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

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.