Abstract
Rainbow trout (Oncorhynchus mykiss) runts (n = 5) and harvest quality fish (n = 5) were sampled from each of all the net cage mariculture facilities in Denmark at the time of slaughter during autumns of 2012 and 2013. Thus, a total of 190 trout were obtained, represented by 95 runts and 95 fish of harvest quality. Trout were examined for macroscopic ectoparasites as well as helminths of the gastrointestinal tract and body cavity by careful visual inspection, and belly flap musculature by pepsin digestion. Stomach content analysis was performed in order to assess the risk of endoparasite transmission to the net cage cultured trout. Low numbers of salmon lice (Lepeophtheirus salmonis) (1–9 lice per fish) were found on 9 trout from western localities off the eastern coast of Jutland characterized by water salinity levels of 21–24‰, whereas no lice were detected on fish from areas of lower salinity. Body cavity and musculature of all trout were free from helminths, and the absence of medically important 3rd stage larvae of Anisakidae was thus confirmed. However, transmission of endoparasites was documented by the finding of the nematode Hysterothylacium aduncum in the intestines of 9.5% of the runts (mean intensity = 2.3) and 2.1% of the harvest quality trout (mean intensity = 2.0). A few cestodes (Eubothrium sp.) located in the pyloric caeca, and one acantocephalan (Neoechinorhynchus sp.) found in the intestine, were collected from four trout. The higher prevalence of H. aduncum among runts was associated with a markedly increased intake of parasite intermediate or paratenic hosts, i.e. small fish (mainly three-spined stickleback) and crustaceans (mainly amphipods), by these fish compared to those of harvest quality. Additionally, a higher number of runts than harvest quality fish had eaten biofouling organisms, e.g. mussels and algae, further indicating a difference in feeding behavior between the two quality classes of trout.
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