Prey selection and dietary overlap among zooplanktivorous jellyfish and juvenile fishes in Prince William Sound, Alaska

The potential for competition for food among zooplanktivorous species of gelatinous zooplankton and fishes has been the subject of much speculation. Here, we evaluate the dietary overlaps among 2 scyphomedusan species, Aurelia labiata and Cyanea capillata, 1 ctenophore species, Pleurobrachia bachei, 1 hydromedusan species, Aequorea aequorea var. albida, and juveniles (Age-0) of 4 forage fish species, walleye pollock Theragra chalcogramma, Pacific sandlance Ammodytes hexapterus, Pacific herring Clupea pallasi, and pink salmon Oncorhynchus gorbuscha. Zooplankton samples collected in July-August 1995 to 1998 showed great similarity among years (86 to 96 %), which allowed valid dietary comparisons between pelagic coelenterates and fishes collected in different years during that period. The predators fell into 2 groups, one that ate primarily crustacean prey (A. labiata, P, bachei, juvenile walleye pollock, sandlance, and herring), and another that ate mostly larvaceans (C, capillata, A. aequorea, and juvenile pink salmon). Species within the first group showed significant positive selection for crustacean prey and 1significant negative selection for larvaceans. The opposite trends were observed in the second group. Dietary overlaps were higher among crustacean-eating species (63 +/- 13 %) and larvacean-eating species (66 +/- 10 %) than when comparing crustacean- with larvacean-eating species (24 +/- 14 %). Dietary overlaps among pelagic coelenterate species (41 +/- 21 %) and among fish species (42 +/- 25 %) were similar, while overlaps between pelagic coelenterate and fish species averaged 50 +/- 21 %. These pelagic coelenterate and fish species were collected together in 45 purse seine sets taken in Prince William Sound in July 1999. We conclude that the diets of pelagic coelenterate and forage fish species overlap substantially, and that the species co-occur spatially and temporally. Therefore, the potential for competition for prey exists for these zooplanktivores in Prince William Sound.