Abstract
There has been a dramatic increase in jellyfish biomass over the eastern Bering Sea shelf since the early 1990s, which was previously hypothesized to have been triggered by changing climate and ocean conditions. We examine the hypothesis that the presence of these large carnivores has affected fisheries resources, either through direct predation on larval stages, or through compe- tition for zooplankton prey. In this paper, we explore the impact of this jellyfish increase on zoo- plankton and fish communities based on field data on the composition of the jellyfish community, and the abundance, size, stable isotopic signatures, and feeding habits of the principal scyphomedusae in the region. These data, together with those on zooplankton biomass, are used to estimate the ecosys- tem impacts of this increase. The center of jellyfish biomass has shifted from the SE Middle Shelf Domain in the early 1980s to the NW in the late 1990s. In recent years, the species composition of large medusae caught in trawls was dominated (>80% by number and >95% by weight) by the scyphozoan Chrysaora melanaster. Dense aggregations of this species occupied the water column in daytime between 10 and 40 m. Their food habits consisted mainly of pelagic crustaceans (euphausi- ids, copepods, amphipods), although other jellyfish and juvenile pollock were also consumed. Based on stable isotope ratios, the trophic level of this scyphozoan is equivalent to, or higher than, that of Age 0 pollock. Preliminary estimates showed that medusae have a moderate grazing impact on zoo- plankton in the area around the Pribilof Islands; C. melanaster was estimated on average to consume seasonally about one-third of the standing stock and 4.7% of the annual production of zooplankton in this region. Daily consumption of Age 0 pollock was estimated to be 2.8% of the standing stock around the Pribilof Islands during 1999. A hypothesis for the increase in jellyfishes observed in the eastern Bering Sea, based on release from competition from planktivorous forage fishes, is proposed.
Highlights
In many of the world’s oceans and seas, large gelatinous zooplankton such as scyphomedusae and hydromedusae represent a conspicuous component of the plankton, especially during the productive summer months
We estimate the impact this jellyfish may have on pollock and zooplankton populations in the eastern Bering Sea, and we examine whether C. melanaster has an important role in the food web in the Bering Sea during summer
From the results of isotope ratios and stomach content analysis, it was found that Chrysaora melanaster and juvenile pollock share similar diets
Summary
In many of the world’s oceans and seas, large gelatinous zooplankton such as scyphomedusae and hydromedusae represent a conspicuous component of the plankton, especially during the productive summer months. Owing to the potential impact jellyfishes have on marine ecosystems, there have been numerous attempts to quantify their consumption of zooplankton or ichthyoplankton, mostly in enclosed bays or seas The SE Bering Sea is an ecologically and economically productive ecosystem (National Research Council 1996), and has been the subject of several coordinated research efforts (Loughlin & Ohtani 1999). The SE Bering Sea pelagic ecosystem has undergone dramatic changes over the last few decades (Hare & Mantua 2000, Hunt et al 2002, Schumacher et al 2002). Two prominent changes include the increased dominance of walleye pollock Theragra chalcogramma, beginning in the late 1970s (National Research Council 1996), and the dramatic increase in large medusae biomass, starting in the late 1980s (Brodeur et al 1999a)
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