Spatial overlap and trophic interactions between pelagic fish and large jellyfish in the northern California Current

Recent studies have indicated that populations of gelatinous zooplankton may be increasing and expanding in geographic coverage, and these increases may in turn affect coastal fish populations. We conducted trawl surveys in the northern California Current and documented a substantial biomass of scyphomedusae consisting primarily of two species (Chrysaora fuscescens and Aurelia labiata). Spatial overlap of these jellyfish with most pelagic fishes, including salmon, was generally low, but there were regions of relatively high overlap where trophic interactions may have been occurring. We compared feeding ecology of jellyfish and pelagic fishes based on diet composition and found that trophic overlap was high with planktivorous species that consume copepods and euphausiid eggs such as Pacific sardines (Sardinops sagax), northern anchovy (Engraulis mordax), Pacific saury (Cololabis saira), and Pacific herring (Clupea pallasi). Moreover, isotope and diet analyses suggest that jellyfish occupy a trophic level similar to that of small pelagic fishes such as herring, sardines and northern anchovy. Thus jellyfish have the potential, given their substantial biomass, of competing with these species, especially in years with low ecosystem productivity where prey resources will be limited.