Jellyfish and forage fish spatial overlap on the eastern Bering Sea shelf during periods of high and low jellyfish biomass

Forage fishes and scyphozoan jellyfish are both voracious planktivores within the productive eastern Bering Sea (EBS) ecosystem. To determine the potential competition between the dominant jellyfish Chrysaora melanaster and forage fishes, we compared the spatial distributions of C. melanaster and 4 forage fish species in the EBS as observed in annual surveys of the upper 30 m. We calculated spatial metrics (centers of gravity, inertia and global index of collocation) of C. melanaster and each fish species and examined the degree of jellyfish-forage fish spatial overlap using several geostatistical methods during 2004 to 2012, a period that included high and low jellyfish biomass. Overall, EBS jellyfish occupied large areas where they overlapped with dominant forage fishes; however, the degree of overlap varied inter-annually with fluctuations in jellyfish and forage fish biomass and with climate conditions on the shelf. The spatial overlap be tween jellyfish and age-0 walleye pollock Gadus chalcogrammus was consistent in both low jelly fish biomass (2004 to 2007) and high jellyfish biomass (2009 to 2012) periods, whereas degree of jellyfish overlap with Pacific herring Clupea pallasii, capelin Mallotus villosus and age-0 Pacific cod Gadus macrocephalus varied with climate regimes. Competition between these 2 mid-trophic level groups is important because, while forage fishes are a critical link between plankton and higher trophic levels, jellyfish support few predator groups. Also, jellyfish are potential predators of the early life stages of fish. In locations where overlap is high, jellyfish predation on plankton, fish eggs and larvae may be important in driving dynamics of commercially important fish species.