Salish Sea Chinook salmon exhibit weaker coherence in early marine survival trends than coastal populations

There is increasing evidence that predation by harbour seals on out-migrating salmon smolts may be responsible for the low return of adult coho and Chinook salmon in the Salish Sea. However, little attention has been given to understanding where and when this predation occurs and the extent to which it might be conducted by few or many seals in the population. We equipped 17 harbour seals with data loggers to track seal movements and used accelerometry to infer prey encounter events (PEEs) following the release of ~384000 coho (May 4, 2015) and ~3 million Chinook (May 14, 2015) smolts into the Big Qualicum River. We found a small proportion (5.7%) of all PEEs occurred in the estuary where salmon smolts entered the ocean—and that only one-quarter of the seals actively fed there. PEE counts increased in the estuary after both species of smolts were released. However, the response of the seals was less synchronous and occurred over a greater range of depths following the release of the smaller-bodied and more abundant Chinook smolts. Harbour seals feeding in the estuary appeared to target coho smolts at the beginning of May but appeared to pursue predators of Chinook smolts in mid-May. PEE counts in the estuary increased as tide height rose and were higher at dusk and night—especially during full moonlight. Such fine-scale behavioural information about harbour seals in relation to pulses of out-migrating smolts can be used to design mitigation strategies to reduce predation pressure by seals on salmon populations.

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Decoupling outmigration from marine survival indicates outsized influence of streamflow on cohort success for California’s Chinook salmon populations

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/cjfas-2018-0140 ◽

2019 ◽

Vol 76 (8) ◽

pp. 1398-1410 ◽

Cited By ~ 12

Author(s):

Cyril J. Michel

Keyword(s):

Chinook Salmon ◽

Oncorhynchus Tshawytscha ◽

Primary Source ◽

Environmental Drivers ◽

Life Stages ◽

Cohort Size ◽

Seaward Migration ◽

Productivity Indices ◽

Marine Survival ◽

Marine Productivity

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.

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Combining Migration History, River Conditions, and Fish Condition to Examine Cross-Life-Stage Effects on Marine Survival in Chinook Salmon

Transactions of the American Fisheries Society ◽

10.1080/00028487.2017.1281166 ◽

2017 ◽

Vol 146 (3) ◽

pp. 408-421 ◽

Cited By ~ 1

Author(s):

J. L. Gosselin ◽

J. J. Anderson

Keyword(s):

Chinook Salmon ◽

Life Stage ◽

Fish Condition ◽

Migration History ◽

Marine Survival

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Rapid growth in the early marine period improves the marine survival of Chinook salmon (Oncorhynchus tshawytscha) in Puget Sound, Washington

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f10-144 ◽

2011 ◽

Vol 68 (2) ◽

pp. 232-240 ◽

Cited By ~ 105

Author(s):

Elisabeth J Duffy ◽

David A Beauchamp

Keyword(s):

Body Size ◽

Chinook Salmon ◽

Oncorhynchus Tshawytscha ◽

Puget Sound ◽

Fork Length ◽

Adult Survival ◽

Local Conditions ◽

Size Dependent ◽

Marine Survival ◽

Growing Conditions

We examined the effect of early marine entry timing and body size on the marine (smolt-to-adult) survival of Puget Sound Chinook salmon ( Oncorhynchus tshawytscha ). We used data from coded wire tag release groups of hatchery Chinook salmon to test whether hatchery release date, release size, and size in offshore waters in July and September influenced marine survival. Marine survival was most strongly related to the average body size in July, with larger sizes associated with higher survivals. This relationship was consistent over multiple years (1997–2002), suggesting that mortality after July is strongly size-dependent. Release size and date only slightly improved this relationship, whereas size in September showed little relationship to marine survival. Specifically, fish that experienced the highest marine survivals were released before 25 May and were larger than 17 g (or 120 mm fork length) by July. Our findings highlight the importance of local conditions in Puget Sound (Washington, USA) during the spring and summer, and suggest that declines in marine survival since the 1980s may have been caused by reductions in the quality of feeding and growing conditions during early marine life.

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