Salish Sea Marine Survival Project

47 SUMMARY OF RESULTS TO DATE Results to date suggest survival increases with distance from the estuary, and that wild fish have a consistently higher survival than hatchery conspecifics by a factor of approximately 3:1 (Figure 2). The surprising finding was the still very low survival of fish caught in September-October of their first year at sea. These fish were captured and tagged by microtrolling (using modified troll gear to capture small Chinook) around the Southern Gulf Islands. This suggests that significant mortality occurs during the first winter and beyond. Thus, there is evidence of two key critical periods, one at marine entry, and the second over the first winter at sea. Kevin Pellett, DFO, produced preliminary "decay curve" models starting with 1,000 fish, illustrating how numbers would compare over time given the survival estimates determined above for the 2015 tag cohort. The curves for wild (Figure 3) and hatchery (Figure 4) are shown below. The data are based on PIT tag returns through 2019 which are essentially complete, while Age 2 data (green dots) are estimated from coded wire tag returns in the fishery. The curves are similar, though the initial drop in abundance of wild fish is steeper than for hatchery fish. The reason for this is unknown, though it may be related to the short period that hatchery fish remain in the river and estuary environment and their initial greater body size. Other key findings to date include the following: Freshwater mortality appears to be confined to the downstream migration of smolts and not in-situ rearing. Mortality increases with both migration distance and declining flow, and can be over 75% during low flow years. This appears related to heron predation, and hatchery fish were more vulnerable to this predation (perhaps due to naivety). This has led to changes in hatchery practices, and hatchery releases now occur in the lower river. Outmigration timing for both hatchery and wild Chinook appears to be related to size. Hatchery fish were larger than wild fish in every year and generally migrated to the ocean within five days of release. Wild fish tended to reside longer in-river before migrating with the longest residency correlated to the smallest fish. Size does not appear to influence downstream survival of either hatchery or wild Chinook. Cursory investigations of size-selective mortality suggest all size classes are repre- sented in tag returns (of recovered fish) from all stages. NEXT STEPS AND SUGGESTIONS Data collection will be continued until November 15, 2020 when all surviving Cowichan Chinook that were PIT tagged during SSMSP will have returned to the Cowichan River. This project highlights the importance of the first winter in the ocean, a period that has been studied very little anywhere. Future research should focus on this period of Chinook life history. Given the efficacy of methods and new information gathered during the Cowichan programs, expansion of these studies is now taking place in other river systems. Figures 3 (left) and 4 (right). Chinook mortality "decay curves" showing how abundance of 1000 fish in the river changes throughout their life cycle based on PIT tag survival data. Graphs provided by Kevin Pellett, DFO. FISH REMAINING AVERAGE FORK LENGTH (MM) 2015 WILD MORTALITY 1000, River 825, Beach 540, Purse 202, Microtroll 53, Age 2 (est.) 13, Return 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 93.5% mortality September to return (189 dead, 13 live) 79.8% mortality May-September (798 dead, 202 live) 1000, River 858, Purse 878, Beach 208, Microtroll 20, Age 2 (est.) 5, Return 0 100 200 300 400 500 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 FISH REMAINING AVERAGE FORK LENGTH (MM) 2015 HATCHERY MORTALITY 97.6 % mortality September to return (203 dead, 5 live) 79.2% mortality May-September (792 dead, 208 live) Figure 2. Summary of return data by location for hatchery and wild Chinook tagged in 2015. Graph provided by Kevin Pellett, DFO. 0.5% 0.6% 0.6% 2.6% 1.3% 1.6% 2.4% 6.5% 0% 1% 2% 3% 4% 5% 6% 7% River Beach Purse Micro Troll <300 mm AGE 2-5 RETURN RATE - 2015 TAGS Hatchery Wild

• Cover