P. 1999. Distribution of the non-indigenous Cercopagis pengoi in the coastal waters of the eastern Gulf of Finland. -ICES Journal of Marine Science, 56 Supplement: 49-57.We studied the distribution and population structure of the immigrant predaceous cladoceran species Cercopagis pengoi and its role in the zooplankton community during a 1-week cruise at the beginning of September 1997. Samples were collected at 20 stations within a coastal area of 500 km 2 in the Gulf of Finland. Temperature above the thermocline was about 16-18 C. Phytoplankton was dominated by filamentous blue-green algae, metazooplankton by rotifers, cladocerans, cyclopoids, and calanoids. Ciliates (maximum density 2 10 5 L 1 ) were dominated by oligotrich ciliates and Mesodinium rubrum. The maximum density of different metazoan taxa varied from a few (Cercopagis) to a thousand (rotifers) per litre. Most of the Cercopagis population consisted of maturing parthenogenic females with a low percentage of males and gametogenic females. On the basis of population structure, we suggest that the population was still in the growth phase, parthenogenesis being the dominant reproduction strategy. Variation in zooplankton community structure and density was associated with differences in salinity, the stations nearest to the coast favouring cladocerans and cyclopoids. Rotifers and ciliates were most abundant at the eastern stations. Since density of Cercopagis was also highest at the eastern stations, this predator may have been an important factor in the structure of the zooplankton community. The potential effects of Cercopagis on the pelagic community structure and dynamics in the Gulf of Finland are discussed. 1999 International Council for the Exploration of the Sea
The non‐indigenous predaceous cladoceran Cercopagis pengoi (Ostroumov) was found for the first time in the Gulf of Finland in 1995. After this invasion, the diet of the Baltic herring Clupea harengus mebras and its seasonal dynamics were investigated in the coastal waters of SE Finland. The specimens of C. pengoi were not present in plankton samplings until the end of July when the surface water temperature reached 13.5 °C. From the beginning 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 of Eurytemora affinis and Bosmina coregoni maritima. During August to mid‐October, C. pengoi was the main food source. In November, after the disappearance of C. pengoi from the plankton, B. coregoni maritima became the main prey object. Both the smallest and largest size classes of the herrings investigated 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 annual temperature fluctuations. The Baltic herring population has substantially changed its diet in this area, and now it prefers the new‐comer C. pengoi.
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