Effects of release size, location, and timing on the freshwater residence and early survival of Chinook salmon (Oncorhynchus tshawytcha) on the west coast of Vancouver Island

Hatchery enhancement of declining Pacific salmon populations has long been a foundation of salmon management; however, the influence of hatchery release practices on freshwater residence time and survival to estuarine entry is not well-understood. This study employed Passive Integrated Transponder (PIT) tags to evaluate the relationships between the size of individuals at release, release location, and release timing on the duration of freshwater residence and survival of hatchery-produced Chinook salmon (Oncorhynchus tshawytscha) in the 103 km2 Toquaht River watershed, located on the west coast of Vancouver Island, British Columbia. The Toquaht River mainstem is ~22 km in length, divided into upper and lower sections by the 118 ha Toquaht Lake. A total of 4848 PIT-tagged Chinook salmon smolts were released on three dates (May 23rd, June 9th, June 19th), and during each release, fish were split evenly between three different locations within the watershed (Toquaht Lake: 7.9 km from the detection array; upper river: 11.9 km from the array; and the lower river: 4.1 km from the array). The detection data were analyzed using an integrated model of freshwater residence and capture–recapture fitted using Bayesian inference. The duration of freshwater residence between release and detection ranged from 0.5–60 days, with a mean of 14 days, and exhibited a negative relationship with fish size. The estimated median survival probability for all locations and dates was 0.39, with the highest probability of survival (0.78) found in fish released into the lake on June 19th and the lowest (0.06) for lower-river fish released on June 9th. Changes in streamflow were likely responsible for the observed pattern in survival by date. Survival probability decreased as fish size increased. Overall, the study provides further evidence that hatchery release strategies can significantly influence freshwater residence and survival, and these practices should be well-understood and flexible in the face of changing environmental conditions. The results of this study may have significant implications for the management of Toquaht River Chinook salmon, particularly in the context of ongoing efforts to rebuild the naturally spawning population.