Prospects and pitfalls of using heart rate bio-loggers to assess the welfare of rainbow trout (Oncorhynchus mykiss) in aquaculture

Investigating real-time stress responses during common aquaculture practices is necessary for improving the welfare of farmed fish. Recently, Star-Oddi DST milli-HRT bio-loggers capable of measuring heart rate (as an indicator for stress) in freely swimming fish have proved to be useful in this regard. However, despite the growing popularity of these devices, they have not yet been stringently evaluated and validated for use in fish. The present study is the first to do so by simultaneously recording heart rate of a commercially important salmonid species (rainbow trout, Oncorhynchus mykiss) using the abovementioned bio-loggers, as well as an established non-invasive wireless heart rate recording technique that has been demonstrated to provide more accurate recordings than those collected using traditional methods requiring a physical connection to recording equipment. The measurement error of the bio-loggers was < 1 beat min(-1) irrespective of the sampling period when only using measurements graded with the highest quality index (QI(0)) and it is advised that only QI(0) measurements be used as inclusion of poorer quality measurements (QI(1-3)) markedly reduce measurement accuracy. Furthermore, sufficiently long recovery periods should be employed prior to collecting data with these devices in salmonids, as anaesthesia and surgical implantation of bio-loggers resulted in transiently elevated resting heart rates for > 72 h. Following the successful validation of these bio-loggers, the effects of common aquaculture practices such as netting, grouping and enforced activity on the cardiac performance of freely swimming rainbow trout were investigated. Prior to the subsequent exposure to the abovementioned stressors, mean heart rates of trout were similar to 42 beats min(-1). Following netting, trout increased heart rates by 57%, which recovered within 3 h. Grouping subsequently induced a more pronounced and prolonged stress response, as heart rates initially increased by 86% and then remained elevated throughout the 8 h recovery period. Consequently, trout were only able to further increase heart rates by 7% following enforced activity. This diminished cardiac response demonstrates a situation where the cumulative allostatic load induced by prior stressors is approaching a level where the animal struggles to perform normal physiological processes in the face of further challenges. These findings are important for the aquaculture industry as exposure to multiple stressors without sufficient recovery times has the potential to result in the collapse of cardiac scope, which combined with unfavourable environmental conditions could lead to increased mortality.