Canned salmon are not the heroes of a natural history museum haphazardly assembled in a barn where decades of Alaska’s marine ecology are preserved in brine and tin. Parasites can tell us a lot about an ecosystem because they often deal with the affairs of more than one species. But historically we haven’t paid much attention to them unless they cause serious problems for humans.

That’s a problem for parasite ecologists like Natalie Mastic and Chelsea Wood at the University of Washington, who are looking for a way to retrospectively track the impact of parasites on marine mammals in the Pacific Northwest. When Wood got a call from the Seattle Seafood Association asking if he wanted to take away his dusty, old, expired cans of salmon from the 1970s, his answer was an emphatic yes.

The jars were set aside for decades as part of the society’s quality control process, but in the hands of conservationists, they have become an archive of beautifully preserved specimens; not from salmon but from worms. Although the thought of worms in your canned fish may make you a little nauseous, these marine parasites, anisakids, which are about 0.4 inches (1 centimeter) long, are harmless to humans if they are killed during the canning process.

“Everyone assumes that worms in your salmon are a sign that things are going wrong,” Wood said when the study was published this year. “But the life cycle of anizakida connects many components of the food web. I see their presence as a sign that the fish on your plate comes from a healthy ecosystem.”

When anisakids are eaten by krill, they enter the food web, and the krill are in turn eaten by larger species. In this way, anisakids first reach the salmon and then the intestines of marine mammals, where the worms complete their life cycle by reproduction. Their eggs are released into the ocean by the mammal and the cycle begins again.

“If there is no host such as cetaceans, anisakids cannot complete their life cycle and their numbers will decline,” he said.

The 178 boxes in the “archive” contained four different species of salmon caught in the Gulf of Alaska and Bristol Bay over a 42-year period (1979–2021), including 42 boxes of catfish. (Oncorhynchus keta), 22 coho (Oncorhynchus kisutch), 62 pink (Oncorhynchus gorbuscha), 52 eyes (Oncorhynchus nerka).

Although the methods used to preserve salmon fortunately did not preserve the worms in their original form, the researchers were able to cut the fillets and count the number of worms per gram of salmon.

They found that helminths increased over time in chum and grayling, but not in salmon or coho.

“Seeing their numbers increase over time, as we do with humpbacks and chums, shows that these parasites can find the right hosts and reproduce,” said Mastic, the lead author of the paper. “This may indicate a stable or recovering ecosystem with sufficient numbers of correct hosts for anisakids.”

But the level of sessile worms in coho and sockeye is more difficult to explain, especially because the canning process has made identification of specific anisakid species difficult.

“Although we are confident in our identification at the family level, we have not been able to identify it yet. [анізакід]”Thus, parasites of a growing species may tend to infect grayling and chum, whereas parasites of a stable species may tend to infect coho and sockeye.”

Mastic and his colleagues believe that this new approach, in which dusty old boxes are transformed into an ecological archive, could contribute to many other scientific discoveries. Looks like they’ve opened quite a can of worms. This study was published on: Ecology and Evolution.