New data on the distribution and feeding habits of jellyfish in the Northwest Pacific

Abstract

In June and July of 2012, the jellyfish catches in the northeastern portion of the surveyed Pacific waters off the Kuril Islands substantially exceeded those in the southwestern portion. This indicates that jellyfish disperse over the studied area predominantly from the southern Bering Sea and from the eastern coast of Kamchatka. Their strobilation probably takes place as well on the shelf and continental slope of eastern Kamchatka. The distribution of jellyfish with medium-sized bells does not show any geographic pattern; the aggregations that are formed are mixed regarding the original locality of individuals. Jellyfish occur within a broad range of surface water temperatures and their catches have declined significantly only in the southeast of the area of surveys near the Subarctic Front. As is seen from the data we compared, not only the abundance of jellyfish, but their feeding activity and, as a consequence, the amount of consumed food decreased by an order of magnitude during the cold season (in the spring 2011). However, irrespective of the season, the largest quantities of food were recorded in the largest and most numerous jellyfish (Phacellophora camtschatica, Chrysaora melanaster). The quantitative results of the studies on the diet of jellyfish may be somewhat underestimated, as fragile jellyfish bodies are easily damaged in trawl nets and evaluating the diet is possible only for intact individuals. Use of specialized catching gear in the future may help us to specify the feeding dynamics in jellyfish, as well they may provide an opportunity to observe their feeding behavior. At the same time, quantitative estimates of the daily-food intake in jellyfish can be obtained only by taking the rates of digestion and prey consumption found under laboratory conditions into account, with their subsequent verification in balance models.