Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2009
Publication Date: 9/1/2009
Citation: Good, C., Davidson, J., Welsh, C., Brazil, B., Snekvik, K., Summerfelt, S. 2009. The impact of water exchange rate on the health and performance of rainbow trout Oncorhynchus mykiss in recirculation aquaculture systems. Aquaculture. 294:80-85.
Interpretive Summary: At The Freshwater Institute in 2005, fish mortality increased rapidly during a period of high feed loading and low flushing in a single recirculating aquaculture system (RAS). At the time, all water quality parameters measured were at acceptable levels, and no infectious disease was diagnosed. This mortality event could represent a significant barrier for producers operating RAS at low flushing rates, and hence controlled studies have been carried out to further investigate this phenomenon in an attempt to understand the etiology of the observed fish health decline. This study reproduced the conditions (high feed, low flushing) that existed during the original mortality event, but was unable to reproduce this event over the 6-month study period. Surprisingly, fish reared under these conditions performed equally well compared to control fish raised in high flushing RAS, and although there were some significant differences in histopathology between sampled treatment and control fish, the etiology of the original mortality event was not solved through this study. This study led to follow-up research carried out to investigate the role of dissolved carbon dioxide and its effects on fish health in high feed, low flushing RAS.
Technical Abstract: Fish mortality in recirculating aquaculture systems (RAS) has been observed by the authors to increase when RAS are managed at low makeup water exchange rates with relatively high feed loading. The precise etiology of this elevated mortality was unknown, all typical water quality parameters were within safe limits, and no infectious diseases or opportunistic infections were diagnosed. To investigate this phenomenon, a controlled six-month study was conducted in six replicated RAS to assess fish health indicators in relation to high feeding / low flushing conditions. Juvenile rainbow trout (Oncorhynchus mykiss) were stocked in six identical RAS (1000 fish per system), and were maintained over the study period between 25 kg/m3 (minimum) and 80 kg/m3 (maximum) densities. Three RAS 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 overall but was significantly different between the 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, and the previously observed fish health decline was not reproduced in this experiment.