Open Collections

Abstract

The Fraser River is the major producer of Pacific salmon (Oncorhynchus spp.) in Canada with millions of juveniles out-migrating through its estuary each year. Estuaries provide critical rearing habitat for juveniles, in particular Chinook salmon (O. tshawytscha), which can reside in estuaries for several weeks. The diet of juvenile Chinook salmon in this habitat is a crucial component of their early development and survival. In 2021 and 2022, I examined size, stomach contents, and stomach fullness of 142 juvenile Chinook salmon of wild and hatchery origin from eelgrass habitat at Roberts Bank (Fraser River estuary). I examined an additional 37 wild Chinook salmon from marsh and sandflat habitats. GSI analyses revealed 83% of all wild fish originated from lower Fraser River populations and PBT analyses revealed 91% of all hatchery fish originated from lower Fraser River populations. In eelgrass habitat, hatchery fish were on average 6.1 mm (~7%) longer than wild fish. Stomach fullness increased with fish size and was ~34% higher in wild fish compared to hatchery fish. Wild fish trended towards fuller stomachs in marsh habitat and emptier stomachs in sandflat habitat compared to those in eelgrass habitat. Stomach content analysis suggested size and time of year were indicators of juvenile Chinook’s transition to a primarily piscivorous diet, which was seen in wild fish with fork lengths as small as 63 mm. Liver and dorsal muscle samples from 130 wild and hatchery Chinook salmon captured in eelgrass habitat were analyzed for δ¹⁵N and δ¹³C revealing overlapping trophic ellipses. Wild fish had greater niche widths compared to hatchery fish and tended to consume higher-quality prey and utilized more diverse habitats compared to hatchery fish highlighting the importance of conserving eelgrass and marsh habitats and the prey they support. To improve foraging competency of hatchery fish in estuaries, naturalization of feeding systems in rearing facilities should be implemented.