The parasite faunas of snails, mussels, crustaceans and small-sized fishes were investigated over a period of six months in the Schlei fjord, on the Baltic coast of Schleswig-Holstein. Two sites differing in salinities were compared: Missunde with 5-9 %o and Olpenitz with 12-20 %~ Prevalences, number of host-parasite combinations, numbers of core and secondary species were generally higher in Olpenitz than in Missunde. In the latter site, only prevalences of cestodans in planktic copepods and the number of rare species were apparent. Specificity of parasites was relatively high in snails, mussels and fishes, but lower in benthic crustaceans. Parasites may survive in an extreme environment like brackish water by means of special strategies which differ from those acquired in the marine milieu: suspension of specificity, adaptation to hosts which are genuine brackish water species, extension of host spectra, and shortening of life-cycles. Although the parasite communities of the Schlei fjord were primarily influenced by the prevailing salinities, the influence of other factors, e

A comparative parasitology study of gobiid fishes from two sites on the western Baltic Sea was done to get information on the quality of the respective environments. The parameters used were the infracommunity, the species numbers and abundance with respect to the size classes of the hosts, and the abundance with respect to the seasons. The hosts, Gobius niger, Gobiusculus flavescens, Pomatoschistus minutus, and P. pictus, were caught at Blank Eck (Kiel Bight) and Dahmeshöved (Lübeck Bight) during 1992 and 1993. The infracommunities of Dahmeshöved attained higher rates of prevalence of helminth parasites than did those of Blank Eck, as was valid in all host specimens harboring one or two parasite species. The group bearing three parasite species was present in Dahmeshöved but absent in Blank Eck. Parasite species numbers, which were similar in the two sites, increased from the smallest to the largest hosts but changed with growth. With one exception, this result is also valid for the abundance, which clearly differed between the hosts and between the sites. Whereas species numbers as well as abundance rates were low in spring, they increased in summer. The results reveal that the composition of infracommunities depends on the local parasite fauna and on the strength of general host defense mechanisms, whereas that of the component community is dependent on the susceptibility of hosts to single parasite species and on the density of intermediate parasite stages. This is also the reason for the stronger infestation of hosts from Dahmeshöved as opposed to those from Blank Eck, where the degree of eutrophication is lower and, therefore, fewer herbivorous intermediate hosts can exist.

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Parasite communities of the inner and middle Schlei Fjord were investigated from 1992 to 1994. Host groups, comprising small‐size or young fishes, benthic and planktonic crustaceans, and snails were taken from 4 localities (Selk, Haddeby, Borgwedel and Missunde) of differing salinities (1–8 ‰). The host and parasite fauna turned out to be very sparse, comprising 10 (Selk) to 16 (Missunde) host species of which only 7 and 9, respectively, harboured parasites. Parasites numbered 4 in Selk and 8 in Borgwedel and Missunde. Hysterothylacium sp. (Nematoda) was the only parasite present in all localities. High prevalences were attained by Proteocephalus percae (Cestoda) and Cryptocotyle concavum (Digenea) in Borgwedel and Missunde, whereas in Selk and Haddeby Caligus lacustris prevailed. With the exception of Cryptocotyle concavum intensities were mostly 1, and multiple infestations were rare. On the basis of comparisons with former investigations a faunal impoverishment can be stated during the course of 18 years in the inner Schlei Fjord. Increasing eutrophication causing deep mud layers on the bottoms as well as the ensuing high oxygen deficiencies even in the free water column during summer and autumn can be possible reasons for this phenomenon. As further consequence, the parasite spectrum was reduced to species which have simple developmental cycles and to species which prefer planktonic or suprabenthic, not benthic, hosts. The environment in Missunde with salinities of 6–8 ‰, can be designated as that described by Remane as an area of species minimum. In former times the parasite communities in Missunde were characterised by a species richness which was in contrast to the abundance of free living organisms.