Straying and Life History of Adult Steelhead in a Small California Coastal Stream Revealed by Otolith Natural Tags and Genetic Stock Identification

Abstract

Straying has been difficult to study directly in natural steelhead Oncorhynchus mykiss populations. We analyzed an opportunistic sample of seven adult steelhead from a small basin on the Big Sur coast of California to determine their life history traits, including whether they were strays. Otolith natural tags (87Sr/86Sr) and genetic stock identification indicated that all seven adults were strays from at least six different sources. Three adults strayed from nearby streams (<72 km) on the Big Sur coast, while three strayed from distant sources, including the Klamath River (680 km to the north); the source of one stray could not be identified. Six strays were progeny of steelhead mothers, but one was the offspring of a nonanadromous mother. Six were female and one was male, and all that could be genotyped were homozygous (n = 4) or heterozygous (n = 2) for the anadromy‐associated haplotype in a migration‐associated genomic region. While the opportunistic nature and small size of the sample prevents us from inferring the rate of straying into the basin, our study nonetheless demonstrates that steelhead may stray across greater distances than generally has been appreciated and that nonanadromous females can produce anadromous offspring that stray, thus providing connectivity among basins.

The Genetic Composition of Wild Steelhead Based on Spatial Proximity to a Hatchery

“…Our results corroborate other studies that have documented straying in natural‐origin steelhead at varying rates and spatial scales (Pearsons and O'Connor 2020; Donohoe et al. 2021). These findings illustrate that some level of natural mixing of wild steelhead populations has likely occurred for eons and should be expected to continue.…”

“…Clearly, some level of natural mixing, which may be interpreted as straying, is needed to recolonize populations after catastrophic events, avoid genetic bottlenecks during periods of low abundance, and expand range based on changing environmental conditions (Keefer and Caudill 2014). Our results corroborate other studies that have documented straying in natural-origin steelhead at varying rates and spatial scales (Pearsons and O'Connor 2020;Donohoe et al 2021). These findings illustrate that some level of natural mixing of wild steelhead populations has likely occurred for eons and should be expected to continue.…”

Counting the Parts to Understand the Whole: Rethinking Monitoring of Steelhead in California’s Central Valley

“…This could reduce population resilience by promoting the movement of fish into highly-variable, lowsurvival environments (i.e., the Delta and ocean), while decreasing the number of fish in a fairly stable environment (i.e., dam tailwaters). Further, resident individuals may produce offspring that later adopt an anadromous life history (Christie et al 2011;Donohoe et al 2021), and the importance of this contribution to maintaining the Steelhead contingent of O. mykiss populations in anadromous waters is not well understood.…”

High dispersal rates in hybrids drive expansion of maladaptive hybridization

“…For example, coastal steelhead trout (i.e. rainbow trout), have demonstrated high stray rates in natural populations [39], which suggests that pRBT may be a driver in the straying phenotype. Additionally, natal site fidelity (also known as philopatry) is thought to be guided by olfactory recognition in juvenile imprinting that occurs before outmigration from nursery habitats and complex interactions in adult maturation processes including reproductive behaviors, environmental conditions and senescence physiology [36,40].…”