A sample of 7266 fish from 31 species was obtained from the north-eastern Bothnian Bay, Baltic Sea, from 1975 to 1982. Twenty-six fish species were found to be infected with seven acanthocephalan species identified as Echinorhynchus salmonis, E. bothniensis, E. borealis, E. gadi, Acanthocephalus anguillae, A. lucii and Neoechinorhynchus rutili. Acanthocephalus lucii was the only acanthocephalan not to attain sexual maturity in at least one species of fish. Fourteen new host records were found for E. bothniensis. A total of 1576 acanthocephalan infections was found; in most cases these were single species infections, but in 108 cases infections of two, and in two cases of three species were observed.
The most prominent host-parasite relationship involved E. salmonis and Coregonus widegreni, where the total of 11 505 worms taken from 1164 fish was roughly ten times the number of any of the other acanthocephalan parasites taken from all its hosts. Echinorhynchus gadi was encountered only in Gadus morhua. The fish of the Bothnian Bay were not considered to offer an important environment for E. gadi, E. borealis or A. lucii. Acanthocephalus anguillae was common only in Leuciscus idus while the range of hosts for N. rutili was found to be broad with 15 of 18 infected fish species being genuine definitive hosts.
Larger host fish tended to be more frequently and heavily infected than smaller ones with all acanthocephalan species except for N. rutili. Female fish were usually more heavily infected than males and the frequency distributions of numbers of acanthocephalans per fish were often observed to be overdispersed. The occurrence of non-acanthocephalan species of helminth in the alimentary tracts of 23 species of fish was recorded during the survey. On the basis of estimates of Berger-Parker indices, acanthocephalans were considered to be dominant in seven species of fish, cestodes in 12, digeneans in three and nematodes in one host species. Observations were also made on acanthocephalans in lampreys. The relevance of some of the findings for acanthocephalan transmission and adaptation to new environments is discussed briefly.