Spatial variability in zooplankton consumption by the Pacific oyster (Crassostrea gigas) relative to native bivalves in the Gulf of Alaska

In the Northern Gulf of Alaska (NGA), the diet of the non-native Pacific oyster (Crassostrea gigas) lacks thorough characterization, despite the species' significant contribution to the state's mariculture industry. Previous observations indicated that C. gigas occupies a higher trophic level than the native Pacific blue mussel (Mytilus trossulus), consuming zooplankton in addition to phytoplankton. Using bulk stable isotopes (δ¹³C and δ¹⁵N), we characterized the spatial patterns in C. gigas resource across five sites in the NGA spanning 300 km (Jakolof Bay, Bootleggers Cove, Peterson Bay, Simpson Bay, and Bridget Cove). Individuals of M. trossulus and C. gigas were collected from each site, and in Jakolof Bay, five additional native bivalve species were also examined. C. gigas occupied a higher trophic level than M. trossulus across all sites, and a stable isotope mixing model revealed that zooplankton contributed 29 %-94 % of C. gigas diet in spring 2024. The isotopic space of C. gigas in Jakolof Bay did not overlap with any of the native clam species, indicating that C. gigas occupies a distinct isotopic niche. Furthermore, C. gigas DNA gut content analysis detected M. trossulus DNA in C. gigas from two of four bays sampled, suggesting that C. gigas consumed M. trossulus larvae. These results indicate that in Alaska, C. gigas may occupy a distinct dietary niche relative to other native bivalves, including predation on bivalve larvae. Impacts of the predation pressure on native species should be closely monitored as mariculture operations expand.