Migration phenology of adult Chinook salmon: tradeoffs among acute and cumulative thermal exposure risks.

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Citation abstract

Migratory coldwater species like anadromous salmonids are highly sensitive to river temperatures and methods are needed for estimating their exposure and risk. We built an agent-based model (ABM) for a threatened spring-run Chinook salmon population over 220-km of their adult migration in Oregon's Willamette and North Santiam River basins. Upstream movement rules for the ABM were derived from a companion study of 161 radio-tagged salmon that moved through five study reaches before reaching prespawn holding sites. Tagged fish moved faster when river temperatures were warmer in all reaches, informing movement rules. We used the ABM to simulate continuous thermal histories for salmon migrating in two years (2011, 2015) with sharply contrasting thermal regimes and used the simulations to address hypotheses about thermal exposure and prespawn mortality risks. In both cool (2011) and warm (2015) years, early-run salmon had high cumulative exposure, whereas late-run salmon encountered acutely high temperatures, indicating substantive risk trade-offs within annual runs. On average, total degree day (DD) accumulations during migration and holding were ∼1100 (2011) and ∼1500 DD (2015), with 56 % and 69 %, respectively, accruing during holding in the terminal reach. Approximately 3 % (2011) and 24 % (2015) of simulated salmon encountered temperature maxima ≥21 °C, a threshold with known behavioral and physiological impacts. Estimates of prespawn mortality from an independent carcass dataset were ∼29 % (2011) and ∼61 % (2015), but it is unknown whether cumulative or acute processes (or both) precipitated the mortality. ABMs can help fisheries managers quantify environmental exposure, identify risks, and develop temperature mitigation strategies.