2. Requirements of Pen Culture and Site Selection

There are two separate aspects to be considered, one is the requirement of the species (biological) and the other is the structural requirements of the pen (physical). Both these in turn are linked with the process of site selection. We have already referred to the inter-tidal and sub-littoral coastal areas (marine and brackishwater) and also the shallow freshwater bodies as suitable sites for pen culture.

2.1 Biological factors:

The enclosure conditions should be suitable to the species to be cultured. Success of the culture system would depend on the understanding the optimal requirements of the species. Now even the fast swimming yellow tail and the highly demanding Atlantic salmon are being cultured in pens.

For better survival and growth rate of the species under culture optimal conditions of the following factors should be known.

1. Temperature

2. Salinity (brackish waters)

3. Oxygen

4. Other water conditions (e.g. pollution)

5. Feeding rate and pattern

6. Crowding (stocking density)

7. Predator control

8. Fouling by algae and mussels

The temperature, oxygen requirement, water quality (pollution) feeding and crowding are closely related. The optimal stocking of fish in an enclosure is dependant on the availability of oxygen, the rate of oxygen demand of feed remains and excreta (increase of B.O.D). Oxygen content is also as well recognized related to temperature and salinity. The abundance of other organisms (plankton, benthos) within the pen also would deplete oxygen. Since temperatures and O₂ demand of fish are high in summer the oxygen lack problem will be acute then.

Thus in the enclosed area better water quality can only be maintained if there is a current of water - “a strong current of about two knots, however, requires a strong cage or enclosure structure to hold the net in place” (Moller, 1979). Recourse to artificial pumping of water to have a good circulation in the enclosure is also suggested.

Pollution (increased B.O.D. from feed loss and excreta) of the farm (enclosure) area would depend on the species of fish, their size, feeding ration etc. Moller (1979) states that the magnitude of pollution from a fish farm may be as much as 60% of the total feed given. To cite Moller, “more than half of this value represents feed loss. Faeces and food particles sink to the bottom. If the water flow is low and enclosure bottom bounded by reefs or other thresholds of the main basin, the wastes will accumulate and may contaminate the farm. Small fjords (or bays or coves for the matter) are not good localities for intensive fish farms above a certain size”.

Fouling is a biological problem. Since the waters around the enclosure are rich in nutrients algae and mussels also grow well attached to the framework and nets of the enclosure. To quote again Moller, “fouling makes the net heavier, increases the drag in current and prevents the exchange of water in the pen”. The excess nutrient problem (which causes fouling) would be less if there is a good circulation of water through the enclosure, but the nutrients can also be utilized economically through polyculture. Moller suggests introduction of European oyster (Ostrea edulis) in Norwegian fish farms for the purpose, but low temperature prevents development of sexually mature individuals. This may not be a serious problem in the tropics. Fouling can be reduced by cleaning the nets in chemicals. The shipworms (Teredo) also cause difficulties by boring into untreated wood in enclosure structures.

Predation causes heavy mortality in pens, even upto half the total loss of fish (Moller and Bjerk, 1975). Birds (herons, gulls and cormorants), Mammal (seals, otters) and larger fish are the major predators. Moller (1979) states with understanding, “predatory animals often demonstrate unbelievable cunning in reaching the penned fish, and they acquaint themselves with the protective devices which have been installed”. To protect the pens, the enclosures have to be completely sealed by nets and wires - safety nets are to be used against predatory fish and seals and birds are to be scared away by shot guns.

Protection against predators and poaching have been discussed at the “International workshop on pen and cage culture”, and a list of predators and their most satisfactory control as summarised therein is given hereunder:

Tropical Countries:

Rats: dogs or rat guards/traps

Turtles: Perimeter nets

Others: dogs or perimeter nets as control, a mono-filament perimeter net used to entangle and drown animals.

Monitor lizards: perimeter net or nets with feet buried in the earth

Birds: monofilament net laced cover on the cage top

Puffer fish: netting is stretched tight.

