Cercopagis pengoi - New important food object of the Baltic herring in theGulf of Finland

The non-indigenous predaceous cladoceran Cercopagis pengoi (OSTROUMOV) was found for the first time in the Gulf of Finland in 1995. After this invasio n, the diet of the Baltic herring Clupea harengus mebras and its seasonal d ynamics were investigated in the coastal waters of SE Finland. The specimen s of C. pengoi were not present in plankton samplings until the end of July when the surface water temperature reached 13.5 degreesC. From the beginni ng of August to the end of October Cercopagis was found in all the plankton samples. Before the occurrence of Cercopagis, the diet of herring consisted mostly o f Eurytemora affinis and Bosmina coregoni maritima. During August to mid-Oc tober, C. pengoi was the main food source. In November, after the disappear ance of C. pengoi from the plankton, B. coregoni maritima became the main p rey object. Both the smallest and largest size classes of the herrings inve stigated consumed C. pengoi. Females with overwintering eggs are the most attractive prey for herrings. The overwintering eggs of C. pengoi, due to their very hard capsules, seem to pass undamaged through the stomach and intestines of herring. C. pengoi started to play a highly important role in the zooplankton community in the waters of SE Finland, and its abundance is not too dependent upon the annu al temperature fluctuations. The Baltic herring population has substantiall y changed its diet in this area, and now it prefers the newcomer C. pengoi.