Physiological condition and salinity preference in freshwater migrating juvenile sockeye salmon

The amount of time that juvenile salmon remain in an estuary varies among and within populations, with some individuals passing through their estuary in hours while others remain in the estuary for several months. Underlying differences in individual physiological condition, such as body size, stored energy, and osmoregulatory function, could drive individual variation in the selection of estuary habitat. Here we investigated the role of variation in physiological condition on the selection of estuarine and ocean habitat by sockeye salmon (Oncorhynchus nerka) smolts intercepted at the initiation of their 650 km downstream migration from Chilko Lake, Fraser River, British Columbia (B.C.). Behavioural salinity preference experiments were conducted on unfed smolts held in fresh water at three time intervals during their downstream migration period, representing the stage of migration at lake exit, and the expected timing for estuary entry, and ocean entry (0, 1, and 3 weeks after lake exit..., The focal population is sockeye salmon from Chilko Lake located 650 river km north of the Fraser River estuary at an elevation of 1174 m, in the interior of British Columbia, Canada (Figure 1). Out-migrating yearling sockeye salmon smolts were collected from Chilko Lake between 22:00 and 4:00 April 30 – May 1, 2019, during the peak outmigration period (Macdonald et al., 2020). Smolts were collected by dip net at a smolt fence and transferred to a riverside transport tank with continual flow-through of aerated river water. In total, 263 smolts were collected for salinity preference experiments. All fish collection, holding, and experimentation protocols were reviewed and approved by the Animal Care Committee at Simon Fraser University (Protocol number 1238B-17).
Behavioural experiments were conducted on smolts at three time intervals during their downstream migration period, representing their lake-exit, estuary-entry, and ocean-entry outmigration stages (0, 1, and 3 weeks after lake exi..., , # Physiological condition and salinity preference in freshwater migrating juvenile sockeye salmon

This dataset provides physiological and behavioural data on migrating Chilko sockeye yearling smolts. Fish tested in the river treatment group were tested lake-side 1 day after lake-exit. Fish tested in the estuary and ocean treatment group were transported to a holding facility and held in freshwater for 2-4 weeks. Salinity preference experiments were conducted over to infer habitat choice among migrating smolts. Fish with that spent the majority of the experiment time in a chamber were determined to prefer the salinity of that chamber. Immediately after the experiment, fish were euthanized and preserved for physiological analysis. Size metrics, energetic density, and gill NKA activity were measured for each fish and used to predict fish behaviour during migration.

## Description of the data and file structure

| variable name | variable type | unit | leve...