Temperate Countries:

2.2 Physical (structural) Factors

Those as we pointed out refer to the structural requirements of the enclosure, but it is hard to distinguish between the interactions of physical and biological factors involved. As given above some physical problems have already been discussed. The structural details of the enclosure will be discussed separately. Rightly the physical requirements will be dealt with under “Site exposure” (see below), but we shall discuss below general characteristics of ‘site selection’ first and then take up ‘site exposure’, as dealt by Milne (1979a). Since the general characteristics of the site are discussed under “cage culture” we shall refer here only to certain salient points pertaining to pen culture.

2.3 Site Selection

As observed before, enclosure culture is practiced in the intertidal and sublittoral areas in the brackish and marine waters and in the shallow freshwater bodies in areas closer to the shore.

Primarily the hydrography of the waters must be studied to know the seasonal changes in water temperature, salinity, oxygen content, water currents, pollution levels etc. If brackish water fish are to be farmed in the intertidal area there is no need to control salinity, but if the marine froms are to be farmed (e.g. plaice in Artdoe, Scotland), then salinity has to be controlled. The rainfall from the catchment must also be assessed, since it is “essential that the fresh water does not become impounded” in the enclosure (sealed embankment); thus in the intertidal area of the freshwater draining into it must be diverted (Fig. 3).

Fig. 3. An intertidal embanked pen with a rockfilled dam (after Milne, 1979a)

In the ‘intertidal’ enclosure embankment type (Fig. 4) it should have as large a volume between high and low water neap tides as possible. If water level is kept at high water neap tide level water filling and draining can be effected at any time in the spring-neap cycle. And if the pond is chosen above the low water neap tide mark the pond may be drained for harvesting/maintenance anytime. This type of intertidal enclosure (very similar to ponds - but as per our definition, they are pens) can be manipulated to vary salinity as required. If salinity control is not required a net enclosure instead of a sealed embankment can as well serve the purpose in the intertidal area.

Sublittoral enclosures are relatively inexpensive, but additional costs have to be incurred in maintenance of nets and water circulation. The site selected should have “the variation between high and low water as small as possible, bearing in mind that there should also be adequate circulation within the enclosure at neap tides. The reason for this is that it is the low water volume which determines the density of stocking of the enclosure” (Milne, 1979a). As already mentioned the area selected should have adequate physico-chemical conditions and should be free from pollution.

2.4 Site Exposure

The site chosen is to be first cleared from the point of view of the biological requirements of the species to be cultured and now the site should be considered from the aspect of exposure to winds and waves, and it would be necessary to design the structures above and below water, adequate to withstand hurricanes and typhoons as well.

For studying the effect of wind forces on structures above water it is essential to know the mean hourly wind speeds and maximum gust speeds occurring in the vicinity of the site - such information is usually available from the meterological office of the countries concerned.

For structures below water level wave force induced by wind blowing over a period of time are to be considered. For design of solid structures the estimates are done easily by engineers, but for mesh retaining structure, the evaluation of design forces is more complex. A method of ready calculation is given in Appendix 1 in Milne (1979a). For further details of designs of structures in the sublittoral area Appendix 2 and 3 Milne (1979a) may be seen.

Various factors to be considered in the selection of pen (also cage) sites, as summarised in the “International Workshop on cage and pen culture”, 1979 (Philippines), for Marine, brackishwater and freshwater conditions, are listed in Table I.

Among these, we have already, referred to protection from elements, (wind, waves, tides and currents), and water quality and soil types (chemical, physical, biological, pollution). In addition an aspect considered in the table is “artificial protection” from wind, water currents, floods and typhoon, and the use of freshwaters and deflectors and also a more important socio-economic problem, i.e. access and security, which include supplies of materials, feeds and fingerlings, markets (live and fresh sales), labour (availability and cost), monitoring (easy access for regular monitoring visits), security and others including frequency of navigation, property rights, policies and laws and social aspects. Some of these have already been considered under “cage culture” and will not be dealt with here again.

Table I. Factors to be considered in the selection of cage/pen sites

(From SEAFDEC/IDRC, 1979)

Fig. 4. Ideal beach location for intertidal enclosure (after Milne, 1979a)