Genetic differences between hatchery and wild steelhead for survival, growth, dispersal, and male maturation in a natural stream (Study site: Twenty-Mile Creek; Stocks: Dworshak hatchery and Selway River wild; Year classes: 1994 and 1995)

Abstract

This study was initiated in the early 1990s to provide managers with data comparing genetic fitness for natural rearing, as measured by survival of juveniles in freshwater, between steelhead Oncorhynchus mykiss from Dworshak National Fish Hatchery and wild steelhead from the Clearwater River, Idaho. We artificially spawned hatchery steelhead and wild steelhead from the Selway River, a Clearwater River tributary, released the resulting genetically marked (at the PEPA allozyme locus) progeny (HxH, HxW from hatchery females and wild males, and WxW) as unfed fry in a second order tributary of the South Fork Clearwater River, and monitored fish residing in the stream or emigrating from it for five years. Barrier falls prevented access to the stream by naturally produced steelhead. Over 90% of the emigrants were one or two years of age and too small to be smolts (mean fork length at age-2 = 103 mm). Per fry released, the HxH cross produced 0.64-0.83 times as many emigrants as the WxW cross (P<0.05). The HxH cross produced 0.63 times as many age-4 residuals as the WxW cross for one year-class (P=0.051) and 0.68 times as many for the other (ns). Survival from age-1 to age-4 was lower for HxH than for WxW residuals of one year-class (P<0.05) and survival from age-2 to age-4 may have been lower for HxH than for WxW residuals of the other (P=0.062). Collectively, these results indicate lower survival for HxH than for WxW fish. Size was often greater for HxH than for WxW fish indicating faster growth for the former, and condition factor was also usually greater for HxH than for WxW fish. Dispersal of fry from release sites and emigration of one- and two-year olds from the study stream were greater for WxW than for HxH fish, and apparently neither was from competitive displacement of small by larger fish. Incidence of flowing milt was higher for HxH than for WxW fish at age-2. Peak incidence of flowing milt for older residuals was similar among crosses (about 50%), but the peak occurred at greater size and age for WxW than for HxH residuals. HxW fish were intermediate between HxH and WxW fish, not similar to HxH fish, in survival, growth, condition, dispersal, and maturation, so differences among crosses likely resulted from additive genetic differences between the hatchery and wild populations rather than from maternal differences between hatchery and wild females. During our study, local managers decided against supplementing most wild steelhead populations in the Clearwater basin. Our study indicates that supplementing with Dworshak Hatchery fish is likely to reduce the fitness of wild populations through interbreeding and therefore supports that decision.