Physiological indicators of divergent stress responsiveness in male striped bass broodstock

Abstract

Future expansion of the hybrid striped bass ( Morone saxatilis) industry is limited in part by continued reliance upon wild sources for seedstock and the absence of domesticated broodstocks. Inherent in the development of a domesticated stock is the ability to identify and select individuals with superior performance attributes. Since the response to stress is a characteristic that may be of value in a culture environment, we evaluated a population of male striped bass for relative stress responsiveness. Male broodstock comprised of three families (MD, MD36 and NC) were exposed to a 1-min net challenge monthly for six consecutive months and then bled 1-h following the stressor. Mean plasma cortisol was highest (295±20.4 ng/ml) in July and reached its lowest level (88±10.9 ng/ml) in December at the end of the study suggesting that the fish adapted to the stressor. When fish were ranked for stress responsiveness based on their mean cortisol levels, high responders (HR; n=6) could be distinguished from low responders (LR; n=6) on the first three sample dates. When the fish were segregated by family, NC fish had significantly lower post-stress plasma cortisol and glucose levels when compared to MD and MD36 fish suggesting that the stress response is a genetically linked trait in striped bass as shown for other species. Selected HR fish were significantly longer than LR fish throughout the study but their weights were not significantly different. However, LR individuals had significantly better condition than HR fish throughout the study suggesting that there may be differences in energy partitioning between the two groups. Selected HR fish injected with bovine corticotropin releasing hormone (bovine corticotropin releasing hormone (bCRH); 50 μg/kg BW) had significantly greater plasma cortisol levels than injected LR fish 1 and 3 h following injection. Cortisol in bCRH-injected LR individuals did not change from pre-injection values. Plasma glucose in bCRH-injected animals followed a profile commonly observed following a stressful event, but there was no difference between HR and LR individuals. These results indicate that post-stress plasma cortisol levels are useful for identifying divergent stress responsiveness in male striped bass.