The impact of water exchange rate on the health and performance of rainbow trout Oncorhynchus mykiss in water recirculation aquaculture systems

A controlled six-month study was conducted in six replicated water recirculation aquaculture systems (WRAS) to assess fish health indicators in relation to high feeding/low flushing conditions. Juvenile rainbow trout (Oncorhynchus mykiss) were stocked in six identical WRAS (1000 fish per system), and were maintained over the study period between 25 kg/m3 (minimum) and 80 kg/m3 (maximum) densities. Three WRAS received a relatively high makeup water exchange rate (2.6%), while three systems received only 0.26% exchange; mean feed loadings of 0.39 and 4.1 kg/day per m3/day of makeup water flow were maintained for the high and low exchange treatments, respectively. At the end of the study period, there were surprisingly no significant (p < 0.05) differences in average fish weight between treatment groups. Percentage survival was excellent in both high and low makeup system populations (99.5% ± 0.1 and 98.9% ± 0.4, respectively). Histopathological evaluation revealed a significant increase in splenic and skin lesions in low makeup system fish relative to those sampled from the high makeup systems, while lesions in the posterior kidney were more prevalent in the high makeup system fish. Plasma chloride and blood urea nitrogen were also significantly different between fish from the two treatment groups, and caudal fin quality assessment showed significantly greater fin erosion in low makeup system fish. Despite these clinical and subclinical differences, there appeared to be no major treatment effect on overall fish performance.