Salish Sea Marine Survival Project

83 Our measures of marine survival for Chinook, Coho, and steelhead are determined by the number of smolts entering the sea divided by the subsequent number of adults that return to coastal waters in fisheries or that avoid fisheries to return to hatcheries or natural streams. The marine survival of tagged hatchery-produced Chinook, Coho and steelhead for over 40 years in the Salish Sea have been invaluable in monitoring survival and measures of variability between populations, but as presented below the variation between populations, species and years is substantial (Figure 2). While the trends in marine survival declines are apparent, the variability between populations within any year is equally important to our analyses of determinants. Our review of historical data on survival of multiple populations of Chinook 1 Salmon in the Salish Sea indicated that Chinook survival rates varied significantly within the Sea and popu- lations from nearby rivers tended to show similar patterns of survival. However, Coho 2 Salmon demonstrated similar survival patterns within a year throughout the Sea, regardless of river of origin. These observations are consistent with our sampling of oceanographic conditions within the Sea, and with the distribution patterns of juvenile Chinook and Coho Salmon determined from trawl catches and DNA analyses to identify populations of origin. Importantly, Chinook Salmon revealed consistent patterns of distribution between years with specific populations consistently rearing in different areas of the Salish Sea. Coho Salmon were more widely distributed and mixed. Within the Canadian Salish Sea, we are unable to assess steelhead patterns of survival due to inadequate data but their abundance has been declining. Further, for both Chinook and Coho Salmon, the patterns of survival in the Salish Sea differed from the patterns in outer coastal populations but were not consistently lower than those other populations. In considering the primary determinants of salmon production, we have repeatedly returned to the concept of a 'critical period' as hypothesized at the beginning of the SSMSP, and whether one critical period is sufficient to explain variations between years and trends over time. Certainly within the duration of the SSMSP, we saw clear evidence of mortalities throughout the salmon life cycle: Extensive mortality of Fall Chinook Salmon juveniles within the Cowichan River associated with extreme low flows during the spring of 2015 and 2016 (higher survival during normal flows in 2017); Effects of degraded/lost habitats in estuaries; 3 Survival of all sizes classes tagged in the nearshore and subsequently returning to the Cowichan River but at differing rates for hatchery and wild fish; and Unprecedented environmental conditions in the North Pacific Ocean that would affect the maturing portions of the Pacific Salmon at sea. Figure 2. Plot of marine survival estimates for hatchery-reared Chinook, Coho, and steelhead in the Salish Sea since 1972 Ocean Entry Year (OEY), presented as individual estimates by species, location and year (preliminary analysis by Dr. Kathryn Sobocinski (Western Washington University, Bellingham, WA). 1. Ruff, C. P. et al. 2017. Fisheries Oceanography, 2017: 1-13. 2. Zimmerman, M.S. et al. 2015 Marine and Coastal Fisheries, 7: 116-134. 3. C.D. Levings. 2016. Ecology of Salmonids in Estuaries around the World. UBC Press, Toronto, Ont. (OL638.S2L49)

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