Associate Professor University of Washington Seattle, Washington, United States
How has the risk of parasitism changed for marine mammals over the past several decades? Parasitological assessments of marine mammals are rarely performed and are biased toward unhealthy animals. A more practical method for assessing long-term change in risk might be to measure the abundance of parasite infectious stages in marine mammal prey, like salmon. Parasitic nematodes of the family Anisakidae (anisakids) use salmonids as intermediate or paratenic hosts in life cycles that terminate in marine mammal definitive hosts. These infections can cause acute gastritis and peritonitis in cetaceans. To assess whether anisakid burden – and infection risk for marine mammals – has changed in salmonids over time, we used a novel data source: salmon that were caught, canned and thermally processed in Alaska, USA for human consumption. We examined canned filets of pink (Oncorhynchus gorbuscha, n = 58), sockeye (Oncorhynchus nerka, n = 45), and chum salmon (Oncorhynchus keta, n = 35) processed between 1979 and 2019. We dissected each filet and quantified the number of worms per gram of salmon tissue.
Anisakid burden increased over time in chum (p < 0.0001) and pink (p < 0.01), but not sockeye salmon (p = 0.56). These results suggest that marine mammals consuming chum and pink salmon have experienced an increasing risk of intestinal parasitism over the 40-year study period. On their own, anisakids have minor impacts on marine mammal host health, though in large quantities or in concert with other stressors they can result in mortality. Marine mammals face a myriad of stressors, resulting in the endangerment that several Alaskan marine mammals populations face today. An increasing risk of anisakid infection may have deleterious effects on the health of both individuals and at-risk populations; in populations small enough, infection may make the difference between a conservation success or failure. Canned fish hold the potential to serve as a window into past fish and marine mammal health that would otherwise be lost. Our study is the first study to use cooked, canned fish to detect a trend in parasite burden over time, showing that canned fish hold potential to glimpse into ecosystems of the past, and detect changes that may put ecosystems of the present at risk.