Dietary analysis of wild and hatchery juvenile Chinook Salmon ( Oncorhynchus tshawytscha ) in the Fraser River estuary reveals ontogenetic and foraging differences

Estuaries provide critical rearing areas for both wild and hatchery juvenile Chinook Salmon ( Oncorhynchus tshawytscha) but there has been limited examination of differences in diets between these two groups of fish, and how diets change during the months long estuarine rearing period. In 2021 and 2022, we examined size, stomach contents, and stomach fullness of 142 juveniles of wild and hatchery origin from eelgrass habitat in the Fraser River estuary. An additional 37 wild origin fish from marsh and sandflat habitats were obtained—hatchery origin fish were rarely found in those habitats. In eelgrass habitat, stomach fullness increased with fish size and was ~51% higher in wild fish compared to hatchery fish, despite hatchery fish being ~3.5% longer on average than wild fish. Size and time of year were indicators of juvenile’s transition to a primarily piscivorous diet, which was observed earlier and at smaller sizes in wild fish than hatchery fish.