Aggregations of the jellyfish Aurelia labiata: abundance, distribution, association with age-0 walleye pollock, and behaviors promoting aggregation in Prince William Sound, Alaska, USA

Aurelia labiata medusae occurred in aggregations with hundreds to millions of jellyfish. The aggregations were widely distributed in inlets of Prince William Sound (PWS), Alaska. Aerial surveys of PWS in May to August in 1995 , 1996, and 1997 showed marked interannual variation in the numbers of aggr egations observed, from a minimum of 38 in 1997 to a maximum of 557 in 1996 . Acoustic surveys showed that the aggregations extended from near-surface (0 to 5 m) to 15 m depth. Schools of young-of the-year walleye pollock Ther agra chalcogramma were associated with A. labiata, both within and below th e aggregations. All seine catches that contained juvenile pollock also cont ained jellyfish. Medusa swimming was analyzed from underwater videotapes in order to elucidate how aggregations might be formed and maintained. Medusa e did not swim in circles in the aggregations. Medusae all swam vertically in the same direction, either up or down, in crowded parts of the aggregati ons, suggesting that they had become concentrated in flow features, like co nvergences, in the water column. Reduced swimming, due primarily to frequen t collisions among medusae in the aggregations, also may have caused the me dusae to become concentrated. The potential advantages of aggregation inclu de increased fertilization success, retention near shore where planula sett lement sites and zooplankton prey may be more abundant, retention in conver gences where zooplankton are concentrated, and reduced predation.