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Title: Observations on side-swimming rainbow trout Oncorhynchus mykiss in water recirculation aquaculture systems

item GOOD, CHRISTOPHER - Freshwater Institute
item DAVIDSON, JOHN - Freshwater Institute
item KINMAN, CHRISTIN - Freshwater Institute
item KENNEY, P. BRETT - West Virginia University
item SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2014
Publication Date: 9/17/2014
Citation: Good, C., Davidson, J., Kinman, C., Kenney, P., Baeverfjord, G., Summerfelt, S. 2014. Observations on side-swimming rainbow trout Oncorhynchus mykiss in water recirculation aquaculture systems. Journal of Aquatic Animal Health. 26(4):219-224. DOI: 10.1080/08997659.2014.938870.

Interpretive Summary: "Side-swimming" fish, i.e. those that swim in a controlled manner but are oriented perpendicular to the direction of gravity, are occasionally seen during rainbow trout growout trials at The Freshwater Institute. While this condition is generally not related to health or welfare issues in our experience, the presence of side-swimming fish in cultured fish populations is inherently conspicuous, and questions regarding these fish are often raised during tours of our facility. Therefore, investigating the causes of side-swimming could help in efforts to reduce it's overall prevalence, and consequently assist in improving public perception of the aquaculture industry and the quality of products from fish farms. We sought to investigate side-swimmers and normal swimmers in order to develop a baseline understanding of important anatomical and physiological differences between these two populations. Side-swimmers were found to be significantly smaller in length, weight, and fillet yield, while blood gas and chemistry analyses revealed differences suggesting increased swimming exertion in side-swimming fish. Most side-swimmers exhibited malformed or abnormally positioned swim bladders; however, abnormal swim bladders were also observed in a portion of the regularly swimming trout. Despite the differences between side-swimmers and normal swimmers that were determined in this study, no pathognomonic physiological or anatomical difference(s) was elucidated, and therefore further research to investigate this phenomenon is required in order to assist farmers in reducing side-swimmers in their fish populations.

Technical Abstract: During a controlled 6-month study using six replicated water recirculation aquaculture systems (WRAS), it was observed that rainbow trout Oncorhynchus mykiss in all WRAS exhibited a higher-than-normal prevalence of side-swimming (i.e. controlled, forward swimming, but with misaligned orientation such that the fish's sagittal axis is approximately parallel to the horizontal plane). To further our understanding of this abnormality, a sub-study was conducted wherein side-swimmers and normally swimming fish were selectively sampled from each WRAS, and growth performance (length, weight), processing attributes (fillet yield, visceral index, ventrum (i.e. thickness of the ventral “belly flap”) index), blood gas and chemistry parameters, and swim bladder morphology and positioning, were compared. Side-swimmers were found to be significantly (p<0.05) smaller in length, weight, and fillet yield, and had higher ventrum indices. Whole blood analyses demonstrated that, among other things, side-swimmers had significantly lower whole blood pH and higher pCO2. Side-swimmers typically exhibited swim bladder malformations, although the positive predictive value of this subjective assessment was only 73%. Overall, this study found several anatomical and physiological differences between side-swimmers and normally swimming rainbow trout. Given the reduced weight and fillet yield of market-age side-swimmers, producers would benefit from additional research to reduce side-swimming prevalence in their fish stocks.