Distribution and relative importance of jellyfish in a region of hydrothermal venting

Net sampling to 3000 m depth at Endeavour Ridge in the northeast Pacific in July 1991-1994 shows that medusae in the immediate vicinity of the hydroth ermal vent fields often make up a larger proportion of the total zooplankto n abundance and biomass from mesopelagic to bathypelagic depths than in the surrounding waters. This was particularly evident in the dominant Trachyme dusae, and least evident in the siphonophores. In addition, the large red S cyphomedusa Stygiomedusa gigantea was a major biomass component in the regi on of the deep (1000-1800 m depth) migrating scattering layers at the vent field, but was not found in any net tows greater than 10 km away from vents . There is no concurrent increase in relative or percent biomass of fish or chaetognaths, which are the other major predators in the community. We hyp othesize that predaceous medusae respond opportunistically to the enhanced zooplankton biomass throughout the water column around vents in spring to e arly summer, in a way that other predators do not. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.