LAUSR

Ichthyophonus spp. are cosmopolitan parasites causing proliferative, systemic disease in a number of marine and freshwater fish, including several commercially important species such as Atlantic and Pacific herring (Clupea harengus, C. pallasii), rainbow trout (Oncorhynchus mykiss), Chinook salmon (O. tshawytscha) and sockeye salmon (O. nerka) (Gregg et al., 2016; Kocan et al., 2006; Rahimian & Thulin, 1996; Tierney & Farrell, 2004; Zuray et al., 2012). There is some uncertainty regarding both species diversity and host specificity within the Ichthyophonus genus, and at present only two species have been formally described, I. hoferi Plehn and Mulsow (1911) from rainbow trout and I. irregularis Rand et al., 2000 from yellowtail flounder (Limanda ferruginea). There are, however, strong genetic indications that the genus comprises more species than the two described so far (Hershberger et al., 2016; Rasmussen et al., 2010). Atlantic mackerel (Scomber scombrus) is known to be susceptible to Ichthyophonus infections (Gregg et al., 2016; Johnstone, 1913; Murchelano et al., 1986; Sproston, 1944), but the prevalence has not been extensively monitored. A few studies indicate differences between geographic areas, seasons and individual shoals of mackerel. Sproston (1944) observed varying Ichthyophonus sp. (as I. hoferi) prevalence across different catches in the North Sea, ranging between 0% and 100% over a 3year period with annual means of 38– 70%, whereas Rahimian (1998) did not find any infected mackerel during a survey in the adjacent Skagerrak and Kattegat. Murchelano et al. (1986) observed infected individuals both in the eastern and western North Atlantic, but the general prevalence could not be determined due to low sample sizes. The diverging results of these studies indicate large differences in the prevalence of Ichthyophonus infections in Atlantic mackerel, possibly due to temporal fluctuations or variations in infection pressure in different geographic regions. Recent studies indicate frequent intermixing between the different spawning components within the Northeast Atlantic (NEA) mackerel stocks (Henriksen, Nøttestad, Olafsdottir, Slotte, & Sánchez, 2020; Jansen & Gislason, 2013), and infected individuals in some components may thus potentially spread parasites to other spawning components. NEA mackerel is also found increasingly further north and west, most likely due to changes in the migration pattern following climate change (Nøttestad et al., 2016; Nøttestad et al., 2020). Parasites infecting the mackerel, such as Ichthyophonus spp., can thus potentially spread and infect new fish host species with little or no inherent resistance to them, which could have great ecological and commercial ramifications. The diversity and prevalence of Ichthyophonus sp. in NEA mackerel should be monitored closely. The present study details our observations of the prevalence of Ichthyophonus infections in mackerel obtained from the Northeast Atlantic. A total of 960 NEA mackerel were sampled during research cruises and from commercial catches in the North, Norwegian and Greenland Seas in 2019– 2021. To assess the prevalence of Ichthyophonus sp., freshly caught or defrosted fish were examined macroscopically for visible signs of infection in the form of granulomas in the heart, kidney, spleen or red muscle tissue (Hodneland