Comparison of the parasite communities of brown trout (Salmo trutta) from two coastal lakes in central Norway

Abstract

There are several studies concerning parasite communities in brown trout (Salmo trutta) and other salmonids in Norwegian lakes. These are mainly conducted in deeper oligotrophic inland lakes with a few coastal exceptions. Coastal lakes with eutrophic characteristics have received less attention. Therefore, this study aims to investigate similarities and differences in parasite diversity and community composition, within a costal system containing both eutrophic and oligotrophic characteristics. A total of 60 brown trout were examined for metazoan parasites in June 2021 from two coastal lakes in Fremstadvassdraget, central Norway. Litlvatnet - a shallow lake with eutrophic characteristics, and Storvatnet - a deep, oligotrophic lake. The study revealed that parasite diversity, evenness and total abundance of allogenic and autogenic parasites were similar between the lakes. However, differences were revealed in terms of infracommunity parasite composition between the lakes. Larval nematode Eustrongylides sp. and the metacercaria stage eyefluke Diplostomum sp. were more abundant and/or prevalent in Litlvatnet trout, whereas adult stage kidney digenean Phyllodistomum umblae, larval tapeworm Dibothriocephalus sp. and an unknown nematode (sp1) had higher abundance and/or prevalence in Storvatn trout. Adult stage intestinal digenean Crepidostomum brinkmanni also differed between the lakes, being more aggregated in Litlvatnet. Comparison with oligotrophic inland systems indicates that P. umblae may favor oligotrophic systems, whilst Eustrongylides sp. thrive in systems with eutrophic characteristics. The other parasite taxa found may be more adaptable to different lake characteristics. Stable carbon and nitrogen isotope- and diet content analysis were also conducted to see if the prey the trout had consumed in the last period reflected the patterns in parasite community. Litlvatnet trout had higher signatures of both nitrogen and carbon isotopes than Storvatnet trout. This may be caused by more marine and agricultural input in Litlvatnet, being the downstream located lake. The diet content also differed between the lakes, with Storvatnet trout consuming more on zooplankton. Fish prey were not abundant, but the high nitrogen isotope values and infection of many parasites possibly transmitted through piscivory, indicates that some trout may have eaten fish prey. This study suggests that differences in lake characteristics like size, depth and trophic status has little effect on parasite diversity. The parasite community composition, however, may be affected, and can partly be explained by the diet and trophic niche differences observed between the lakes.