Connecting physiological condition with salinity preference behaviour to infer estuary habitat choice in sockeye salmon smolts (Oncorhynchus nerka)

The time period in which juvenile salmon remain in an estuary varies greatly among and within populations, with some individuals passing through estuaries in a matter of hours, while others remain in the estuary for several months. This individual variation in estuary use suggests that there may be underlying differences in individual salmon condition that temporally mitigate the selection of habitat, such as smolt size (fork length, mass, condition factor), stored energy (lipids and proteins), and osmoregulatory function (gill N+-K+-ATPase activity, NKA). I investigated the role of physical and physiological condition on the selection of estuarine and ocean habitat by sockeye salmon (Oncorhynchus nerka) smolts intercepted at the initiation of their downstream migration from Chilko Lake, Fraser River, B.C.. Since juvenile salmon energetic costs increase with rising salinity, I expected that smolts of lower physiological condition (i.e. low condition factor, poor energetic status and low NKA) would prefer to remain in the freshwater environment of the estuary, while smolts of higher physiological condition would prefer saline waters in the estuary and potentially indicate more rapid ocean entry. Behavioural salinity preference experiments were conducted on unfed smolts (n = 263) held in freshwater at three time intervals during their downstream migration period, representing the expected timing for lake exit, estuary entry, and ocean entry, at 0, 1, and 3 weeks respectively. Smolt condition factor (K), energetic stores and NKA predicted salinity preference behaviour in the estuary and ocean outmigration stages, but not at lake exit. Our results suggest that smolt physiological condition upon reaching the estuary may influence migratory behaviour and habitat selection, providing novel evidence on the temporally dependent interplay of physiology, behaviour and migration in wild juvenile Pacific salmon. As juvenile migratory behaviour is linked to physiological condition, and physiological condition is determined by productivity and competition within the rearing habitat, the importance of estuaries likely varies across years and within a population cohort; thus estuaries may be of heightened importance for wild juvenile salmon in years of poor freshwater growth conditions. These findings support the growing body of evidence on the importance of conserving both rearing habitat for juvenile growth potential and estuarine habitat for smolt refugia before ocean entry.