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Researchers used tinned fish to reconstruct parasitic population change, giving new meaning to the phrase “opening a can of worms”
BY RACHEL NUWER
Chelsea Wood, a parasite ecologist at the University of Washington, and her then graduate student Natalie Mastick had been pondering for months how they could reconstruct fluctuations over time in the risk parasites pose to marine mammals in the Pacific Northwest. No long-term datasets exist for the vast majority of parasitic species, so Wood and Mastick knew they’d have to get creative. One idea they had was to use salmon’s parasitic loads as proxies for marine mammal infection.
Killer whales, seals and belugas prey on salmon, which are an intermediate host for various nematodes that complete their several-step life cycle in those predators. The parasites cannot breed and enter the environment without marine mammals, so the level of infection in salmon is intimately tied to that of their predators.
While the logic behind the researchers’ idea of using salmon parasites to understand marine mammal parasites was sound, there was still one problem: Where would they get a bunch of old salmon that they could dissect to look for parasites?
They seemed to be out of options when, “out of the blue,” Wood says, she received a call from the Seafood Products Association, a trade organization in Seattle. The group told her it was cleaning out its basement and was getting rid of stacks of dusty boxes of long-expired canned salmon. Before throwing the cans away, the association wondered if Wood might want them.
“We were like, ‘Worms could be in there!’” Wood recalls. “That’s how we started on this study: [the Seafood Products Association] asking us if we wanted this trash from their basement and us saying, ‘Absolutely, yes.’”
Wood and Mastick’s hunch about canned salmon possibly serving as a time capsule for worms turned out to be correct: the tinned fish contained lots of parasites. As the researchers reported this week in Ecology and Evolution, they were able to use the recovered parasites to reconstruct how infection burden has changed over the course of 42 years in four salmon species and found that it increased in two. While this might sound like a bad thing, the researchers suspect that the uptick in worms signals “a conservation success story” for marine mammals, Wood says. “It’s possible as marine mammals have come back, their parasites have come back, too.”
Canned salmon gets cooked as it’s sealed, so going into the study, the researchers were unsure whether they would be able to detect any anisakids, the type of parasitic nematodes that infect the muscle of Alaskan salmon. The worms, however, “were very noticeable—at least for us,” Wood says. “We were really surprised.”
Anisakids find their way into salmon through smaller intermediate hosts such as krill or fish. After the salmon eat those smaller hosts, the parasites burrow into the salmon’s muscle, making “a little pocket,” Wood says. She and her colleagues carefully picked through the salmon muscle tissue with forceps, and when they opened one of the pockets, the worms tended to “spring out,” Wood says. “They were very easily visible.” (These worms occur in many types of seafood, she adds, and because they are killed in the cooking process, they pose no danger to humans unless food is undercooked or a person has a particular allergy to them.)