Thermal selection and delayed migration by adult sockeye salmon (Oncorhynchus nerka) following escape from simulated in-river fisheries capture

Two hypotheses were tested concerning the consequences to adult sockeye salmon (Oncorhynchus nerka) of escape from commonly used fishing gear (gillnet, seine net and tangle net). First, by experimentally exposing 214 fish to three commonly used fishing gear types (gillnets, tangle nets, or seine nets) and releasing to complete migration after PIT-tagging, we tested the hypothesis that migration success and behaviour are differentially impacted according to the type of fishing gear interaction and escape. Also, by fitting salmon with thermal loggers, we tested whether salmon behaviourally selected cooler lake water following an escape encounter. Migration success was unaffected after escape from the simulated gear types compared to control fish. Surviving sockeye salmon that had gillnet or seine net encounter, but not a tangle net encounter, on average took approximately 2 days longer to migrate to the spawning area (averaging 14.7 days and 14.9 days, respectively) compared to control fish (12.6 days). Furthermore, escaped fish migrated at cooler temperatures through a lake system (average of 14.0 °C) compared to control fish (15.3 °C), which would reduce their absolute cost of transport during this migration by reducing the standard metabolic rate by about 10%. Consequently, in addition to demonstrating that fish escapement from a fishing gear will alter their subsequent migration behaviour, we introduce the possibility that the associated increase in energy expenditure and migration delay can be partially compensated for by behavioural selection of cooler water, if it is available, to lower basic energy turnover in a fish that is entirely reliant on energy stores to fuel its spawning migration.