Impact and Adaptation Responses of Okanagan River Sockeye Salmon (Oncorhynchus nerka) to Climate Variation and Change Effects During Freshwater Migration: Stock Restoration and Fisheries Management Implications

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Abstract
We summarized existing knowledge on behavioural and physiological responses of Okanagan sockeye salmon (O. nerka) adults to annual and seasonal variations in aquatic thermal regimes during migration. This enabled us to identify an underlying set of ’decision rules’ as a biophysical model of how temperature mediates en-route delays as a specific element of annual migrations by sockeye salmon. Several sets of results indicate that adult sockeye migrations stop as seasonal water temperatures increase and exceed 21 °C and then restart when temperatures decrease and fall below 21 °C. Model predictions of annual variations in the duration of migratory delay exhibited close agreement with independent estimates of observed delays available from a subset of years (predicted delay = 1.23 observed delay + 2.08, r2 = 0.92, p < 0.001, n = 10). We applied the model in a retrospective analysis of the likely impacts of climate variation and change events on adult sockeye migrations in freshwater over the 70 plus year interval between 1924 and 1998. Results indicate that migration delays for a significant portion of the sockeye population averaged 29 days per year (range 0–55). Average annual migration delays roughly equal the 33 day estimate of time required, given continuous migration, to traverse the 986 km distance from the Columbia River mouth to terminal spawning grounds near Osoyoos Lake, BC. Alternating intervals of above-average and below-average migration delays corresponded closely with ’warm-phase’ and then ’cold-phase’ periods of the Pacific Interdecadal Oscillation. Circumstantial evidence suggests alternating periods of sub-average and above-average productivity for salmon on the southern end of their range are linked to climate variation and change events in both freshwater and marine environments. Climate impact and adaptation responses that register first at the level of salmon, propagate rapidly through both salmon resource users and fisheries managers. Consequently, future climate warming episodes will complicate the manageability and threaten the sustainability of many salmon populations in the southern end of their range (Georgia Basin and the Pacific Northwest). This requires strategies that minimize the impact of uncertain climate variability and change scenarios on the resilience of the salmon resource, and maximize our adaptive capacity for both short- and long-term fisheries planning and management decisions.