Estimating the Ratio of Hatchery‐Produced to Wild Adult Steelhead on the Spawning Grounds using Scale Pattern Analyses

Abstract Hatcheries produce Pacific salmon and trout Oncorhynchus spp. for many purposes, including fishery enhancement. The genetic integrity of wild populations spawning near such hatcheries may depend on the efficacy of their spatial or temporal separation from hatchery fish. We describe a simple, novel approach based on the examination of scales from an iteroparous species, steelhead O. mykiss , to evaluate whether the ratio of hatchery‐produced adults to wild adults on the spawning grounds met recommended levels. In this river, migrating steelhead are diverted into the hatchery by a weir. Hatchery‐produced fish are manually spawned and killed in the hatchery, whereas wild fish are passed over the weir and allowed to spawn naturally upstream from the hatchery. Therefore, in principle, all hatchery‐produced adults should be captured at the hatchery on their first spawning migration. However, scales from 8.3% (58 of 699) of female and 2.6% (22 of 844) of male hatchery‐produced steelhead adults showed evidence of previous spawning migrations. Although this frequency was lower than the incidence of repeat spawning by wild fish (males, 11.0%; females, 20.3%), these records nevertheless indicated significant reproductive opportunities in the wild for hatchery‐produced fish. Combining these frequencies of repeat spawning with estimates of the survival and numbers of wild fish, we modeled the ratio of hatchery to wild fish on the spawning grounds using data from eight return years. This ratio exceeded the recommended levels under all reasonable scenarios modeled (14.7‐89.6% hatchery fish). Combined with evidence of spatial and temporal overlap, our data suggest that unless changes are made to increase the capture efficiency of adults at Washington State's Forks Creek Hatchery and similar hatcheries relying on segregated stocks, the risks of genetic introgression and ecological interactions with wild steelhead populations will remain high.