Observational Evidence of Spatial and Temporal Structure in a Sympatric Anadromous (Winter Steelhead) and Resident Rainbow Trout Mating System on the Olympic Peninsula, Washington

Abstract We documented the spawning distribution and male mating tactics of sympatric anadromous rainbow trout Oncorhynchus mykiss (winter steelhead) and resident rainbow trout in the Calawah and Sol Duc River basins, Washington. Snorkel surveys and in situ behavioral observations were used to determine the spatial and temporal distribution patterns and male mating tactics of anadromous, resident, and hatchery residual fish across the spawning season. In general, male steelhead entered our survey reaches earlier than female steelhead, and both entered earlier than the wild resident and hatchery residual forms. Spatially, wild residents represented the greatest proportion of the population in the middle and upper survey reaches. Those differences coincided with mating attempts primarily between male and female steelhead early in the spawning season and primarily between female steelhead and wild resident males at the end of the season. Most of the mating attempts that we observed involved a single female and a single male steelhead, but attempts commonly included multiple male steelhead, wild resident males, or both, and behavioral tactics differed between forms. The patterns suggest a strong temporal structure and a lesser spatial structure to the distribution of O. mykiss during the spawning season, which has important implications for future studies of this complex species.