The following discussion on environmental impact of pen culture is largely based on the study of Beveridge (1984).
Pen and cage culture can be classified into 3 categories on the basis of feeding; extensive, semi-intensive, and intensive. Extensive culture relies solely on natural food such as plankton and benthic drift and there is no supplemental feeding. In semi intensive culture, a low protein (less that 10% feed such as rice bran is given. The cultured fish still depend to a large extent on the naturally available food. In intensive culture, the fish depend almost entirely on a high (more than 20%) protein feed which usually contains fish meal, and little nutrition is gained from the natural environment.
Intensive culture is largely restricted to temperate, developed regions, where carnivorous species such as salmon and trout are grown using expensive, high-protein feeds (see earlier discussions). Intensive feeds are not essential in tropical fish culture, since many of the farmed fish such as tilapias, carps, and milkfish feed on natural macrophyte, plankton, and detrital production. Supplementary feeding with low cost, low protein agricultural by-products is widely used to improve production. Even in tropical countries where complete diets suitable for intensive feeding have been formulated, they are not in common use since they are too expensive. Most of the cultured species have low retail prices with small profit margins for the producer.
Both pen and cage farming differ significantly from other fish culture systems (ponds, raceways) as the there is little or no control over the aquatic environment in which the fish are kept. We have discussed these in detail earlier and hence would not repeat them, but as aspect which needs further discussion is the environmental impact of pen culture. The relationship of the effect of the operation on the environment itself and the effect of the environment on the operation.
Pens can compete with lake and river fisheries for space, and can bother navigation. Probably the best example of this is Laguna de Bay in the Philippines. Pen culture was introduced in the late 1969's, and became very popular Laws passed in an attempt to limit the pen culture area to about 15,000 ha were passed to reduce conflict between the fish farmers and fishermen. However, there has been a rapid expansion of the pens outside the legal zone, and now there are approximately 34–40,000 ha of pens now covering almost half of the lake. (See also “milkfish pens” given earlier).
In lakes and reservoirs, pens are restricted to the shallow zones near the shores. This can interfere with the spawning of many species of fish, and the shelter needed by the young fish.
The flow of water through pens in effected by drag forces caused by the framework and netting. The reduction in flow is dependent on a number of variables, including current speed, water density, enclosure size and shape, mesh size and type (knotterd/knottless), netting material, degree of fouling, and stocking density.
In intensive culture system, the increased nutrient levels introduced into the environment may influence the development of disease causing agents, particularly intermediate hosts of parasites.
Because of the high concentration of fish, pens act as a magnet to fish eating birds, reptiles, and mammals. In West Africa, it is not uncommon to see herons occupying many of the wooden posts of an enclosure support. Predators not only consume fish directly, but can also seriously damage nets and other pen structures. In Africa, pens can be seriously damaged by monitor lizards causing massive loss of fish. It would of course be fool hardy to place enclosures where crocodiles are present.
Predatory fish are also attracted to the pens. Large barracude will damage nets in brackksh water lagoons (See also “Predators” discussed earlier).
Cultured fish frequently escape from pens, and this can introduce exotic species into the environment. In any commercial pen farm, it is almost inevitable that some fish will escape, escape. The effect on the natural fish population of introduced species is well documented, be it favourable or not. Impact includes extermination of existing species either through predation of competition, habitat distruction, or the outbreak of disease (Epidemics).
Wild fish can also enter the enclosure, competing for available spece, oxygen, and feed. This can be very significant and have an economical impact if the quantities of invading fish are large. Also, small predatory fish can enter through the mesh. If the stocked fish are of a small enough size, the predator can consume an enormous number over a length of time.
As there is no control over the environment, toxic substances in the water body will affect the cultured fish. This includes all kinds of industrial and petroleum pollution as well as more nutural fish toxicants such as toxic algae, hydrogen sulphide, free ammonia, etc.
In West Africa, deliberate poisoning of fish in large water bodies by using insecticides (Lindane, Thiodan) is increasingly common. The effects of such poisonings can be felt several kilometres away, and can decimate pen culture operations.
With intensive culture techniques, there is an increase in the nutrient level of the environment because of the quantity and quality of the given feed. While at the present time, most imtensive culture is confined to temperate, developed countries, an understanding of the effects on the environment are equally important in tropical countries.
Intensive pen farming has several effects on the water quality, including increase in levels of suspended solids and nutrients such as total phosphorus (P), phosphate (PO4-P), ammonia nitrogen (NH3-N), organic nitrogen (N) and carbon (C). There is also a decrease in dissolved oxygen in and around the enclosures. In the sedimentation around the pens and cages, considerable increase in the oxygen consumption and in the total - N, total -P, and organic content of the mud occur (see g following). There are also quantitative and qualitative changes in the flora and fauna of the surrounding environment including bacteria, protozoa, plankton, benthos, and fish.
The opposite occurs in extensive farming, where the cultured fish feed solely upon nutritional sources derived from the environment. If carried out a large scale, extensive farming will remove a significant amount of nutrients form the ecosystem in the form of plankton, benthos, and detritus.
Sunlight and temperature can effect the materials used in pen construction. Among materials used in nets, polypropylene and polyethylene (materials commonly used in feed bags) will deteriorate quickly under intense sunlight, where as nylon netting is much more resistant.
Phosphorous is an essential mineral for fish, as well as usually being the limiting nutrient in aquatic ecosystems.
Fish obtain P almost exclusively from their diet. Most diets used in intensive culture have P in excess to the needs of the fish, or in a form which partially unavailable to the animal. Surplus P is excreted either by the kidneys or in the feaces.
In intensive pen or cage farming, for every kg of fish harvested, the environment can be enriched by as much as 0.75 kg of Carbon, 0.023 kg of Phosphorus, and 0.1kg of Nitrogen. This can be calculated in a variety of ways, using measurements on the water body, or by using the published data on P content of feeds, the FCR (Food Conversion Ratio), and the P content of fish carcasses.
The concentration of total P in a water body is determined by the P loading, the size of the water body (surface area, mean depth), the flushing rate (what perdentage of the total volume is lost anually through over flow) and the part of P permanently lost in the sediments.
In temperate climates, it is best to determine the total P concentrations during the spring overturn, as during most of the year stratification of the water column might result in faulty measurements. In tropical climates, total P should be determined as the mean measured surface P over one year.
The maximum eccetable limits for total P concentrations for lentic inland water bodies used inenclosure culture of fish depend on the climate and the species raised. In temperate climates and the species raised. In temperate climates and during salmonid species, the maximum acceptable P is about 60 mg/m3. In the same climate, carp can support 150 mg/m3. In tropical waters, the acceptable P can be as much as 250 mg/m3.
The capacity of the water body for intensive farming is to be estimated then from the difference between the initial P concentration and the acceptable limit.
Predicting the effects of extensive farming on an aquatic ecosystem is very imprecise. It is based on the relation between the primary productivity of the water body and the production of fish. Theoretically, 10–15% of the primary production could be converted into fish (tilapia) tissue. This theoretical assimilation value is 20 timew better than what happens in well managed fish ponds, which in itself is much more efficient than in nature. In highly productive ponds, the figure is about 1.4%, and about 1.3% in less productive systems. Estimates of extensive sage farming, where the fish are more concentrated and than in enclosures and depend largely on plankton, very 1.0% to 3.5% of the primary productivity.
