Burst swimming in sockeye salmon during their 1100 km upstream migration

In the Fraser River (British Columbia, Canada), some sockeye salmon (Oncorhynchus nerka) populations migrate upwards of 1100 km to reach their spawning grounds and encounter challenging river conditions. Understanding how sockeye salmon partition their finite energy budget into aerobic (sustained) and anaerobic (burst) swimming could help us understand why some fish successfully make it to the spawning grounds while others do not. We tagged sockeye salmon upon their entry in freshwater with archival accelerometer radio tags and recovered 16 tags on the spawning grounds, providing us with the first full migration datasets from which we can quantify burst swimming. We found that the type of bursts (e.g. duration, maxima, peak count and effort) varies along their migration, likely reflecting changing flow dynamics and channel morphology, but that on average, fish only spend 1.2% of their migration bursting. Unsurprisingly, the Fraser Canyon required significantly more bursting than the remainder of their migration, representing 75.3% of the cumulative bursting effort despite representing only 35% of the distance. In this reach, sockeye salmon performed on average 4.5 bursts per km and 117 bursts per day, representing an effort of 40.0 g km^-1, which is significantly more than in any other reach. Moreover, we found that in the early phases of the migration (Lower Fraser and Canyon), salmon avoided bursting at night-time. We did not however find evidence of individual repeatability in the types of bursts that individual fish perform. Our study is the first to quantify burst swimming over a 1100 km migratory route in sockeye salmon. Our findings indicate that despite the repeated need to burst, salmon are equipped to do so in an energy-conserving way. This knowledge is useful for resource managers as they make decisions about mitigation actions that could make migrations easier especially in the context of a warmer future.