Heart deformities, impaired physiological performances and reduced stress tolerance in adult rainbow trout (Oncorhynchus mykiss) farmed under intense conditions

Abstract

In salmonid aquaculture, an increasingly important food production sector, intense and warm rearing conditions boosting growth rate and reducing production time may cause irreparable consequences. In juvenile rainbow trout (Oncorhynchus mykiss), warm rearing inducing rapid growth has resulted in abnormal and deviating heart morphologies compared to juvenile trout reared at cooler temperatures. This was associated with reduced cardiac function, metabolic performance and an impaired warming tolerance. However, it is unknown how the negative consequences of warm rearing develop over time and if they persist in adult fish. In this project, I studied differences between warm reared adult and juvenile (assessed in a previous experiment) rainbow trout in terms of their physiological performances and stress tolerance. Fish were instrumented with electrodes to determine heart rate and cardiac activity, and fish metabolic rate was assessed using respirometry. The fish were subjected to a chase protocol where the maximum cardiac and metabolic capacities were evaluated, to determine their scope to elevate heart rate and metabolic rate above resting levels when needed during stress. Fish were then subjected to an acute warming protocol to determine the warming tolerance of the fish. The heart of each fish was then dissected for morphometrical analyses. My results showed that cardiac deviations persisted in adult rainbow trout. Adult fish exhibited lower maximum heart rate and metabolic rate following physical stress, reducing their heart rate and metabolic scopes. Adult fish also presented a lower warming tolerance than juvenile fish. This was associated with an inability to maintain a high heart rate and metabolism at critically high temperatures in the adult rainbow trout. The impaired physiological capacities and reduced stress resilience caused by warm rearing observed here constitute an animal welfare concern and could explain mortalities and production losses during stressful events of salmonid fish, including rainbow trout, in aquaculture.