Gene Flow between Sympatric Life History Forms of Oncorhynchus mykiss Located above and below Migratory Barriers

Abstract

Oncorhynchus mykiss have a diverse array of life history types, and understanding the relationship among types is important for management of the species. Patterns of gene flow between sympatric freshwater resident O. mykiss, commonly known as rainbow trout, and anadromous O. mykiss, commonly known as steelhead, populations are complex and poorly understood. In this study, we attempt to determine the occurrence and pathways of gene flow and the degree of genetic similarity between sympatric resident and anadromous O. mykiss in three river systems, and investigate whether resident O. mykiss are producing anadromous offspring in these rivers, two of which have complete barriers to upstream migration. We found that the population structure of the O. mykiss in these rivers appears to be influenced more by the presence of a barrier to upstream migration than by life history type. The sex ratio of resident O. mykiss located above a barrier, and smolts captured in screw traps was significantly skewed in favor of females, whereas the reverse was true below the barriers, suggesting that male resident O. mykiss readily migrate downstream over the barrier, and that precocious male maturation may be occurring in the anadromous populations. Through paternity analyses, we also provide direct confirmation that resident O. mykiss can produce offspring that become anadromous. Most (89%) of the resident O. mykiss that produced anadromous offspring were males. Our results add to the growing body of evidence that shows that gene flow does readily occur between sympatric resident and anadromous O. mykiss life history types, and indicates that resident O. mykiss populations may be a potential repository of genes for the anadromous life history type.

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

“…Anadromous females typically outnumber anadromous males in larger streams, and the same is reported from other species of partly migratory trout and charr, such as rainbow trout(McMillan, Katz, & Pess, 2007;Rundio, Williams, Pearse, & Lindley, 2012;Van Doornik, Berejikian, & Campbell, 2013), Arctic charr(Nordeng, 1983), whitespotted charr (Salvelinus leucomaenis, Salmonidae;Tamate & Maekawa, 2004) and some Atlantic salmon populations(Jonsson, Jonsson, & Hansen, 1998), although exceptions occurs (J.-L. Bagliniere, personal communication) Dodson et al (2013). noted an increase in male anadromy of brook trout towards the north.…”

Temporal dynamics of migration‐linked genetic variation are driven by streamflows and riverscape permeability

“…This may be due to the presence of intermittently permeable barriers, which could force fish to breed outside of their natal stream in years when they are inaccessible. Apgar et al () also found that migration‐linked loci frequencies showed greater divergence than neutral loci in O. mykiss , and Van Doornik, Berejikian, and Campbell () reported high rates of gene flow between sympatric anadromous and resident O. mykiss . The result also aligns with evidence that anadromous and resident O. mykiss within a watershed are more closely related to each other than to fish with the same life history in neighbouring watersheds (Clemento, Anderson, Boughton, Girman, & Garza, ; Leitwein et al, ; Olsen, Wuttig, Fleming, Kretschmer, & Wenburg, ).…”

Restricted gene flow between resident Oncorhynchus mykiss and an admixed population of anadromous steelhead

“…Based on characteristics of both Ekone Falls and the falls in Quartz Creek (e.g., height, flow, plunge pools), it is thought that neither is a complete barrier to upstream migrating steelhead, but we could not conclusively confirm this. Even with obstructed upstream movement, individuals in upstream reaches can become entrained (fall) or swim downstream (Hayes et al., ; Pearse et al., ; Van Doornik, Berejikian, et al., ; Van Doornik, Eddy, et al., ). In fact, approximately half of our Ekone Falls samples were taken below the falls.…”

“…Rundio et al () found that sex ratios of outmigrants were skewed toward females in a sympatric resident and anadromous population. However, Van Doornik, Berejikian, et al., (), Van Doornik, Eddy, et al., () showed that male resident O. mykiss have a greater tendency to descend falls, inferring that sex ratios are likely to be skewed in favor of females in above barrier populations where sympatry between resident and anadromous fish is limited. Gender ratios in the upper watershed of Rock Creek were only slightly greater than 50% male and comparable to the lower watershed groups, thus limiting our ability to infer relative life history origins based on sex ratios (see Ohms et al., ).…”

“…Fisheries literature is replete with examples of sympatry and gene flow between resident and anadromous O. mykiss populations. For sympatric, interbreeding forms, the resident life history is typically viewed as a genetic “cache” to be considered in conservation and recovery planning for declining steelhead populations (Hayes et al., ; Holecek & Scarnecchia, ; McPhee et al., ; Van Doornik, Berejikian, et al., ; Van Doornik, Eddy, et al., ; Weigel, Connolly, & Powell, ). There are, however, local or watershed‐specific exceptions as exemplified in our study in which different migratory phenotypes experience restricted gene flow (Docker & Heath, ; McMillan et al., ; Narum, Contor, Talbot, & Powell, ).…”