Temporal and ontogenetic variability in trophic role of four groundfish species - Walleye Pollock, Pacific Cod, Arrowtooth flounder, and Pacific Halibut - around Kodiak Island in the Gulf of Alaska

Changes in trophic position estimates of commercial fishery catches are used as an ecosystem-based indicator for sustainability, but often these estimates do not incorporate species-specific seasonal feeding dynamics and ontogenetic diet changes. Using stable isotope analysis, we obtained a fine-scale resolution of ontogenetic and temporal(interannual and seasonal) variations in the trophic roles of four commercially and ecologically important groundfish species in the Gulf of Alaska: walleye pollock Theragra chalcogramma, Pacific cod, Gadus macrocephalus, arrowtooth flounder, Atheresthes stomias, and Pacific halibut, Hippoglossus stenolepis. For each groundfish taxon, the nitrogen stable isotope signature (d15N) increased with total length. In contrast, the lipid-normalized carbon stable isotope signature (d13C) significantly varied with size-class only for walleye pollock. There were species-specific differences in trophic position; adult Pacific cod fed at the highest trophic position, and walleye pollock fed at the lowest trophic position. Walleye pollock also had the lowest d13C, indicating a mainly pelagic diet, while Pacific halibut and Pacific cod had the highest d13C, indicating a mostly benthic diet. Interannual differences in trophic position were detected for each species. Pacific cod, arrowtooth flounder, and Pacific halibut fed at a significantly lower trophic position in 2003 than in 2001, 2002, or 2004. All species had a significantly lower average d13C (i.e., a more benthic diet) in 2001 and a higher average d13C (a more pelagic diet) in 2003. Pacific cod, Pacific halibut, and walleye pollock had a significantly more pelagic diet in the summer. Walleye pollock, arrowtooth flounder, and Pacific halibut showed a significantly more benthic diet in the fall, which probably corresponded with their seasonal migrations. Temporal variations in stable isotope signatures were observed but were relatively small (<1.25‰). Length-based ontogenetic diet shifts, however, were much larger (up to 6‰) and must be considered when trophic position is used as an ecosystem indicator of sustainability.