Could new records of parasitic cestode <i>Nybelinia surmenicola</i> in Dolly Varden (<i>Salvelinus malma</i>) from the Beaufort Sea indicate increased presence of salmon shark (<i>Lamna ditropis</i>) in the Arctic?

Parasitological investigations of anadromous Dolly Varden ( Salvelinus malma) (Walbaum, 1792) harvested along the Canadian Beaufort Sea coast in summer 2021 and 2022 revealed infections by the parasitic cestode Nybelinia surmenicola (Okada in Dollfus, 1929) (2% and 0.8% prevalence, respectively) whose only documented final host is salmon shark ( Lamna ditropis) (Hubbs and Follett, 1947), which to date has been documented only once in the Arctic. Infection rates were low as only two fish, captured in consecutive years, were infected with a single worm at the plerocercoid stage in their stomach. These infections are the first record of the parasite in the Arctic Ocean and for Dolly Varden in North America. The source of infection in both fishes was likely from consuming a euphausiid, presumably Thysanoessa raschii (M. Sars, 1863), which is an infrequent prey of Dolly Varden and a known first host of the parasite, that became infected by consuming N. surmenicola eggs expelled by salmon shark. The euphausiids presumably originated from the Bering Sea and were actively transported by currents into the Arctic Ocean yet it is unknown where their infection occurred. Given the amount of time for exogenous feeding to begin, the presumed rate of transport and time spent feeding in Arctic waters (i.e., Chukchi and Beaufort seas), we infer the infection of euphausiids most likely occurred in the Arctic, which suggests the presence of salmon shark. Additionally, the findings provide evidence that salmon shark may becoming increasingly prevalent in Arctic waters in recent years due to climate change. Our observation underscores the utility of parasitological information for surveillance for detecting climate-related change in Arctic marine biodiversity.