Smolt Transformation in Two California Steelhead Populations: Effects of Temporal Variability in Growth

—We tested the effect of temporal patterns in food supply on life history decisions in coastal steelhead Oncorhynchus mykiss irideus from a Central California coastal (CCC) population (Scott Creek) and a Northern California Central Valley (NCCV) population (upper Sacramento River basin). We manipulated growth through feeding experiments conducted from May to the following March using warm (2006 cohort) and cool (2007 cohort) temperature regimes. Survival in seawater challenges just before the time of typical juvenile emigration provided an index of steelhead smolt versus nonsmolt life history pathways. Survival varied significantly with fish size (with larger fish being more likely to survive than smaller fish) and by source population (with CCC steelhead being more likely to survive than NCCV steelhead of the same size). The timing of increased food supply (treatment group) did not significantly affect seawater survival rates in either NCCV or CCC steelhead. For both strains, the eventual survivors of seawater challenges (putative smolts) diverged from the eventual mortalities (putative nonsmolts) in both size and growth rate by June in both years, suggesting that the initial growth advantages were maintained throughout the experiments. A significant divergence in condition factor between smolts and nonsmolts by December matched the expected morphological transition of smolts, which showed faster growth in length than weight compared with nonsmolts. The apparent timing of the decision window, several months before the typical period of smolt emigration, matches the patterns observed for other salmonids. In coastal California, this decision must occur * Corresponding author: [email protected] Received August 13, 2009; accepted April 6, 2010 Published online July 19, 201