Cage and pen fish farming. Carrying capacity models and environmental impact.
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The use of cages and pens to rear fish in inland waters is an increasingly popular method of fish culture, involving relatively low initial costs, and simple technology and management methods. However, these water-based culture methods differ from land-based operations such as ponds and raceways in that they are open systems, where interaction between the fish culture unit and the immediate environment can take place with few restrictions, and they are often sited in publicly-owned multipurpose water bodies. Thus any impacts may lead to a conflict of interests. A number of studies have demonstrated that the cage and pen structures can affect the multi-purpose nature of water bodies, by restricting space which might otherwise be used for fisheries, recreation or navigation, and by interfering with currents and sediment transport. In some circumstances predators and disease-bearing organisms have been introduced or attracted to the site. However, the most significant impacts are due to the method of culture employed. Intensive operations can affect water quality, and influence the biomass and diversity of the benthos, plankton and nekton. It is argued that the P loadings to the environment are the most important components of the wastes. The role of P in the diets of fishes is reviewed, total-P loadings are quantified for both intensive tilapia and trout culture operations, and the P loading models developed by Dillon and Rigler (1974) adapted to predict the environmental i mpact of intensive cage culture on the aquatic environment. Tentative development limits are also proposed. Following a review of current information on energy transfer from plant to herbivorous fish in ponds and lakes, efficiencies of 1.0 – 3.5% plant carbon : fish carbon are suggested as attainable from extensive cage or pen culture. This is considerably higher than yields from lentic bodies managed for fisheries. The efficiency of transfer will vary with productivity, and the relationship b etween primary production and fish yield is likely to be sigmoid, as suggested by Liang et al (1981) for fisheries yields. The carrying capacity of freshwaters for semi-intensive culture depends upon the quality and quantity of feed used, and the productivity of the site. A simple model, combining extensive and intensive-type models is proposed. The models proposed for use in predicting the environmental impact of cage and pen culture are in the initial stages of development and have yet to be validated and calibrated. Several methods for reducing the impact of intensive culture methods are proposed, and these include combining with extensive operations. Finally, it is proposed that some categories of water body may be unsuitable for large scale culture operations.