Spatial and Temporal Patterns in Smolt Survival of Wild and Hatchery Coho Salmon in the Salish Sea

Abstract

Understanding the factors contributing to declining smolt‐to‐adult survival (hereafter “smolt survival”) of Coho Salmon Oncorhynchus kisutch originating in the Salish Sea of southwestern British Columbia and Washington State is a high priority for fish management agencies. Uncertainty regarding the relative importance of mortality operating at different spatial scales hinders the prioritization of science and management activities. We therefore examined spatial and temporal coherence in smolt survivals for Coho Salmon based on a decision tree framework organized by spatial hierarchy. Smolt survival patterns of populations that entered marine waters within the Salish Sea were analyzed and compared with Pacific coast reference populations at similar latitudes. In all areas, wild Coho Salmon had higher survival than hatchery Coho Salmon. Coherence in Coho Salmon smolt survival occurred at multiple spatial scales during ocean entry years 1977–2010. The primary pattern within the Salish Sea was a declining smolt survival trend over this period. In comparison, smolt survival of Pacific coast reference populations was low in the 1990s but subsequently increased. Within the Salish Sea, smolt survival in the Strait of Georgia declined faster than it did in Puget Sound. Spatial synchrony was stronger among neighboring Salish Sea populations and occurred at a broader spatial scale immediately following the 1989 ecosystem regime shift in the North Pacific Ocean than before or after. Smolt survival of Coho Salmon was synchronized at a more local scale than reported by other researchers for Chinook Salmon O. tshawytscha, Pink Salmon O. gorbuscha, Chum Salmon O. keta, and Sockeye Salmon O. nerka, suggesting that early marine conditions are especially important for Coho Salmon in the Salish Sea. Further exploration of ecosystem variables at multiple spatial scales is needed to effectively address linkages between the marine ecosystem and Coho Salmon smolt survival within the Salish Sea. Since the relative importance of particular variables may have changed during our period of record, researchers will need to carefully match spatial and temporal scales to their questions of interest. Received October 12, 2014; accepted January 20, 2015

Declining patterns of Pacific Northwest steelhead trout (Oncorhynchus mykiss) adult abundance and smolt survival in the ocean

“…1. e-folding scale (v) is the distance apart at which population correlation coefficients would be reduced to 37% (i.e., e -1 × 100%) of the expected mean correlation at a distance of 0 km, 0 (the intercept). The e-folding scale is an arbitrary measure but provides a common metric to compare the scale of spatial synchrony in smolt survival with that in other studies (e.g., Kilduff et al 2014;Ruff et al 2017;Teo et al 2009;Zimmerman et al 2015). Larger v values suggest similar patterns for populations that are further apart, whereas smaller values imply similarities for close-together populations.…”

“…Thus, it might be expected that they would exhibit lower rates of variation in recruitment and marine survival over time than many marine fishes. However, research shows that marine survival for many Pacific salmonids varies greatly over time (e.g., Kilduff et al 2014;Peterman et al 1998;Pyper et al 2002;Teo et al 2009;Zimmerman et al 2015), though it has not been fully examined across species. With persistent pressures on Pacific salmonids by humans, the increasing threat of climate change, and continued calls for recovery, monitoring population abundance and survival trends over space and time is essential for identifying the factors affecting population dynamics to guide appropriate management and conservation actions (Bisbal and McConnaha 1998;Burke et al 2013).…”

“…Recently, there has been increasing concern that poor marine survival of Pacific Northwest (including British Columbia (BC), Washington, and Oregon) steelhead may contribute to their declining abundance and lack of recovery (Moore et al 2015;Ward 2000 Beamish et al 2000;Beetz 2009;Coronado and Hilborn 1998a;Teo et al 2009;Zimmerman et al 2015; and Chinook salmon (Oncorhynchus tshawytscha): Coronado and Hilborn 1998b;Duffy and Beauchamp 2011;Kilduff et al 2014;Ruff et al 2017;Sharma and Liermann 2010;Sharma et al 2013), such an analysis has been done for only one steelhead population -the Keogh River of BC (Ward 2000;Ward and Slaney 1988). There are few large-scale mass mark-recapture or life-cycle monitoring studies (e.g., coded wire tag studies) on steelhead, and few populations are intercepted in marine fisheries, making it more difficult to estimate marine survival rates.…”

“…We then estimated smolt-to-adult return (SAR) rates since the late 1970s for 14 wild populations and 34 hatchery populations from Oregon, Washington, and BC. Finally, we measured spatial and temporal covariation in SARs to help us understand the spatial and temporal scales at which environmental variables may be related to these trends (as in Zimmerman et al 2015).…”

Residency, partial migration, and late egress of subadult Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) in Puget Sound, Washington

“…The reduced growth of resident salmon compared with those feeding along the coast is balanced against the higher survival rates of resident fish. Survival rates of coho salmon entering the Salish Sea are higher on average than coho salmon of coastal populations but the survival rates in these regions have shown different trajectories over the past decades (Zimmerman et al, 2015). Assuming similar patterns with Chinook salmon, the resident component of the population complex might enjoy higher overall fitness in some regimes and lower in others, effectively buffering the complex as a whole.…”