Abstract
Steelhead Oncorhynchus mykiss are ecologically, economically, culturally, and recreationally important throughout the Pacific Northwest. The potential influence of recreational fisheries on wild steelhead is poorly understood and is a function of the abundance of wild fish, how many are encountered by anglers (i.e., encounter rate), and the mortality of fish that are caught and released. In Idaho, estimates of wild steelhead encounter rates are derived using the number of wild and hatchery steelhead passing Lower Granite Dam, the number of hatchery steelhead harvested, and the number of hatchery steelhead caught and released. Currently, managers assume hatchery and wild steelhead have equal encounter rates and apply a 5% catch-and-release mortality rate to the portion of the wild steelhead population caught by anglers. I sampled, tagged, and released 1,277 spawn-year 2020 (SY2020) and 2,072 spawn-year 2021 (SY2021) adult steelhead at Lower Granite Dam with T-bar anchor tags and passive integrated transponder (PIT) tags to apply novel methods to estimate hatchery steelhead encounter rates and catch-and-release mortality. Tagged fish moved into fisheries where 312 SY2020 and 639 SY2021 fish were caught and reported by anglers. Estimated encounter rates were 30.2% (95% confidence interval; 22.2, 39.5) for wild fish and 57.4% (20.7, 87.4) for adipose-clipped fish in SY2020. In SY2021, encounter rates were 37.0% (31.9, 43.6) for wild fish and 52.4% (44.9, 59.9) for adipose-clipped fish. Differences in survival of caught steelhead and those not reported as caught were evaluated using detections at various locations (e.g., PIT arrays, weirs). Based on this analysis, catch-and-release mortality of wild fish tagged with high reward tags (i.e., US $100 and $200 tags) was 3.9% (95% credible interval; 0.2, 16.0) and averaged 3.8% (± SE; ± 8.1%) across all reward values. Results of my research provide important information that will be useful in guiding management of hatchery and wild steelhead in Idaho and the region.