Genetic Diversity in Steelhead before and after Conservation Hatchery Operation in a Coastal, Boreal River

Abstract

Allelic variation at 10 DNA microsatellite loci was assayed in scale samples of steelhead Oncorhynchus mykiss (n = 333) collected between 1976 and 2003 from the lower main stem of the Kitimat River, British Columbia. Our objectives were to (1) investigate the genetic diversity of wild steelhead populations in the river before hatchery stocking began in 1984 (baseline samples: 1976–1977, 1983–1984; n = 145) and (2) assess the potential genetic impacts of interbreeding of returning hatchery adult fish with wild spawners over almost 20 years of large-scale hatchery operation (1987–2003; n = 188). The annual target number of wild broodstock adults used for hatchery production was 40 (20 of each sex) but varied from 9 to 39 among years. The level of population subdivision (ϑ) among Kitimat River samples was low (0.004) and not significantly different from 0. Tests of ϑ between prehatchery and posthatchery operation indicated no significant changes. Similar results were obtained using other measures of genetic differentiation (principal components analysis of microsatellite allele frequencies and Cavalli-Sforza genetic distance). Our data, however, did indicate a slight but significant reduction in allelic richness after hatchery stocking. Pairwise tests for genetic differentiation among samples from different year-classes were nonsignificant. We conclude that for the current management regime there is little apparent impact of hatchery practices on either the genetic structure or variation within the lower main-stem Kitimat River steelhead, but there may be a reduction in rare alleles. The practice of using substantial numbers of wild fish and multiple year-classes in the hatchery may have minimized genetic changes via genetic drift.