Fine‐Scale Population Structure of Rainbow Trout in the Spokane River Drainage in Relation to Hatchery Stocking and Barriers

Abstract

We used 13 microsatellite loci to examine population structure in rainbow trout Oncorhynchus mykiss collected from 20 tributaries and 3 main stems in the greater Spokane River drainage. Populations displayed some excess homozygosity and linkage disequilibrium, which was more pronounced in upper tributary collections and probably the result of small effective population sizes or structuring within tributaries. In general, population structure followed geographic structure; collections from creeks within sub‐drainages were the most closely related, and collections from different tributaries were genetically distinct. Comparisons with cutthroat trout O. clarkii indicated little to no introgression. Comparisons with steelhead (anadromous rainbow trout), coastal rainbow trout O. mykiss irideus, and inland rainbow trout from hatcheries suggested introgression by hatchery fish into some wild populations. Introgression was suspected in populations from stocked tributaries and tributaries that lacked barriers to escaped hatchery fish. Populations from tributaries above barriers that had not been stocked were genetically distinct from hatchery fish and appeared to be native inland redband trout O. mykiss gairdneri.

A genetically distinct wild redband trout (Oncorhynchus mykiss gairdneri) population in Crane Prairie Reservoir, Oregon, persists despite extensive stocking of hatchery rainbow trout (O. m. irideus)

“…In the Spokane River drainage of Washington, Small et al (2007) describe introgression of local wild populations in some areas exposed to hatchery stocking, whereas other stocked areas show minimal genetic relationship to hatchery collections. Taylor et al (2007) found low levels of introgression among divergent hatchery and wild O. mykiss in the Athabasca River that was restricted to a select few localities within the basin.…”

“…Despite apparent restricted geneflow between OSH and CPR, there are no geographic or physical barriers that preclude the opportunity to interbreed, and it is probable that interbreeding does occur to some extent. The persistence of a genetically distinct CPR population is likely due to differences in life histories and/or utilization of spawning habitat or assortative mating behavior between populations (Narum et al 2004;Small et al 2007). The CPR redband trout population maintains a relatively strict adfluvial life history utilizing reservoir tributaries beginning in January and extending through May.…”

“…Since 1985 the reservoir has been stocked with domesticated Oak Springs Hatchery rainbow trout (OSH) in numbers ranging from 25,000 to 170,000 fingerlings annually, but a steady and significant decline in the trout fishery since the early 1990s has led to a major decrease in use of Crane Prairie Reservoir by recreational anglers. Although it is accepted that productivity and genetic distinctiveness of wild populations may be impacted through behavioral and genetic interactions with hatchery fish Small et al 2007), the nature of these interactions between OSH and CPR in Crane Prairie Reservoir are poorly understood.…”

Deep divergence and apparent sex-biased dispersal revealed by a Y-linked marker in rainbow trout

“…The remaining inland areas that have coastal haplotypes are all in eastern Washington (localities 18, 24–26). Localities 18 and 25 represent areas that have documented stocking activities for the past 70 years (Small et al 2007) and coastal haplotypes in the area may represent introgression of coastal-derived hatchery stock into the native inland populations. Locality 24, the Wells Steelhead Hatchery, also contained coastal haplotypes and may have used milt from coastal steelhead to propagate inland populations (Thorgaard 1983).…”

“…The coastal and inland lineages are recognized by differences in coloration and numbers of pyloric caecae, scales along the lateral line, vertebrae and gill rakers (Behnke 1992). Differences in microsatellite and allozyme frequencies have often been used to study introgression and hybridization between these subspecies (Utter 2001, Knudsen et al 2002, Small et al 2007). …”