Increasing Dominance of Odd-Year Returning Pink Salmon
The hypothesis that abundance patterns differ between even- and odd-year returning Pink Salmon Oncorhynchus gorbuscha was examined using data from the eastern and western North Pacific Ocean, northern and southern British Columbia, and biologically based conservation units, which are Canadian groupings of salmon that are genetically and/or ecologically distinct from each other. Detailed data from (mostly) southern British Columbia were examined to test hypotheses that the differences between even- and odd-year broodlines were due to fishing, broodline interactions, limitations in freshwater or the ocean, and/or density dependence. The odd-year broodline has become increasingly predominate over the genetically distinct even-year broodline on both sides of the Pacific and in five of six British Columbia regions. Five analytical approaches revealed abundances were generally increasing for odd-year conservation units and declining or stable for even-year conservation units. Recent increases in odd-year spawner abundance in southern British Columbia were correlated with decreased fishery exploitation, but exploitation was higher for odd-year than for even-year salmon, refuting the hypothesis that differential exploitation is responsible for the changing dominance. Significant negative interactions between even- and odd-year broodlines were found in several of the British Columbia regions tested, but there was little evidence of competition between broodlines in the marine environment. Odd-year populations in the Fraser River increased despite density-dependent reductions in freshwater production, while there was no indication of changes in marine productivity. Our results, combined with literature findings indicating a more southerly glacial refugium for odd-year than for even-year Pink Salmon and temperature-related survival differences between these broodlines, suggest that recent climate conditions are benefiting odd-year returning Pink Salmon more than even-year salmon, especially in the southern part of their range.
Received July 6, 2013; accepted January 21, 2014