A matter of timing: the role of ocean conditions in the initiation of spawning migration by late-run Fraser River sockeye salmon (Oncorhynchus nerka)

摘要

Year 1995 marked the start of a major shift to earlier river entry of late-run Fraser River sockeye salmon (Oncorhynchus nerka) en route to the spawning grounds. Earlier entry has typically been accompanied by considerably greater in-river mortality. We examine this behavioral change using correlation analyses between the entry timing of the Adams River and Weaver Creek stocks and the surface marine conditions encountered by the stocks during their homing migration from the northeast Pacific several months earlier. For Adams stocks, maximum correlation is between entry timing and offshore winds, such that the weaker the wind stress in the direction of the prevailing surface currents in early July, the earlier the river entry in late summer. For Weaver stocks, maximum correlation is with salinity, such that the lower the surface salinity along the coast in August, the earlier the river entry. We hypothesize that oceanic changes lead to changes in late-run sockeye physiology which then leads to changes in behavior. Physiological changes are postulated to arise from two types of preconditioning: Type-1 occurs in the offshore region, whereby the weaker the prevailing currents that normally hinder eastward migration, the more endogenous energy available for maturation and the earlier the river entry. Type-2 occurs in coastal regions, whereby the lower the salinity, the more rapid the osmoregulatory adaptation to freshwater (and possible susceptibility to water-borne pathogens) and the earlier the entry. Results suggest that the earlier entry that began in the mid-1990s is linked to weaker ocean currents and lower coastal salinities.