Little impact of hatchery supplementation that uses native broodstock on the genetic structure and diversity of steelhead trout revealed by a large-scale spatio-temporal microsatellite survey

Abstract

Artificial breeding programs initiated to enhance the size of animal populations are often motivated by the desire to increase harvest opportunities. The introduction of non-native genotypes, however, can have negative evolutionary impacts. These may be direct, such as introgressive hybridization, or indirect via competition. Less is known about the effects of stocking with native genotypes. We assayed variation at nine microsatellite loci in 902 steelhead trout (Oncorhynchus mykiss) from five rivers in British Columbia, Canada. These samples were collected over 58 years, a time period that spanned the initiation of native steelhead trout broodstock hatchery supplementation in these rivers. We detected no changes in estimates of effective population size, genetic variation or temporal genetic structure within any population, nor of altered genetic structure among them. Genetic interactions with nonmigratory O. mykiss, the use of substantial numbers of primarily native broodstock with an approximate 1:1 male-to-female ratio, and/or poor survival and reproductive success of hatchery fish may have minimized potential genetic changes. Although no genetic changes were detected, ecological effects of hatchery programs still may influence wild population productivity and abundance. Their effects await the design and implementation of a more comprehensive evaluation program.

Keywords: effective population size; gene flow; genetic drift; historical DNA samples; microsatellite DNA; native broodstock; salmonid; temporal analysis.

Figure 1

Map showing the geographic location of the rivers from which steelhead trout (Oncorhynchus mykiss) were sampled in southwestern British Columbia, Canada. 1, Chilliwack; 2, Chehalis; 3, Alouette; 4, Seymour; and 5, Capilano rivers. *Denotes the Coquihalla River, which is the source of some broodstock used in hatchery supplementation of the Chehalis River. The symbol denotes the Kitimat River, study site for Heggenes et al. (2006).

Figure 2

Average measures of population size, genetic variation, and structure in steelhead trout (Oncorhynchus mykiss) within rivers before (empty bars) and after (filled bars) the inception of hatchery supplementation using native broodstock. Based on variation of 902 samples that were genotyped at five or more of nine assayed microsatellite loci. Refer to Table 2 for population codes; ALL refers to all rivers combined; ALL-CA refers to all rivers except the Capilano River. Standard deviations given where applicable. (A) Mean relatedness (r_xy); (B) Variance in r_xy; (C) Effective population size (N_e); (D) Mean number of alleles (N_A); (E) Allelic richness (R); (F) Gene diversity (H_E); (G) F_ST (θ).

Figure 3

Plot of mean factorial correspondence scores along the first three axes for time point samples of 902 steelhead trout (Oncorhynchus mykiss) based on variation at five or more of nine assayed microsatellite loci. Refer to Table 2 for population codes. The status of time points highlighted as before (empty circles) or after (filled squares) the initiation of hatchery supplementation using native broodstock.