Scientists open 40-year-old salmon and find a surprising sign of ocean recovery
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Scientists open 40-year-old salmon and find a surprising sign of ocean recovery
Date:
April 1, 2026
Source:
University of Washington
Summary:
Old canned salmon turned out to be a time capsule of ocean health. Researchers found that rising levels of tiny parasitic worms in some salmon species suggest stronger, more complete marine food webs. Because these parasites depend on multiple hosts—including marine mammals—their increase may reflect ecosystem recovery over decades. What looks unappetizing may actually be a sign of a healthier ocean.
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FULL STORY
A photo of an anisakid worm — circled in red — in a canned salmon fillet. Credit: Natalie Mastick/University of Washington
Alaska's waters support one of the world's most important salmon fisheries, sustained by complex marine food webs. Scientists are eager to understand how these systems are changing as the climate shifts. The challenge is that reliable samples from decades ago are hard to find.
"We have to really open our minds and get creative about what can act as an ecological data source," said Natalie Mastick, currently a postdoctoral researcher at the Peabody Museum of Natural History at Yale University.
Scientists Turn to an Unusual Source: Decades-Old Salmon
While working on her doctorate at the University of Washington in Seattle, Mastick took an unconventional approach. Instead of relying on modern samples, she examined old cans of salmon.
These cans contained fillets from four species collected over a 42-year span in the Gulf of Alaska and Bristol Bay. Mastick and her team opened 178 cans and carefully dissected the preserved fish, counting tiny parasitic worms known as anisakids embedded in the flesh.
Although these parasites had been killed during the canning process and would not harm consumers, they hold valuable scientific information.
Why "Sushi Worms" Can Signal a Healthy Ecosystem
At first glance, finding worms in fish may seem alarming. But scientists say the opposite can be true.
"Everyone assumes that worms in your salmon is a sign that things have gone awry," said Chelsea Wood, a UW associate professor of aquatic and fishery sciences. "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 came from a healthy ecosystem."
Because these parasites depend on multiple species to survive, their presence can reflect the overall strength of the marine ecosystem.
Rising Parasite Levels Reveal Long-Term Ocean Trends
The team's findings, published in Ecology & Evolution, showed that anisakid levels increased in chum and pink salmon between 1979 and 2021. In coho and sockeye salmon, parasite levels remained steady.
"Anisakids have a complex life cycle that requires many types of hosts," said Mastick, who is lead author on the paper. "Seeing their numbers rise over time, as we did with pink and chum salmon, indicates that these parasites were able to find all the right hosts and reproduce. That could indicate a stable or recovering ecosystem, with enough of the right hosts for anisakids."
How Parasites Track the Entire Marine Food Web
Anisakids begin life as free-floating organisms in the ocean. They enter the food chain when small creatures like krill consume them. As predators eat those infected animals, the parasites move up the food web.
For example, krill may be eaten by small fish, which are then eaten by larger fish such as salmon. Eventually, the parasites reach marine mammals, where they reproduce. Their eggs are released back into the ocean, restarting the cycle.
"If a host is not present -- marine mammals, for example -- anisakids can't complete their life cycle and their numbers will drop," said Wood, who is senior author on the paper.
Are These Parasites Dangerous to People?
Humans are not part of the anisakid life cycle. Eating properly cooked fish poses little risk because the worms are already dead.
However, anisakids, also called "sushi worms" or "sushi parasites," can cause illness if consumed alive in raw or undercooked seafood. Symptoms may resemble food poisoning or, in rare cases, a condition known as anisakiasis.
Where Did the Salmon Samples Come From?
The canned salmon used in the study came from the Seafood Products Association, a Seattle-based trade group. The organization had preserved these cans over many years for quality control but eventually no longer needed them.
Mastick and co-author Rachel Welicky, an assistant professor at Neumann University in Pennsylvania, tested different techniques to analyze the samples. The worms, about a centimeter (0.4 inches) long, often curl up inside the fish muscle. By gently pulling apart the fillets with forceps and using a dissecting microscope, the team was able to count them accurately.
Why Are Parasite Levels Increasing?
There are several possible explanations for the rise