Koi herpesvirus (KHV; cyprinid herpesvirus‐3) and carp oedema virus (CEV) are important viruses of common and koi carp (Cyprinus carpio); however, the distribution of these viruses in wild common carp in North America is largely unknown. During the summers of 2017 and 2018, 27 mass mortalities of common carp were reported from four states in the USA (Minnesota, Iowa, Pennsylvania and Wisconsin), the majority of which were distributed across eight major watersheds in southern Minnesota. Samples from 22 of these mortality events and from five clinically healthy nearby carp populations were screened for KHV, CEV and SVCV using real‐time polymerase chain reaction (qPCR). KHV was confirmed in 13 mortality events, CEV in two mortality events and coinfections of KHV/CEV in four mortality events. Nucleotide sequence analysis revealed that the KHV and CEV detected here are closely related to European lineages of these viruses. While molecular detection alone cannot conclusively link either virus with disease, the cases described here expand the known range of two important viruses. This is also the first reported detection of KHV and CEV coinfections in wild carp populations.

The influence of zebra mussels (Dreissena polymorpha) on phytoplankton abundance is well known, but their community‐level impact on energy flow is less clear. Reduced phytoplankton abundance could increase reliance of fish and aquatic invertebrates on alternative energy sources such as epiphyton and benthic algae. We assessed impacts of zebra mussels on energy flow by comparing food webs in two Minnesota, USA, lakes during summers of 2013 and 2014. Lake Carlos had a dense population of zebra mussels, while upstream Lake Ida was free of zebra mussels until this study began and maintained low densities during our study. We used baseline‐corrected (BC) δ13C to test whether fish and littoral invertebrate primary and secondary consumers were more reliant on littoral carbon in Carlos compared to Ida. We also used BC δ15N to determine if trophic position of fish species differed between lakes. Lastly, we compared isotopic niche space by estimating standard ellipse areas for fish species in Carlos and Ida lakes, and tested whether the community‐level range of trophic levels, reliance on littoral carbon and standard ellipse area differed between lakes. Results showed invertebrate secondary consumers had more enriched BC δ13C in Carlos than in Ida, indicating greater reliance on littoral energy. Mixing models indicated that 10 of 11 fish species were more reliant on littoral carbon in Carlos, with littoral carbon use in the 10 species 1.5‐fold higher in Carlos. Isotopic niche analysis also showed increased littoral reliance in Carlos fish, as the same 10 fish species in Carlos had statistically distinct ellipses that were enriched in δ13C. Mixing models also indicated that seven of 11 fish species analysed had significantly higher trophic positions in Lake Carlos. In contrast, community‐scale metrics for fish showed no difference between lakes in the range of trophic levels, range of reliance on littoral energy, or size of standardised ellipse area of isotopic niche space. This indicates that, despite most individual fish species increasing their reliance on littoral energy and shifting upwards in trophic position, the overall size of the community isotopic niche area remained similar between lakes. Our results indicate that zebra mussels have community‐wide impacts on energy flow in lakes, with invertebrate predators and many species of fish increasing their reliance on littoral energy sources, and most species of fish shifting to higher trophic positions. A key question is whether increased water clarity associated with zebra mussels can increase littoral production sufficiently to compensate for higher demand. If not, it is plausible that invertebrate and fish production will decline due to increased intra‐ and inter‐specific competition.

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