Earlier migration timing of salmonids: an adaptation to climate change or maladaptation to the fishery?

Recent studies reporting shifts in the timing of salmonid migrations have suggested global warming to be a cause. However, the specific mechanisms underlining the evolution of earlier migration timing in salmonid fishes are unknown. In this paper, I present a hypothesis by which fishery-induced selection works to advance the timing of salmonid migration, given that the timings of migration and breeding are genetically controlled heritable traits. Although late-spawning salmon brood lines enter rivers after early-spawning brood lines, there is evidence that all brood lines arrive in coastal fishing grounds at similar times. As such, late-spawning brood lines would be fished for longer periods of time, with their increased harvest rate imposing directional selection on earlier-spawning brood lines. Thus, fisheries-induced evolution could favor the earlier timing of river entry to escape coastal fisheries. Should earlier migration timing not be an adaptation to global warming-should it be a maladaptation to fisheries-induced selection instead-then it will have a negative impact on the sustainability of salmonid resources.