Spatial and temporal isolating mechanisms: the formation of discrete breeding aggregations of sockeye salmon (Oncorhynchus nerka)

This study examined the spatial and temporal isolation of breeding aggregations of sockeye salmon (Oncorhynchus nerka) along a single island beach in Iliamna Lake, Alaska. The density and spatial extent of these aggregations varied among years, but one of the sites (Fuel Dump Point) was consistently separated from another major spawning area (Fuel Dump Bay) by at least 15 m of unused but generally similar habitat. Females settled first at the Point and this site supported higher spawner densities than the adjacent Bay. Male breeding opportunity, estimated from operational sex ratios, was higher in the Bay soon after spawning commenced. However, few males moved from the Point into the Bay and all movement that occurred was late in the season, when breeding opportunities for males were very limited. The reproductive success of males that moved from the Bay to the Point was estimated to be 0.003% of the total reproductive success of all males at the Point. The reproductive success of males that moved from the Point to the Bay was estimated to be 3% of the total reproductive success of all males at the Bay. Thus, spatial and temporal variation in settling largely isolated the two spawning aggregations within a breeding season. We suggest that variation in habitat quality may interact with the heritability of spawning date, contributing to the formation of distinct breeding groups.