INFLUENCE OF TANK DESIGN AND HYDRAULIC LOADING ON THE BEHAVIOR, GROWTH, AND METABOLISM OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Subadult rainbow trout (Oncorhynchus mykiss) stocked at 48 kg/m(3) (3 lb/ft(3)) were subjected to treatments of tank design (rectangular plu g flow, circular, and cylindrical cross flow) and water exchange rate (1.5 and 2.5 exchanges/h) to determine their effects on fish behavior, growth, and metabolism. Ambient light levels and current velocities w ere also measured in each of three tank sectors (upstream, middle, and downstream) to determine their relative contributions to behavioral e ffects. Tank design significantly affected fish orientation to current , contact time with tank surfaces, and frequency of agonistic encounte rs, though aggression levels were relatively low overall. Gradients in fish distribution by sector were greatest in plug-flow tanks. Effects were either modified or eliminated by increasing the water exchange r ate front 1.5 to 2.5/h. Multiple-regression analysis showed the follow ing hierarchy of independent-variable effects on fish distribution: ta nk type> exchange rate> aggression level> current velocity> light leve l. Significant effects of tank design were also observed on fish growt h in terms of biomass gain (cross flow> plug flow> circular). These re sults were marched in metabolic studies, where both oxygen consumption and ammonia excretion were highest in circular and lowest in cross-fl ow tanks. Reduction (cross-flow compared with circular tanks) in oxyge n consumption averaged 13.6%, ammonia excretion 17.5%. These results w ere also modified by an increase in water exchange rate. Tank-design e ffects on fish metabolism and growth may be mediated, at least partly, through changes in fish behavior.