Facilitated migration could bolster migrant passage through anthropogenically altered ecosystems

Abstract

Anthropogenic habitat change frequently outpaces the adaptive capacity of migratory taxa, placing many species and populations at risk of extirpation or extinction due to the mismatch of natural migration phenology and suitable conditions. While dynamic protection can greatly benefit migratory species, it is contingent on the flexibility of relevant management actions. For regulated ecosystems where advanced management planning is required, we present a new framework—facilitated migration—for actively matching natural migration phenology with suitable conditions for successful migrant passage. Using a case study of juvenile Chinook salmon (Oncorhynchus tshawytscha) oceanward migration in the Sacramento River, a major bottleneck to the recovery of imperiled populations in California, we show how the conditions associated with migration preparation, migration initiation, and successful migrant passage could be regulated to benefit migrants. Thermally shifted preparation, flow pulse-mediated initiation, and passage protection via increased flow, all accomplished by controlling the release of water from storage reservoirs, could increase the number of natural-origin Chinook salmon populations successfully migrating to the ocean by 43%–479% compared to the status quo management scenario. To further inform expected outcomes, we find that the temporal scope and diversity of juvenile salmon lifestages studied via acoustic telemetry should be expanded, and that the range of flows and flow changes that juvenile salmon experience should be increased. Facilitated migration works by synchronizing migration—thus, a prudent course of action when implementing this strategy would be to favor event quality over event quantity. In forcing scientists and managers to critically evaluate how migrations can be supported and manipulated, facilitated migration will help migratory species persist in anthropogenically altered ecosystems.