Origin, Postrelease Survival, and Imprinting of Pen‐Acclimated and Direct‐Stocked Chinook Salmon in Lake Ontario

We determined the stocking origin of the Chinook Salmon Oncorhynchus tshawytscha harvest and compared the relative survival and tributary returns of pen-acclimated and direct-stocked salmon at eight sites in Lake Ontario. Chinook Salmon parr were coded-wire-tagged in 2010, 2011, and 2013, and adult salmon were recovered in the lake and tributaries from 2011 to 2017. Lake harvest counts indicated that pen acclimation provided significantly better smolt-to-lake harvest survival, with averages of 1.7, 2.3, and 2.3 times greater contribution (SE = 0.15, 0.21, 0.23), respectively, to the lake fishery than direct stocking. After accounting for lake survival, pen acclimation also provided significantly better smolt-to-adult return rates to tributary harvest than direct stocking at some sites, with average return ratios of 0.8, 1.1, and 1.6 (SE = 0.2, 0.2, 0.3), respectively, for the three year-classes. Port-specific lake harvest from June to August consisted of salmon stocked at sites throughout the lake, indicating a well-mixed population prior to the prespawn staging period. Of the eight lake ports analyzed, a minority of the Chinook Salmon harvested by anglers in June-August were stocked at each site (mean = 13%, SE = 2%) or at nearby sites (mean = 22%, SE = 4%), with most originating from stocking sites greater than 30 km away. In September, stocked Chinook Salmon became more segregated in the lake, with harvest at each site consisting of a higher percentage (mean = 38%, SE = 4%) of salmon stocked at those sites and nearby sites (mean = 29%, SE = 5%), suggesting that salmon began staging in this period. In contrast, most salmon harvested by tributary anglers in October were stocked at those sites (mean = 67%, SE = 4%), indicating good imprinting of both pen-acclimated and direct-stocked Chinook Salmon. Most strays in tributaries were from nearby stocking sites. Increased Chinook Salmon catch rates in the lake fishery coincided with increased use of pen acclimation and a declining prey fish population, underscoring the importance of evaluating outcomes of new stocking policies that may increase predator survival and predation pressure on prey.