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Harvest quality rainbow trout ( Oncorhynchus mykiss Walbaum, 1792) raised in an integrated multi-trophic aquaculture (IMTA) unit off the German coast in the Baltic Sea were studied for ectoparasites and endoparasites. One hundred five specimens were sampled between November 2013 and 2014. Four endoparasite taxa were found, while no zoonotic or fish pathogenic ectoparasites could be detected. Molecular identified metacercariae of Diplostomum baeri were recorded at a non-critical level (18.1% prevalence, 1–4 intensity), getting introduced into the Baltic Sea through the cultured rainbow trout from the freshwater-based hatchery. The isolated acanthocephalan Echinorhynchus truttae was of freshwater origin, too. Pomphorhynchus laevis (Acanthocephala) and Brachyphallus crenatus (Digenea) seem to be transmitted into the net cage from Baltic Sea intermediate hosts. The parasite fauna was compared with those of wild living sea trout, Salmo trutta trutta studied at the immediate area (Unger and Palm in Parasitol Res 115(1):165–174, 2016 ). Stomach content analyses demonstrated that the farmed pellet-fed fish also utilized natural resources, especially fouling organisms. A single escapee, caught from the wild in the vicinity of the cages, had a massive intestinal infection with Baltic Sea parasites, the acanthocephalans Echinorhynchus gadi , P. laevis and the nematode Hysterothylacium aduncum . The salinity (helminths) as well as the culture conditions (trichodinids) at this pilot facility prevent significant transfer of fish pathogenic and zoonotic parasites as well as ectoparasite establishment and reduce the parasite risk for maricultured rainbow trout. This sampled location is particularly suitable for healthy and environmentally friendly aquaculture farming in the Baltic Sea.

Zoographical distribution of metazoan fish parasites in herring, Clupea harengus , from the Baltic Sea was analysed in order to use them as potential biological indicators. A total of 210 herring from six different sampling sites were investigated, harbouring 12 different parasite species [five digeneans (D), one cestode (C), three nematodes (N) and three acanthocephalans (A)]. The distribution of the parasite species differed according to region, with a distinct gradient of decreasing species richness towards the east of the Baltic Sea. The western localities at Kiel Bay, Rügen and Poland had the highest parasite diversity, including the marine parasite species Anisakis simplex (s.s.) (N), Brachyphallus crenatus and Hemiurus luehei (both D). The eastern localities had low parasite species richness, predominated by the freshwater digenean Diplostomum spathaceum . We could identify three different Baltic herring stocks, the spring-spawning herring of the western Baltic reaching from the Kattegat to the German and Polish coast, the stock of the central Baltic proper and the northern stock of C. harengus var. membras of the Gulf of Finland. The limited distribution of the herring parasites within the Baltic Sea enables their use as biological indicators for migration patterns and stock separation. The acanthocephalan Pomphorhynchus laevis that has already been used as an accumulation bioindicator for heavy metals was only recorded for the western herring stocks. However, the presence of mainly generalistic parasites and their uneven distribution patterns make their use as indicators for regional environmental and global change more difficult.

The feeding behavior of juvenile chum salmon ( Oncorhynchus keta ) and pink salmon ( Oncorhynchus gorbuscha ) is given for the period of their joint fattening in the coastal waters of Prostor Bay (Iturup Island). Investigations show decrease of variability in food content and stomach fullness indexes while juveniles move from shallow-water towards the open sea. Feeding selectivity of chum salmon and pink salmon for some species of copepods is studied and is shown to reduce tension in food relationships. High infestation of juveniles with trematode Brachyphallus crenatus is identified.

A total of 200 Maurolicus muelleri from the Mid-Atlantic Ridge (MAR) and the Norwegian Deep (ND) were studied for parasitic helminths. Two different metazoans were isolated from the MAR and five species from the ND. The predominant parasite species in both areas were tetraphyllidean cestode larvae (Scolex pleuronectis) and the anisakid nematode Anisakis simplex sensu stricto (s.s.), the latter being identified using genetic analysis of the internal transcribed spacer (ITS-1, ITS-2) and 5.8S regions of the rDNA. The parasite fauna of M. muelleri from the MAR was less species rich in comparison to ND, due to the deep-sea and oceanic environment. The digeneans Brachyphallus crenatus and Lecithaster confusus as well as the raphidascarid fish nematode Hysterothylacium aduncum were only collected from the ND. This can be explained either by the deep origin of the sampled fish specimens or the lack of suitable intermediate or final hosts in the region. Based on the frequent occurrence of A. simplex (s.s.) around the MAR and the ND, a pelagic life cycle is suggested at both localities, involving baleen and toothed whales as final and pelagic and mesopelagic fish and invertebrates as intermediate or paratenic hosts.

The helminth parasites present in 412 lesser sandeels (Ammodytes tobianus) taken from June 1996 to May 1997 from the Aran Islands on the west coast of Ireland were examined. Ten helminth parasite species were recorded, and more than 92% of the sandeels were infected with at least 1 helminth species. Seven of the species were digeneans, including Brachyphallus crenatus, Hemiurus communis, Derogenes varicus, Lecithaster gibbosus, Opechona bacillaris, Cryptocotyle lingua, and Galactosomum lacteum; 2 nematodes, including Hysterothylacium sp. and Contracaecum sp.; and 1 cestode, Scolex pleuronectis. Three of the 7 digenean species were either larvae or immature. Only 2 species, the digeneans G. lacteum and H. communis, had prevalences greater than 50%. The dominant species was G. lacteum, accounting for 67% of all parasites present. The relationship between spawned groups, host length, and season versus the abundance, prevalence, species richness, and the total number of parasites in the infracommunities was investigated. No difference was found between the parasite communities of the 2 spawning races of the host population. Mean abundance and prevalence of the different parasite species showed seasonal variation. Numbers of parasite species and numbers of parasites increased with fish length. The role of A. tobianus as an intermediate host for helminths was assessed; it was determined that most were infectious to birds or mammals, with the majority of the parasite species being autogenic (infectious to fish). The mean number of parasites per fish was nearly a quarter of the value recorded for A. tobianus in the North Sea, where a much higher intensity of infection was recorded.

Cystophorous cercariae from Retusa obtusa (Montagu) (Gastropoda, Opisthobranchia, Retusidae) develop into adults of Brachyphallus crenatus (Rudolphi, 1802) Odhner, 1905 (Hemiuridae). The free-swimming cercariae were ingested by laboratory-reared Acartia tonsa Dana, and the cercarial body was injected into the hemocoel of the copepod. Two-week-old metacercariae held at 15 C were infective to stickleback Gasterosteus aculeatus Linnaeus. The cercariae of B. crenatus are very similar to the cercariae of Hemiurus luehei Odhner, 1905, and Lecithocladium excisum (Rudolphi, 1819) Lühe, 1901 (Hemiuridae), which develop in closely related opisthobranch snails. Scanning electron microscopy of metacercariae and adults of B. crenatus revealed the annular plications of most of the external surface to be scalelike. The area surrounding the genital pore and the presomatic pit was densely plicated.