Abstract
Historically, marine survival estimates for salmon have been confounded with freshwater seaward migration (outmigration) survival. Telemetry studies have revealed low and variable survival during outmigration, suggesting marine mortality may not be the primary source of variability in cohort size as previously believed. Using a novel combination of tagging technologies, survival during these two life stages was decoupled over 5 years for Sacramento River Chinook salmon (Oncorhynchus tshawytscha). Outmigration survival ranged from 2.6% to 17%, and marine survival ranged from 4.2% to 22.8%. Influential environmental drivers in both life stages were also compared with smolt-to-adult ratios (SAR) for three Chinook salmon populations over 20 years. Streamflow during outmigration had higher correlation with SAR (r2 > 0.34) than two marine productivity indices (r2 < 0.08). The few SAR estimates that were poorly predicted by flow occurred during years with the lowest marine productivity, suggesting most interannual SAR fluctuations are explained by outmigration survival, but abnormally poor marine conditions also reduce SAR. The outsized influence of flow on SAR provides managers with a powerful mitigation tool in a watershed where flow is tightly regulated.
Highlights
Convention is that variability in salmon cohort success is set during the early marine residence period
Outmigration versus marine survival comparison Overall, outmigration survival ranged from 2.6% to 17%, and marine survival ranged from 4.2% to 22.8% for eight late-fall-run Chinook salmon coded-wire tag (CWT) release groups from brood years 2007 through 2010 (Fig. 4)
This study indicates that outmigration survival, and the conditions that affect it, are the primary drivers of smolt-to-adult ratio (SAR) dynamics, and marine survival likely plays a critical role only in years with abnormally unfavorable marine conditions for salmon
Summary
Convention is that variability in salmon cohort success is set during the early marine residence period. Direct evidence of how outmigration (freshwater plus estuarine) survival might be affecting overall cohort success has been scarce throughout the range of salmon populations. Recent telemetry studies have estimated very low survival during the outmigration life stage of certain salmon stocks (Buchanan et al 2013; Michel et al 2015; Clark et al 2016), suggesting that marine survival is likely higher than what the literature indicates. Through the accurate partitioning of outmigration and marine survival, it may be possible to identify new survival bottlenecks, which will require new and different management solutions
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