will be less in that the net barriers will be common at the contiguous limits only). In both these cases difficulties in cleaning and maintenance and also water circulation could drop up. Elevation and sectional views of the pen barrier nets of Megishima are shown in Fig. 7. Anchors placed on the outside only for restraining the nets; note the 40cm diameter piles; the 35mm horizontal steel bars for strengthening; the double wire mesh (galvanized), 25mm square inside and 50mm diamond outside; and the rubble piled on the top of nets stretched on two sides on the floor, for restraining the net, as well as for protection from predators. This enclosure is also provided with as boat lock at one end of the enclosure near the shore for boats to pass and stay. For details of the design of moored support framework for netting barrier in deep water in a Scottish lock (with an inner fish net and outer predator net suspended from the scaffolding framework) and that of the 12 - M square fish enclosures at Faery Isles (Scotland) reference is made to the description in M.Ire (1971a). The latter has a fish netting of 12mm galvanized mesh inside and predator and trash net of 75mm mesh outside. There is also an access catwalk and another catwalk surrounding the enclosures. The catwalk is one metre wide and facilitates access for routine inspection, feeding, cleaning etc. The elevation of the catwalk has to be higher than the high water mark. In this specific case the catwalk is 1.2m higher than the high water ordinary spring tide (HWOST) and a hand rail one metre above the catwalk floor. Over 73 net enclosures (rigid) were constructed in the Inland Sea in Japan, covering vast areas and providing a major problem of cleaning by divers at regular intervals. Some of the Japanese net enclosures were abandoned due to poor circulation of water (poor siting and also due to low tidal range - of less than 1 metre). Also often the standard fishing nets used fouled easily and circulation was restricted and nets damaged by water currents combined with wind and wave action.
Fig. 7. Elevation and section view of net enclosure at yellow-tail fish farm at Megishima, Japan, showing net fixing arrangements (after Milne, 1979a)
Successful enclosures have been sited only after detailed hydrographic studies especially to ensure sufficient water exchange. Success also depends on the design and selection of net ting structure (see selection of netting materials under ‘Cage Culture’). A successful net enclosure is the one built in 1968 at Sakaide (Takamatsu) in Japan. The enclosure is of 8 ha. and uses galvanized chain link mesh for the main fish netting to reduce marine fouling and subsequent maintenance and cleaning (1973) harvest here was 1000,000 yellow tail weighing 300g each) (Milne, 1979b).
In Scotland galvanized wildmesh and galvanized scaffolding were chosen as the most suitable materials for the construction of 12M square fish enclosures at Faery Isles, Loch Sweed in Argyll. In loch Sween there is a tidal range of 1.5m only and therefore, was suitable (Low tide range) for access catwalk. The netting materials chosen was so good that an inspection by divers after 5 years showed that the original galvanized mesh netting was still in good condition (Milne, 1979b). Galvanized scaffolding and weldmesh are acceptable because of the ease of erection and subsequent lesser maintenance.
In Faery Isles the corners of the enclosures were curved to a 1.5m radius, assuming that square corners were not useful in a fish using a sliding screen. Subsequently in the enclosures built in ardtoe in 1972, (7 × 3m enclosures) square corners were adopted.
When fish are stocked at high density predators such as sea birds and others are attracted. To avoid this surface covers of galvanized weldmesh were provided at catwalk level. The surface covers were of 2" (51mm) spacing net and were light and were hinged at the centre of the enclosure enabling easy ascess.
We have already referred to the sublittoral enclosures in Norway (Flogyjolpo and Volkjolpo) for Atlantic salmion farming.
A sublittoral rigid enclosure is the one built in Loch Moidart in 1974. Here there is a tidal range of 5m and therefore any structure rising above the tides will have to be very high. To reduce this problem a 40 M3 pen with a top cover with its upper surface just above the low water neap tide level, ensuring access to the enclosure at the low tide level, was constructed. This is example of an enclosure (pen) with a top cover which can be submerged and by definition a pen with the natural floor as the bottom.
We have already referred to the Osland enclosure (Fig 8A, B & C) of Norway usually installed near a sloping shore. About 25m from the shore line the depth is to be about 10m. Concrete or timber dock piles are driven with the sea bed, forming a horse shoe from of about 25 × 35m. A concrete wall is built on the shore to moor the nets. The piles are higher than spring water level. Two nets are stretched, one inside net would hold the fish, the lower fold would work as an effective fish seal and the outside one would keep off drift wood etc., and predators. The nets are re-inforced by a head line, midline and a lead line. There is a provision for attaching a hauling rope (see Fig. 8C) to the midline so the nets can be pulled up for inspection. The hauling rope move through a hole in the cross bar and a cremp fixed near the bottom of the pile. Other details are given in the figure, (see Fig. 8B, C).
