Salish Sea Marine Survival Project

39 Figure 2. Nootka Rose —a preferred riparian vegetation cover for rearing fry. HOW? 1. Chinook were sampled using a variety of observations and gear in the river and estuary 2. Rotary Screw Trap studies 2014-2016 In the last 6 years, research has been undertaken to monitor the spatial and temporal distribution and ratio of hatchery to wild Chinook entering the Cowichan Estuary and through their first months in the ocean. This information has been used to estimate the early marine survival of hatchery and wild Chinook Salmon from this system. The Rotary Screw Trap (RST ) operated by DFO and Cowichan hatchery staff during 2014 -2016 allowed for a mark-capture population estimate of hatchery fish reaching the lower Cowichan River and their in-river survival (Figure 1). 3. PIT tag methodology 2015-2017 In addition to the rotary screw trap, PIT tagging methods were also used to compare freshwater survivals between years, segments of the river and between hatchery and wild fish. 4. Trail Cameras During 2017, the effect of predation on juvenile Chinook survival during their downstream migration was investi- gated using a combination of PIT tags and trail cameras. Tag detection antennas were used to target their potential predators with the goal of detecting tags (fish) consumed by predators. Predator species and numbers were documented using motion-activated trail cameras. SUMMARY OF RESULTS TO DATE Behaviour Cowichan Chinook fry distributed themselves from upper river spawning areas to occupy all suitable edge habitat, from natal to intertidal reaches, until a minimum size was attained for outmigration. Mainstem and large side channel edge habitats with suitable velocities and intact over-hanging vegetation and/or instream riparian vegetation cover were widely utilized for Chinook fry rearing, particularly early in the season (Figure 2). Chinook fry were observed in virtually all intertidal habitats including blind-end channels (although at lower densities than freshwater habitat). The historic loss of these habitats may be a limiting factor, particularly in years following increased escapements with higher production and potential for fry displacement to the estuary. Many such channels in the Cowichan's inner estuary are cut off or poorly connected due to diking and/or erosion protection. Beach/purse seine catches in Cowichan Bay increase dramatically in May and June as fingerlings emigrate from the river. The fingerlings migrating from the river choose to occupy habitats either along the beach or out in the middle of the bay depending on their size. This may have implications for competition between hatchery and wild fish if the former enter the marine environment at a larger size and crop down food resources in the bay before wild fish move into that habitat. Evidence for size selective habitat use can be found by comparing the proportion of hatchery fish in 2015 beach and purse seine catches where twice as many hatchery fish were captured in offshore habitats.

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