Marine mammal recovery is associated with the resurgence of a nematode parasite.

As the oceans change, the abundance of parasites and the risk of infection to marine mammals may also be changing. Nematodes in the family Anisakidae can harm marine mammals, and recent studies have revealed a global increase in these parasites, but the cause is unknown. We sought to determine how anisakid risk in Puget Sound had changed over 98 years by conducting a parasitological analysis of museum specimens of the prey species of marine mammals. We dissected Pacific Herring, Walleye Pollock, Surf Smelt, Pacific Hake, and Copper Rockfish collected between 1920 and 2018. We found that the larval anisakid Contracaecum spp. was the most abundant marine mammal parasite in these prey fish. We used a state-space model to assess the relationship between Contracaecum spp. abundance and time, with harbor seal abundance and sea surface temperature as potential correlates. We detected an overall decline in Contracaecum spp. abundance with a recent uptick starting in 1989, which was correlated with increasing harbor seal abundance. While these data reveal a regional trend, increases in marine mammal parasites in response to marine mammal protection have occurred elsewhere and suggest that the phenomenon might be more widespread than is currently appreciated. Marine mammals in Puget Sound are probably less burdened by anisakids than they were historically, but the recent recovery of anisakids could impact the health of these hosts, which today face very different stressors than they did in the past.