The parasites of ten fish species, including four Gobiidae, three Gasterosteidae, two Syngnathidae, and one Zoarcidae from the Salzhaff region, Northwest Mecklenburg, Baltic Sea, were investigated in 1995 and 1996. As many as 36 parasite species, represented by diverse groups of helminths and protozoans as well as annelids and copepods, are found during 4 seasons in these hosts. By far the most abundant group is represented by digeneans, comprising 15 species, followed by 7 cestodans and 6 nematodes. With regard to component communities, 8 host-parasite combinations are core and secondary species with more than 40% prevalence in which generalists such as the digeneans Podocotyle atomon and Cryptocotyle concavum (in 3 hosts). C. lingua, Diplostomum spathaceum, and Acanthostomum balthicum, and the nematode Hysterothylacium are involved. Also, specialists such as Aphalloides timmi in Pomatoschistus microps as well as Magnibursatus caudofilamentosa and Thersitina gasterostei in Gasterosteus aculeatus attain high levels of prevalence. A comparison of different investigations reveals greater prevalence of allogenic and autogenic parasite species with 3 host cycles in the Rerik-Riff (free Baltic) and higher levels of prevalence of autogenic parasite species with 1 or 2 host cycles in the entire Salzhaff. The component communities of gobies from Dahmeshöved, Lübeck Bight, attain generally lower degrees of prevalence than those of the Salzhaff region. The infracommunities consist mostly of 1-3 parasite species per host specimen; this value is surpassed on occasion in P. microps (maximum 7 species) and in G. aculeatus (maximum 9 species, which may compete for 5 microhabitats in a host specimen). In this context the theory of empty niches propagated by some parasitologists is critically discussed and substituted by the assumption of variable niche widths. The seasonality of the more abundant parasites is either unclear--as in the case of C. concavum--or evident--as in the case of P. atomon, which prevail in early spring and summer, or A. timmi, which dominate in late summer, as do M. caudofilamentosa, which is absent in spring. The main causes of the infestation of fish hosts may be their ages and the availability of parasites due to the presence of intermediate hosts.

The Baltic Sea is characterized by organisms that can tolerate brackish water. Because of the Sea's history during glacial times, its flora and fauna (and also their parasites) can be traced to marine, freshwater and genuine brackish elements beside glacial relics. Snails, planktonic copepods, benthic amphipods and isopods are important intermediate hosts of diverse helminths; in addition polychaetes, bivalves and fishes may also act as final hosts. The most important final hosts, beside fishes, were seals and birds; these were able to disperse the parasites over the whole of the Baltic. Decreasing salinity from west to east limits the distribution of many parasites. Several marine and genuine brackish water species have almost spread over the whole Baltic. Freshwater species, however, have a lower tolerance than marine species and are only rarely found in the western part. A serious problem in the Baltic is eutrophication which can lead to massive abundances of generalist parasites, in host populations as well as host individuals. The final stage of this influence can cause a general decrease of host abundance and, as a consequence, of all kinds of parasites, due to oxygen deficiency. In comparison with the species spectrum of other brackish waters in Europe, the Baltic presents some endemic parasites as well as sharing parasite species with the Mediterranean and even the Black Sea.

Metazoan parasites of guilds of benthic snails and crustaceans and of four fish families--Gobiidae, Gasterosteidae, Syngnathidae, and Zoarcidae--were investigated off the brackish Salzhaff area (Southwest Baltic) in the semienclosed Salzhaff and the near Rerik Riff in the free Baltic. Comparisons revealed greater similarities in parasite populations and communities within the fish guilds than between them. According to an evaluation of the core-/satellite-species concept using abundance values, the most important parasites of fish were some generalists, such as Cryptocotyle spp., Podocotyle atomon, and Diplostomum spathaceum, as well as some specialists, such as Acanthostomum balthicum, Thersitina gasterostei, and Aphalloides timmi. These specialists revealed high degrees of prevalence in their main hosts and lower degrees in one or two by-hosts. Additional importance is assigned to parasites that cause harm to their hosts due to their large size, e.g., Schistocephalus spp., or via massive infestation, e.g., several digenean metacercariae. Because specialists were more prominent in snails and fish from the Rerik Riff, the correlation of host numbers with prevalence resulted in only a slight increase instead of a more rapid rise in regression among crustaceans and fish from the entire Salzhaff, where the generalists were more prevalent. The selected host guilds demonstrated the entire life cycles of three digeneans (P. atomon, A. balthicum, A. timmi), one acanthocephalan (Echinorhynchus gadi), and one nematode (Hysterothylacium sp.). The prevalence increased in these cycles from host level to host level and attained relatively high values in all guilds. The parasite fauna of the Salzhaff area is influenced by eutrophication stress, which leads to a high level of productivity and, consequently, to great densities in primary consumers such as snails and crustaceans. These are attractive for several secondary consumers such as fish and birds, which is the reason for the existence of at least 24 autogenic and 20 allogenic parasite species at this locality. The slight surplus of the first category indicates a yet-balanced environment in the investigation area.

Between 1990 and 1992 deep-snouted pipefish Syngnathus typhle and black goby Gobius niger were found to be infested by Stage 3 and 4 larvae and preadults of the swim bladder nematode Anguillicola crassus in Wismar Bay (southwestern Baltic Sea) at a frequency of 9.6 and 0.7%. respectively. The high prevalence (59.4%) of nematodes in European eels AnguiUa anguilla in the Baltic may therefore be attributable not only to freshwater habitation of some eels, but also to the possibility that they feed on affected copepods and small fishes in the brackish waters of the Baltic Sea.

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