Salish Sea Marine Survival Project

60 2. What is the relationship between survival rates and the migration route through the Discovery Islands? Seymour steelhead 2015: New acoustic receivers around the Discovery Islands revealed that smolts primarily migrated (77%) through the westernmost route (Discovery Passage — red line in Figure 8), and those that did benefitted from ~97% survival per 100 km compared to ~47% per 100 km for Sutil Channel to the east (blue line in Figure 8). Chilko Sockeye 2016-2018: For Chilko Sockeye, preliminary results from 2016, 2017 and 2018 tagging show that Sockeye preferentially use Discovery Passage and Sutil Channel on their way to Johnstone Strait (Figure 9). Survival rates associated with these different routes were assessed in 2018 with smolts captured within the Discovery Islands, tagged with V5 and V7s tags, and transported south to the central northern Strait of Georgia, near Mittlenatch Island (Figure 10). Survival was modelled using Cormack-Jolly-Seber model variants and indicated that Desolation Sound (84% per 100 km) had the greatest rate of survival per 100 km, while Discovery Passage (71% per 100 km) expressed lower survival rates than those observed with hatchery steelhead smolts. Sutil Channel captured the largest proportion of released migrants (~69%) in 2018 and expressed the lowest survival rate (48% per 100 km) among observed routes. OTHER IMPORTANT FINDINGS 1. Researchers at UBC & DFO found that pathogens and immune function were important to migration success. a. Chilko Sockeye smolts that carried the naturally occurring IHNV (infectious haematopoietic necrosis) virus were less likely to survive to reach the ocean. Interestingly, bull trout in the upper Chilko River selectively predated on smolts carrying this pathogen as well as smolts with other immune issues. Bull trout exhibited binge feeding, consuming up to 20 times their maximum ration in smolt prey. This is some of the first direct evidence that piscivores are selectively predating on prey that are infected with pathogens, and providing an "ecological service" benefitting Sockeye by reducing the likelihood that smolts could spread pathogens. b. Preliminary analyses have found other links between immune- and stress response-related genes and mortality for Chilko Sockeye Salmon smolts tagged in 2016. Thus, it appears that expression of several genes may be predictive of survival during juvenile migration. 2. Kintama Research also conducted a small acoustic telemetry pilot study on hatchery-origin Chilko River (Spring) Chinook, to estimate freshwater survival and migration timing. Freshwater survival to the Fraser River mouth (49%) was comparable to other species which migrate the same distance downstream. However, their downstream migration rate (only 18 km/day) was dramatically slower than that of wild Chilko Lake Sockeye, which migrate rapidly to the ocean after exit from Chilko Lake (100-170 km/day). Thus, Chinook smolts took more than one month on average to reach the SOG, in contrast to wild Chilko Lake Sockeye which generally take under a week. SIGNIFICANCE & NEXT STEPS Survival Rates and Routes Survival rates for both Seymour steelhead and Chilko Sockeye vary geographically and are both segment- and route-specific. There is relatively poor survival close to natal areas and different species/ages appear to respond similarly. This low survival is likely attributed to high levels of predation in freshwater. It is apparent that the physio- logical state of smolts is also important, and these studies provide some of the first direct evidence that piscivores target physiologically compromised prey. Survival is higher in the turbid Fraser River, but drops again when smolts enter the marine waters of the Strait of Georgia. There is some evidence that the routes taken by these fish through the Discovery Islands region to exit the Strait of Georgia are important to survival. Travel rates Smolts travel slowest in clear freshwater areas and fastest in the turbid mainstem of the Fraser River. This is likely related to predator avoidance. They travel more slowly through the Strait, but their rate of travel increases through the Discovery Islands, likely due to the impact of currents and tides in this region. There are large differ- ences among species with respect to travel rates: Chilko River Chinook smolts took more than a month to travel from freshwater to marine entry into the Strait of Georgia, whereas this trip takes only one week for Chilko Sockeye. Meanwhile, steelhead smolts travel 2-3 times faster than Sockeye smolts likely due to their larger size. Next steps include retrospective analyses on ~10 years of acoustic telemetry studies to determine how migratory behaviour and survival are influenced by oceanographic conditions in the Salish Sea. Lastly, individual fish-based models (IBM) will be developed to simulate smolt migrations to test what navigation and/or orientation behaviours smolts use in the early marine environment. Together these studies will help our understanding of salmon smolt migrations and the trends in productivity and survival.

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