by Canned salmon are the unlikely heroes of a fortuitous natural history museum, where decades of Alaskan marine ecology have been preserved in brine and tin.
Parasites can tell us a lot about an ecosystem, because they usually involve multiple species. But unless they cause a major problem for humans, we historically haven’t paid much attention to them.
That’s a problem for parasite ecologists, like Natalie Mastick and Chelsea Wood of the University of Washington, who were looking for a way to retroactively monitor the effects of parasites on marine mammals in the Pacific Northwestern.
So when Wood got a call from Seattle’s Seafood Products Association, asking if she would be interested in taking boxes of dusty, old, expired cans of salmon – dating back to the 1970s – off her hands, her answer was unequivocal: yes.
The cans had been set aside for decades as part of the association’s quality control process, but in the ecologists’ hands they became an archive of excellently preserved specimens; not from salmon, but from worms.
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Although the idea of worms in canned fish is a bit stomach-churning, these approximately 1-inch-long marine parasites called anisakids are harmless to humans if killed during the canning process.
“Everyone assumes that worms in your salmon are a sign that something has gone wrong,” Wood said when the study was published this year.
“But the anisakid life cycle integrates many components of the food web. I see their presence as a signal that the fish on your plate comes from a healthy ecosystem.”
Anisakids enter the food web when they are eaten by krill, which in turn are eaten by larger species.
This is how anisakids get into salmon, and eventually into the intestines of marine mammals, where the worms complete their life cycle by reproducing. Their eggs are excreted into the ocean by the mammal and the cycle begins again.
“If no host is present – marine mammals, for example – anisakids cannot complete their life cycle and their numbers will decline,” said Wood, lead author of the paper.
The 178 cans in the ‘archive’ contain four different salmon species caught in the Gulf of Alaska and Bristol Bay over a 42-year period (1979-2021), including 42 cans of salmon (Oncorhynchus keta), 22 coho (Oncorhynchus kiss), 62 pink (Oncorhynchus Gorbuscha), and 52 sockeye (Oncorhynchus nerka).
Although the techniques used to preserve the salmon fortunately do not keep the worms in pristine condition, the researchers were able to dissect the fillets and calculate the number of worms per gram of salmon.
They found that worm counts 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 and salmon, indicates that these parasites were able to find the right hosts and reproduce,” said Mastick, lead author of the paper .
“That could indicate a stable or recovering ecosystem, with enough suitable hosts for anisakids.”
But it is more difficult to explain the stable levels of worms in coho and sockeye, mainly because the canning process made it difficult to identify the specific species of anisakid.
“Although we are confident in our identification at the family level, we were unable to identify the family [anisakids] we detected at the species level,” the authors write,
“It is thus possible that parasites of an increasing species tend to infect pink salmon and chum salmon, while parasites of a stable species tend to infect coho and sockeye salmon.”
Mastick and colleagues think this new approach – dusty old cans becoming an ecological archive – could lead to many more scientific discoveries. Looks like they’ve opened quite a can of worms.
This research was published in Ecology and evolution.
An earlier version of this article appeared in April 2024.
