Life History Diversity of Snake River Steelhead Populations between and within Management Categories

Grouping populations for management may overlook the fine-scale diversity underpinning the stability and resilience of meta-populations and fisheries. A bimodal timing distribution of summer-run steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) historically was observed at Bonneville Dam (BON), the first barrier to upstream migration in the Columbia River basin. Early mode fish (A-run) tended to be younger and smaller (<78 cm) than later fish (B-run). While A-run fish spawn throughout the Columbia River basin, B-run fish spawn primarily in the Snake River basin. Managers used indices of these modes to make fishery decisions, and later these criteria were adopted for conservation. It is still unclear how life history and body size differences among wild Snake River populations are related to the categories at BON. We examined population parameters characterizing the two categories (date of passage at BON, length) and parameters directly affecting population dynamics (age composition, sex ratio). The life history portfolio of Snake River steelhead is quite diverse. There was broad overlap among populations in several respects, forming a gradient in life history characteristics rather than a dichotomous break. All populations produced adults <78 cm and adults returning after August 25. Median lengths of putative B-run populations were close to the criterion that was supposed to be a defining characteristic. In contrast, few A-run populations produced many adults ≥78 cm. Mean percentage of two-ocean fish was 52.1% for A-run populations and 82.0% for B-run populations. Mean age at spawn was greater in populations producing older smolts. Sex ratio was female biased, and older populations had greater percentages of females. Although the run-type dichotomy was useful for management of fisheries in the past, it is not useful for conservation. A combination of genetic stock identification at main-stem dams and population-specific monitoring in natal streams provides a unified framework for the assessment of fisheries management and conservation objectives. Received March 2, 2016; accepted November 9, 2016 Published online March 10, 2017