Interpreting Lamprey Attacks on Pacific Cod in the Eastern Bering Sea

Adult anadromous lampreys attack several species targeted by large-scale commercial fisheries in the North Pacific Ocean, and the potential negative impact to these host fishes is not well understood. The Arctic Lamprey Lethenteron camtschaticum and Pacific Lamprey Entosphenus tridentatus are anadromous species that feed in the eastern Bering Sea, and lamprey parasitism is evident on Pacific Cod Gadus macrocephalus near the Bering Slope. To examine this parasitic interaction, we first built models using morphological measurements from lamprey oral discs to predict which lamprey species caused the observed wounds on Pacific Cod. We then examined lamprey wounding rates and explored healing patterns related to the severity and location of lamprey wounds. We scanned 8,746 Pacific Cod for lamprey wounds and found that 4.9% of the cod had at least one wound. Lamprey wound morphology was better predicted by an oral disk model built for Pacific Lamprey than by a similar model built for Arctic Lamprey. The occurrence of lamprey wounds that had penetrated muscle tissue but had not completely healed was more prevalent as Pacific Cod length increased. Generalized additive model results indicated that latitude and mean Pacific Cod length were important in predicting lamprey wounding rates at a sampling station. Recently inflicted lamprey wounds that penetrated Pacific Cod muscle tissue were observed four times as often as superficial wounds that did not penetrate muscle tissue, but superficial wounds were twice as likely to reach a completely healed state. No difference was detected in the likelihood of a lamprey wound to reach a completely healed state among different host body regions. While there is a potential for lamprey attacks to negatively affect individual host fish, we emphasize the importance of understanding population dynamics between native lampreys and their hosts, as this could aid in explaining variations in the natural mortality of commercially important fish species in the eastern Bering Sea.