Parasite risk of maricultured rainbow trout (Oncorhynchus mykiss Walbaum, 1792) in the Western Baltic Sea, Germany

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.