Hatchery Steelhead Smolt Release Size Effects on Adult Production and Straying

Studies have demonstrated higher postrelease survival for larger hatchery steelhead Oncorhynchus mykiss smolts than for smaller ones. However, the ideal release size should also maximize adult returns and minimize interactions of hatchery fish with wild fish. We conducted a study over five release years with paired groups of anadromous hatchery steelhead smolts reared to target sizes of either 90 g (small size) or 113 g (large size). After stream release, we monitored out‐migration travel times and survival to a downstream dam using freeze‐brand marks in a subsample of each release group. Steelhead smolts that out‐migrate quickly may interact less with wild fish. Across all release groups, travel time was about 3 d slower for small‐sized smolts than for large ones, but this difference was not statistically significant, and there was no significant difference in the percentage of brands detected at the dam between the two smolt sizes. Coded wire tag recovery was used to compare smolt‐to‐adult survival (SAS), the number of adults produced per 100 kg of smolts released, and the percentage of adults that strayed. Across all years, large‐sized smolts had an average SAS of 1.31% compared with 0.92% for small‐sized smolts, a statistically significant difference of 42.0%. The average number of adult steelhead produced per 100 kg of smolts released also was higher for the large‐sized group (11.4 adults/100 kg of smolts) over the small‐sized group (10.1 adults/100 kg of smolts), though the 12.8% difference was not statistically significant. Adults originating from large‐sized smolt releases strayed at a rate of 13.1% compared with a 7.9% rate for the small‐sized smolt groups, a difference that was statistically significant. In this study, larger hatchery steelhead smolts had a higher survival after release; however, we found tradeoffs between management objectives associated with maximizing adult production and those aimed at minimizing ecological risks.