Predation on zooplankton by large jellyfish, Aurelia labiata, Cyanea capillata and Aequorea aequorea, in Prince William Sound, Alaska

Large jellyfish are conspicuous members of many coastal plankton communities. They are potentially important as competitors for zooplankton prey with fish larvae and zooplanktivorous fish, as well as being predators of fish eggs and larvae. Nevertheless, few data exist on predation effects of large medusae on zooplankton in situ. Biovolumes and densities of Aurelia labiata, Cyanea capillata and Aequorea aequorea medusae combined, measured in fishing seines, were generally low (< 11 per 1000 m(3) and < 10 medusae 1000 m(-3)) at 24 to 44 stations during July in 1998 and 1999 in Prince William Sound (PWS), Alaska. Their diets contained mainly copepods, larvaceans and cladocerans, and also a variety of meroplankton. Few fish eggs and larvae were eaten. Multiple regression analyses showed that the numbers of the main prey taxa in the gut contents usually were significantly correlated with medusa diameter and prey density. Digestion rates for copepods and cladocerans at 14degreesC averaged 3 h for A. labiata, 2 h for C. capillata and 1.5 h for larvaceans by both predators. Calculations using the above data indicated that individual medusa consumed 100s to 1000s of prey daily. Because of high prey densities and low medusa densities, predation effects on small copepods were low (mean less than or equal to 0.3% of the standing stock d(-1)). Larvaceans experienced greater predation at an average of less than or equal to 8.3% of the standing stock d(-1). These predation effects were underestimated in 1998, because sampling did not include the numerous aggregations of A. labiata, and also in 1999, when small hydromedusae were abundant (mean 59 medusae m(-3)). During this study, predation by medusae probably did not reduce prey availability to Age 0 sandlance, herring and walleye pollock, with diets consisting primarily of small copepods; however, medusa predation may have affected larvacean availability to Age 0 pink salmon, which consume them extensively.