In designing the fish pen structure, effects of three forces, namely, wind, waves and drifting water hyacinth (Eicchornia) have to be considered. The wind force acting on the fish pen can be calculated from the wind pressure and the surface areas on which the wind acts.
Fig. 8a. Osland enclosure - plan view
Fig. 8b. Elevation view of netting fixed on piles.
Fig. 8c. Attachment of net in the Osland enclosure (after Milne, 1979a)
Compared to the wave action, which is based on the wind, on the pond dykes (levees) the wave action on fish pen structures on the lake pens in said to be the minimal. However, the waves cause a “sail affect” on the pen structures causing the nets to drag away from the pen. Major damage can be done by drifting water hyacienth mass (1–5 hectares spread on occasions). A single water hyacient plant can weigh 10kg and occupy an area of 0.10M2. A hectare of water hyacienth formed by the wind can hit the fish pen in a force equivalent to 15 tons.
The catfish, Chrysichthys nigrodigitatus and C. walkeri were tested in pens in Cote d'Ivoire by Hem (1979), who found that in properly designed pens, using locally available materials (enclosures made with wooden poles and nylon netting) the catfish can be successfully cultured. More details are given in section 3.6.
Most of the buoyed fish net enclosures known are bag nets with a bottom net - as per our definition this enclosure is a ‘cage’ (and not a pen/enclosure) even if the bottom net may rest on the floor. Such enclosures are limited in their size - the largest being of 55m diameter in Japan.
A generalized plan of a milkfish pen, showed rearing, nursery area and an attached ware house in Philippine is shown in Fig. 9. Figure 10 shows the details of a split-bamboo fencing for a traditional milkfish pen. Different types of framing arrangements, showing bamboo poles as piles and netting used are shown in Figures 11A and 11B. Poles have been supported either with a simple bracing (Fig. 11A) or with double bracing (Fig. 11B). Figure 11B also shows a double enclosure. Design of a fish pen wall defined by the Laguna Bay Development Authority, Philippines is given in Fig. 12. Details of a pen enclosure net in use in (Philippines, described by Alferez (1982), are given in Fig. 13. See Guerrero and Soesanto, 1982 for more details. When the Philippine pen-culture fishery was affected greatly by typhoons, alternate designs were considered. Since the Japanese or Faery isles piled net barrier type could not be adapted, “due to extreme lengths of piling required to withstand typhoon conditions” in soft silt in Laguna de Bay for considerable areas a floating net enclosure for a 10 ha area was designed (Fig. 14) (Miline, 1974; Delmendo and Godney, 1976).
In fabricating the flexible enclosure care was taken to make use of locally available simple materials (cf. Bamboo mat replacement), eventhough Japanese 9.5mm (3/8") nylon net, which has antifouling properties and long life as opposed to the one-year life Philippine nylon net, was used. Concrete block sinkers weighing 500kg were spaced 30m apart and were placed from a boat and a chain link raiser chain was provided from the sinker for attachment to the net to allow settlement in the soft sediment. The average depth of Laguna de Bay varied from 3 to 5m and therefore a 7.5m height of net was chosen to allow billowing due to water current and wave motion. In the case of the bamboo pen the fencing stretched above water surface to prevent fish from jumping, but in the present net, a 2.5m horizontal flap was provided on top with float, which effectively prevented fish jumps and also helped as a fish seal when water level rose unprecedented, for the top floats help adjust the stretching of the net into a vertical wall.
A lattice work of nylon ropes, at 10m × 2.5, crosses, from the top net down to the sinkers was provided to spread the load of the forec ‘due to water movement and wave action’. In the construction the only Underwater work involved is the attachment of the chainlink risers to the chain link foot rope-all the others, the lattice work of nylon ropes, nylon netting and chain link foot ropes can be assembled on the shore and connected to previously placed buoys and sinkers, as explained by Milne (1979).
Fig. 9. A milkfish pen, showing nursery, rearing area and warehouse.
Fig. 10. Split bamboo fencing for milkfish pen.
Fig. 11a. Framing arrangement in traditional milkfish pen - Poles with single bracing.
Fig. 11b. Framing arrangement in milkfish pens - double bracing, edge view (left) and double enclosures (right)
Fig. 12. Design of a fish pen will as defined by LLDA (from Alferez, 1982)
Fig. 13. Details of pen enclosure net (from Alferez)
Fig. 14. Floating fish net barrier for milkfish farming (from Milne, 1979)
The advantages are:
There is no need for allowances for wind forces on the exposed section of netting and structures, especially useful in typhoon-affected areas.
Ease of access of boats, since no boat locks are required and smooth bottom boats may alide over the net barrier and the engine propeller can be lifted clear or shielded (see also boat lock described earlier).
Milne (1979b) who designed this flexible enclosure is of the view that this can be fixed anywhere near the shore line, simplifying design and construction.
