| Summary: | The ecological niche of a population is dynamic and will be affected by changes in the ecosystem and as a population migrates. An ontogenetic niche shift can also occur as organisms grow and can include changes in morphology, habitat, and feeding behaviour. Although they are the two most abundant salmon species, and are further augmented through hatchery stocking, it is unclear the degree to which the niches of juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (O. keta) overlap. Furthermore, juvenile pink salmon and chum salmon undergo a period of rapid growth during their first summer at sea and it is unclear how their ecological niche changes with their ontogeny. Understanding the foodweb dynamics of juvenile salmon in the coastal marine environment is important because a large proportion of the overall mortality of salmon is thought to occur during their first summer at sea. The purpose of this study is to determine the degree to which the niches of juvenile pink salmon and chum salmon overlap, how their trophic position and food source changes as they grow into a new ontogenetic niche, and how these processes are affected by ocean conditions. I expected that years of poorer feeding conditions and increased competition would result in reduced trophic position and greater overlap of the niches of juvenile pink salmon and chum salmon. I hypothesized that juvenile salmon would shift their diet to a more offshore-based foodweb as they grew and that their trophic position would increase with size, but that the shift would be stronger when feeding conditions were improved. Statistical analysis showed evidence that the overlap of the niches of pink salmon and chum salmon increased when the abundance of salmon was high. Contrary to expectations, the trophic position of salmon appeared to decrease under favourable conditions. The trophic position of both pink salmon and chum salmon was higher in the southern portion of the study area, and increased when juvenile abundance was high. I suggest that the ...
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