Ichthyophonus‐infected walleye pollock Theragra chalcogramma (Pallas) in the eastern Bering Sea: a potential reservoir of infections in the North Pacific

Abstract

In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscle. Using molecular and histological methods, we identified the parasite in Bering Sea pollock as Ichthyophonus. Infected pollock were identified throughout the study area, and prevalence was greater in adults than in juveniles. This study not only provides the first documented report of Ichthyophonus in any fish species captured in the Bering Sea, but also reveals that the parasite has been present in this region for nearly 20 years and is not a recent introduction. Sequence analysis of 18S rDNA from Ichthyophonus in pollock revealed that consensus sequences were identical to published parasite sequences from Pacific herring and Yukon River Chinook salmon. Results from this study suggest potential for Ichthyophonus exposures from infected pollock via two trophic pathways; feeding on whole fish as prey and scavenging on industry-discharged offal. Considering the notable Ichthyophonus levels in pollock, the low host specificity of the parasite and the role of this host as a central prey item in the Bering Sea, pollock likely serve as a key Ichthyophonus reservoir for other susceptible hosts in the North Pacific.

The parasite Ichthyophonus sp. in Pacific herring from the coastal NE Pacific

“…An expansive area exists between the two southernmost sampling locations in this study (Puget Sound and San Francisco Bay), and Pacific herring populations throughout this gap tend to be relatively small; however, it is likely that Ichthyophonus occurs in some of these stocks, as the parasite can occur in high prevalence among another clupeid, American shad (Alosa pseudoharengus), from the Oregon/ Washington boarder . Although these results indicate that Pacific herring represent a dominant host species for Ichthyophonus in the NE Pacific, other species appear to maintain the parasite northward into the Bering Sea, where a paucity of infections in Pacific herring (Kocan et al 2004) are replaced by oftenhigh prevalences in Chinook salmon and walleye pollock (White, Morado & Friedman 2014). The geographic extremes of the Ichthyophonus range in Pacific herring likely shift on an interannual basis in response to changing climatic conditions, ocean circulation patterns, water temperatures and invertebrate assemblages.…”

“…Although these results indicate that Pacific herring represent a dominant host species for Ichthyophonus in the NE Pacific, other species appear to maintain the parasite northward into the Bering Sea, where a paucity of infections in Pacific herring (Kocan et al . ) are replaced by often‐high prevalences in Chinook salmon and walleye pollock (White, Morado & Friedman ). The geographic extremes of the Ichthyophonus range in Pacific herring likely shift on an interannual basis in response to changing climatic conditions, ocean circulation patterns, water temperatures and invertebrate assemblages.…”

Incidence of Parasites on Frozen Alaska Pollock (Gadus chalcogrammus, Pallas, 1814) Fillets

“…In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscles. Using molecular and histological methods, White and coauthors [44] identified the parasite in Bering Sea pollock as Ichthyophonus.…”

Ichthyophonus in sport-caught groundfishes from southcentral Alaska

“…Although Ichthyophonus is known to infect many marine fishes, its known host distribution in the marine waters of Alaska is limited to Pacific herring in the Gulf of Alaska, walleye pollock Gadus chalcogrammus in the Gulf of Alaska and Bering Sea, Chinook salmon in the Yukon and Kuskokwim rivers, and Pacific halibut Hippoglossus stenolepis in Prince William Sound, Cook Inlet, and the eastern Bering Sea (Eaton et al 1991, Marty et al 1998Kocan et al 2004, White et al 2014, Hershberger et al 2018. The presence of Ichthyophonus in these fishes, all of which are important prey species at some point in their life histories, suggests that many other predatory fishes in the region are likely exposed to the parasite through trophic in teractions.…”