Ecological and Genetic Implications of Aquaculture Activities

Abstract

Protein is the most expensive component in fish feed and the main source of nitrogenous pollution in fish culture. It is therefore economically desirable to maximize the conversion of feed protein into a valuable biomass, and environmentally desirable to minimize nitrogenous waste. Both ends can be achieved in algal-dependent integrated mariculture. Unlike fish-cage effluents, fishpond effluents can be treated. Use of marketable organisms as biofilters increases both diversification and income of the mariculture operation. Algal biofiltration has a stabilizing influence on water quality since, unlike bacteria, algae counteract the consumption of oxygen and the production of CO2 by the fish. However, algal biofilters probably cost more to build and use a larger land area when compared with the alternative bacterial nitrification- denitrification aquaculture biofilter systems. Biofiltration by algae and shellfish, as has been developed as a part of integrated mariculture at the Israeli National Center for Mariculture, offsets the added cost by added sales of the biofilter organisms. This approach is based on algal sunlight-dependent assimilation of nutrients and their conversion into microalgal or macroalgal biomass. The algae are then consumed by algivore invertebrates such as bivalves, gastropods, and sea urchins. Sea urchins in particular can also be used to remove organic detritus. In the fish-phytoplanktonbivalve integrated culture system, bivalves filter microalgae that develop in fishponds and sedimentation ponds in which fishpond effluents collect. Such systems require large treatment areas relative to fish production, and are therefore suitable for extensive or semi-intensive fish culture with relatively low land costs. In fish-seaweed integrated culture, nutrients are removed from the fishpond effluents by green macroalgae such as Ulva lactuca. The macroalgae (seaweeds) can subsequently be fed to macroalgivores or processed for human consumption. While water from the fishponds passes through the macroalgae ponds, most of the ammonia (up to 90%) and significant fractions of other dissolved nutrients are removed from the water. The macroalgae improve the quality of the water by adding O2 and removing excess CO2, thus making the water suitable for recirculation into the fishponds. The seaweed produced is excellent for feeding high-value macroalgivores such as abalone and sea urchins. Fish-seaweed integrated systems require less extensive land area than fishphytoplankton- bivalve integrated systems and produce cleaner effluents. However, they may require larger inputs of capital and energy. Fish culture integrated with algal and shellfish biofilters has the potential to expand sustainably, meeting economic, environmental, and social concerns.