The evolutionary history of steelhead (Oncorhynchus mykiss) along the US Pacific Coast: Developing a conservation strategy using genetic diversity

Changes in genetic variation across a species range may indicate patterns o f population structure resulting from past ecological and demographic event s that are otherwise difficult to infer and thus provide insight into evolu tionary development. Genetic data is used, drawn from 11 microsatellite loc i amplified from anadromous steelhead (Oncorhynchus mykiss) sampled through out its range in the eastern Pacific Ocean, to explore population structure at the southern edge in California. Steelhead populations in this region r epresent less than 10% of their reported historic abundance and survive in very small populations found in fragmented habitats. Genetic data derived f rom three independent molecular systems (allozymes, mtDNA, and microsatelli tes) have shown that the southernmost populations are characterized by a re latively high genetic diversity. Two hypothetical models supporting genetic population substructure such as observed were considered: (1) range expans ion with founder-flush effects and subsequent population decline; (2) a sec ond Pleistocene radiation from the Gulf of California. Using genetic and cl imatic data, a second Pleistocene refugium contributing to a southern ecoto ne seems more feasible. These data support strong conservation measures bas ed on genetic diversity be developed to ensure the survival of this uniquel y diverse gene pool. (C) 1999 International Council for the Exploration of the Sea.