Abstract
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 aggregations 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 Theragra chalcogramma were associated with A . labiata, both within and below the aggregations. All seine catches that contained juvenile pollock also con-
tained jellyfish. Medusa swimming was analyzed from underwater videotapes in order to elucidate how aggregations might be formed and maintained. Medusae did not swim in circles in the aggregation. Medusae all swam vertically in the same direction, either up or down, in crowded parts of the aggregations, suggesting that they had become concentrated in flow features, like convergences, in the water column. Reduced swimming, due pnmarily to frequent collisions among medusae in the aggregation~,also may have caused the medusae to become concentrated. The potential advantages of aggregation include increased fertilization success, retention near shore where planula settlement sites and zooplankton prey may be more abundant, retention in convergences where zooplankton are concentrated, and reduced predation.