World Aquaculture 49 Summary Sea urchins require high-quality seawater to develop and grow optimally in culture, as indeed do many aquaculture species. The tolerance limits of S. droebachiensis for the water quality parameters described above appear to be as narrow as those for salmonid fishes, which are known to have stringent water quality requirements. Poor water quality in production systems may have negative effects on sea urchin culture performance: reduced growth rate, reduced feed conversion efficiency, increased mortality and down-regulation of different genes involved in test formation and metabolism. However, the lack of response to any of the water quality variables described here on feed intake of sea urchins is remarkable. It indicates that sea urchins have poorly developed appetite regulation when fed to excess and are able to ingest feed far in excess of nutritional and energetic requirements. Sea urchin farmers should be aware that ad libitum feeding of sea urchins under sub-optimal culture conditions can result in overfeeding. In turn, this will have a detrimental effect on water quality and easily lead to a negative spiralling effect with increased overfeeding and a progressive decline in water quality. Ironically it appears that sea urchins are well adapted to being fed at very low feed levels. Recent research suggests that restricted feed rates interspersed with periods of no feeding may provide the optimal feeding regime for cultured sea urchins. Recommendations Although studies that have been conducted on the effects of reduced water quality on sea urchin growth and mortality are not sufficient to set out exact tolerance limits for various water quality parameters, some recommendations for sea urchin farmers can be made. These mostly apply to S. droebachiensis, which is the most studied species with reference to aquaculture. They are also most relevant for holding systems where water supply is limited (e.g., recirculating systems), particularly those systems with limited water renewal. Oxygen A sufficient supply of oxygen is most important to sustain optimal growth of sea urchins in culture. Growth is proportionally reduced with decreasing oxygen saturation. Therefore, water should be fully saturated with oxygen to avoid reduced growth. This can be achieved by supplying sea urchins with large amounts of fully saturated water, aerating tank water, or supersaturating inlet water with oxygen. Moderate oxygen supersaturation does not harm sea urchins, so this represents a feasible way of ensuring optimal oxygen conditions without using too much water. Water movement increases oxygen availability for sea urchins. Holding systems that create water movement across the surface of all urchins held in culture tanks would be advantageous. This may be possible by using gravity, aeration, or by creating water waves in culture tanks, as used in abalone culture systems in several countries. Carbon dioxide In cases where water flow is reduced, it is possible that CO2 accumulates to undesirable levels. CO2 accumulation may be controlled by CO2-stripping through aeration. Simultaneously this will enrich water with oxygen in the case of undersaturation. Ammonia and nitrite Accumulation of ammonia and nitrite to toxic levels may occur in recirculating systems when biofilters do not function properly. Good design and management of biofilters is essential. Bacteria in biofilters also produce acid and contribute to reduced pH, together with CO2. Adding calcium carbonate to the water can increase pH and buffering capacity. Adding calcium may also prevent calcium depletion from slowing down the calcification of sea urchin tests. However, no experiments have been done to evaluate if the addition of calcium has any positive effect on sea urchins under intensive culture conditions. Phosphate In the absence of more specific data for sea urchins, and based on data available for other benthic invertebrates, the level of inorganic phosphate should be maintained below 10 µM. This is relatively easy to achieve in flow-through landbased systems but recirculating systems with limited water exchange should regularly monitor phosphate levels. Moni- (Drawn by Oddvar Dahl)
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