By chance, in a natural history museum in the back of a pantry, scientists have made some intriguing parasite discoveries. Well past its prime, canned salmon preserves decades of Alaskan marine ecology in brine and tin.
Parasites can reveal a lot about an ecosystem because they tend to engage in multi-species business. However, unless they cause major problems for humans, historically we have mostly ignored them.
That’s a problem for parasite ecologists like Natalie Mastick and Chelsea Wood of the University of Washington, who have been looking for a way to track back the effects of parasites on marine mammals in the Pacific Northwest.
So when Wood got a call from the Seattle Seafood Association asking if she’d get her hands on some dusty old cans of expired salmon — some dating back to the 1970s — she said unequivocally, “yes.”
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The cans were stored away for decades as part of the association’s quality control process, but in the hands of ecologists they have become an archive of well-preserved specimens; not salmon, but worms.
Check out the video below for a summary of the study:
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Although the thought of worms in your canned fish may turn your stomach a bit, these approximately 0.4 inch (1 centimeter) long marine anisacid parasites are harmless to humans once they are killed during the canning process.
“Everyone thinks that worms in your salmon are a sign that things have gone wrong,” Wood said when the study was published last year.
“But the anisakid life cycle integrates many components of the food web. I think their presence is a signal that the fish on your plate come from a healthy ecosystem.”
Anisakid worm (circled in red) in a canned salmon fillet. (Natalie Mastick/University of Washington)
Anisakids enter the food web when they are eaten by krill, which in turn are eaten by larger species.
This is how the anisakids end up in the salmon, and eventually in the intestines of marine mammals, where the worms complete their life cycle by reproducing. Their eggs are released into the ocean by the mammal, and the cycle begins again.
“Without a host — such as a marine mammal — the anisakids cannot complete their life cycle and their numbers will decline,” said Wood, senior author of the paper.
The 178 cans 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 cans of suckers. (Oncorhynchus keta), 22 coho (Oncorhynchus kisutch), 62 pink (Oncorhynchus gorbuscha) and 52 sockeye (Oncorhynchus liver).
Although salmon preservation methods fortunately do not preserve the worms in a pristine condition, the researchers were able to dissect the fillets and calculate the number of worms per gram of salmon.
They found that worms increased over time in chum and pink salmon, but not in sockeye or coho.
“Seeing their numbers increase over time, as we did with pink salmon, shows that these parasites were able to find all the right hosts and reproduce,” said Mastick, lead author of the paper.
“This may indicate a stable or recovering ecosystem with sufficient suitable anisakid hosts.”
Distribution of canned salmon samples for each salmon species in each decade. (Mastick et al., Ecology and evolution2024)
However, it is more difficult to explain the stable numbers of worms in coho and sockeye, especially since the preservation process made it difficult to identify specific anisakid species.
“While we are confident that we are identifying at the family level, we have not been able to identify [anisakids] we detected at the species level,” the authors write.
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“So it’s possible that parasites from proliferating species tend to infect sockeye and sockeye salmon, while parasites from stable species tend to infect coho and sockeye.”
Mastick and his colleagues believe that this new approach—turning dusty old cans into an ecological archive—could lead to many more scientific discoveries. Looks like they opened quite a can of worms.
This study was published in Ecology and evolution.
An earlier version of this article was published in 2024. in April.