Mechanisms Influencing the Timing and Success of Reproductive Migration in a Capital Breeding Semelparous Fish Species, the Sockeye Salmon

Abstract Two populations of homing sockeye salmon (Oncorhynchus nerka; Adams and Chilko) were intercepted in the marine approaches around the northern and southern ends of Vancouver Island (British Columbia, Canada) en route to a natal river. More than 500 salmon were nonlethally biopsied for blood plasma, gill filament tips, and gross somatic energy (GSE) and were released with either acoustic or radio transmitters. At the time of capture, GSE, body length, and circulating testosterone ([T]) differed between populations, differences that reflected known life‐history variations. Within‐population analyses showed that in Adams sockeye salmon, plasma glucose ([glu]), lactate ([lactate]), and ion concentrations were higher in the northern approach than in the southern approach, suggesting that the former was more stressful. GSE, [T], and gill Na+,K+‐ATPase activities also differed between the two locales, and each varied significantly with Julian date, suggesting seasonality. Despite these relative geographi...