An evaluation of the marine feeding ecology of Pacific salmon using stable isotopes

Pacific salmon (Oncorhynchus spp.) are a group of species characterized by tremendous biological diversity both between and within populations. Their diversity in freshwater life history traits has been recognized to stabilize fluctuations in stock abundance and the associated ecosystem services they provide in a continuously changing environment. In this dissertation, I used carbon and nitrogen stable isotopes to explore diversity in the marine trophic ecology of Pacific salmon.;Chapter 2 presented a synthesis of stable isotope research on Pacific salmon at the species level, summarizing delta15N and delta 13C values of the marine-derived materials these organisms transport to freshwater ecosystems. This chapter supported established diet studies reporting that Chinook and coho salmon feed at higher trophic positions than sockeye, chum, and pink salmon as revealed by delta15N. Additionally, delta 13C showed that Chinook and coho salmon forage primarily in nearshore ecosystems while sockeye, chum and pink salmon forage in more pelagic waters. Chapter 3 examined how marine foraging strategies in anadromous sockeye salmon are linked to their adult freshwater life-history and population-specific body morphology. Variation in delta15N suggested that individuals from large-bodied populations that spawn in deep freshwater habitats fed at higher trophic positions in the ocean than individuals from small-bodied populations from shallow stream habitats. Conversely, within populations, earlier maturing individuals had higher delta15N than older fish and therefore may have foraged at higher trophic positions, or in more coastal environments. These results indicated that population and life-history diversity evident in the adult spawning phase was linked to diversity in foraging strategies adopted by sockeye salmon during their marine lives. Chapter 4 examined stable carbon and nitrogen isotope characteristics of sockeye salmon scales collected from nine rivers in Bristol Bay between 1964 and 2003 to assess whether the trophic ecology of these fish changed systematically over this period during which there were substantial changes in oceanographic conditions in the North Pacific Ocean. Stock-specific patterns in the variation in stable isotopes through time were evident, and variation among rivers increased between 1977 and 1989, a period of substantially increased salmon productivity in the North Pacific. Additionally, some of the dominant signals of variation in stable isotopes were related to important environmental physical drivers such as wind mixing, sea surface temperature, and the Pacific Decadal Oscillation. This chapter further suggested a marine component to biocomplexity of Bristol Bay sockeye salmon as characterized by the stock specific variation in adult stable isotope characteristics over time.;Together the results presented in this dissertation provided new insights into marine foraging by Pacific salmon, suggesting that biocomplexity expressed during the freshwater life history phase also extends to the marine component of the salmon life cycle. This marine biocomplexity may be critical to stability in abundance of the overall stock complex, and should be considered in management and when modeling responses of Pacific salmon to ongoing climate change.