Toxic contaminants and other chemical tracers in Pacific herring and Pacific salmon provide insights into prey preferences and foraging habitat of southern resident killer whales

Abstract

Knowledge of the foraging habitats and diets of endangered marine species is necessary to define and protect their critical habitat, but such information is often lacking, especially for highly mobile species. Stable isotopes and persistent organic pollutants (POPs) can be used as chemical tracers to infer foraging habitats and diet preferences of marine species. Stable isotopes of nitrogen and carbon are often used to infer trophic levels and inshore/offshore foraging habitats of marine species, while the relative concentrations classes of POPs can reflect time foraging in marine regions with distinct chemical inputs. In this study we used a combination of POPs and stable isotopes as chemical tracers to compare foraging habitats and marine distribution of Pacific herring (Clupea pallasii) and Chinook salmon (Oncorhynchus tshawytscha) along the west coast of North America, and to infer diet preferences of southern resident killer whales (Orcinus orca). The relative abundance of four POP classes in herring and salmon populations provided unique chemical fingerprints associated with their marine distribution and exposure to contaminated prey. For example, we observed relatively high levels of DDTs in Pacific herring and Chinook salmon populations originating from California that migrate and feed northward off the coast of California and Oregon, reflecting greater use of DDT pesticide in that region. We used an analysis of the POP and stable isotope patterns among herring, salmon, and whale populations to describe the relative distribution of these species in their foraging habitats and to evaluate prey preferences of three pods of southern resident killer whales