Selection for improved stress tolerance in rainbow trout ( Oncorhynchus mykiss) leads to reduced feed waste

The magnitude of the cortisol response to a standardised stressor is a heritable trait in salmonid fishes, and selection for stress responsiveness induces differences in both behaviour and neuroendocrine function. For instance, in laboratory studies, fish with a high cortisol response show a greater propensity for stress induced anorexia. Some authors have, however, commented that controlled studies encompassing relatively small groups of fish hold little or no relevance to practical aquaculture. This notion may be flawed, since understanding the mechanisms behind the behaviour of individuals is a proviso to predict behaviour in groups even with the caveat that some behaviors may be modified by group size. As an example, optimal feeding regimes should be easier to predict in a population consisting of individuals whose appetite is relatively less affected by external stressors. In a fluctuating and potentially stressful environment, such a population should also generate less feed waste, if kept on fixed rations. In the present experiment, we tested this hypothesis by monitoring feed waste and feed conversion efficiency in lines of rainbow trout selected for a low (LR) or high (HR) cortisol response to stress. The study was carried out after fish had been transported between rearing sites in the United Kingdom and Norway. There was significantly more feed waste from rearing units containing HR fish, and these fish also showed lower feed efficiency (growth per unit feed consumed). The difference in feed waste became more apparent with increasing time after transport, as rations increased. Simultaneously, size was more variable and growth was slower in HR rearing units. These results suggest that there are several potential benefits of selection for low stress responsiveness in aquaculture.