Individual and Population Level Variation in Growth Parameters for Steelhead Trout Oncorhynchus mykiss in Central California

17 We fit a bioenergetics model based on a balance between mass and temperature-dependent 18 anabolic and catabolic factors to growth data for juvenile steelhead trout (Oncorhynchus 19 mykiss) from California. We grew fish from a small coastal stream (Scott Creek) conser20 vation hatchery and a Central Valley (Coleman National Fish Hatchery on Battle Creek, 21 Sacramento River) production hatchery in a common laboratory setting. Our growth 22 model did not characterize the trajectories of all fish, likely as a consequence of enhanced 23 growth via cannibalism or inhibited growth due to risk averse behavior such as avoidance of 24 dominant individuals. For most individuals, however, it provides a close match and allows 25 us to provide an excellent description of individual and stock-specific variation in specific 26 rates of anabolism and catabolism. We use likelihood methods to conclude that there is a 27 strong difference in the mean rates based on origin of the stock, with Central Valley fish 28 having a higher maximal consumption ability but both strains having similar metabolic 29 needs after standardizing for fish size and temperature. This suggests genetic differences 30 between strains base on local adaptation and/or differing degrees of domestication). As 31 a result, environmental change may affect the growth (and thus survival, life history, and 32 demography) of the strains in different ways. 33