Scientists cracked open 178 cans of salmon from the 1970s — and the parasites inside told an unexpected story about the ocean

Researchers at the University of Washington cracked open 178 cans of salmon — some sitting on shelves since 1979 — and carefully dissected the preserved fillets. What they found inside were tiny parasitic worms, coiled in the fish muscle.

For most people, that would be the end of the appetite. For these scientists, it was the beginning of an answer — one that could reshape how we interpret four decades of change in Alaska’s marine ecosystems.

A time capsule no one expected: worms in canned fish

The cans came from the Seafood Products Association, a Seattle-based trade group that had preserved them over many years for quality control purposes and eventually had no further use for them. That institutional quirk became a scientific opportunity.

Natalie Mastick, then a doctoral student at the University of Washington, recognized what those forgotten shelves actually held: a rare biological record of Alaska’s ocean stretching back to 1979. Her team opened 178 cans spanning 42 years, covering salmon collected from the Gulf of Alaska and Bristol Bay. They dissected fillets from four species — chum, pink, coho, and sockeye — counting anisakid parasitic worms embedded in the preserved flesh using forceps and dissecting microscopes.

“We have to really open our minds and get creative about what can act as an ecological data source,” Mastick said.

Why more worms may mean a healthier ocean

The instinct when finding worms in fish is to assume something has gone wrong. The science points in the opposite direction.

Anisakids — sometimes called “sushi worms” — depend on a multi-host life cycle to survive and reproduce. They begin as free-floating organisms, enter the food chain when krill consume them, and travel upward as predators eat those infected animals. Salmon become infected by eating smaller fish. The parasites can only reproduce inside marine mammals, where their eggs are eventually released back into the ocean to restart the cycle.

That dependency is precisely what makes them useful as ecological indicators. If any key host disappears from the food web, the parasite population declines. Rising worm counts suggest the opposite — that all the necessary players are present and accounted for.

“Everyone assumes that worms in your salmon is a sign that things have gone awry,” said Chelsea Wood, UW associate professor and senior author on the study. “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.”

Four decades of data, two different stories

The results weren’t uniform across species, which added real complexity to the findings.

Anisakid levels increased significantly in chum and pink salmon between 1979 and 2021. In coho and sockeye, parasite counts held steady over the same period. The divergence likely reflects the fact that different anisakid species rely on different combinations of hosts — meaning the food web conditions driving parasite increases in one salmon species may not apply equally to another.

Pinning down exactly which worm species were responsible wasn’t possible. The canning process preserved the outer structure of the parasites well enough to count them accurately, but destroyed the internal features needed for species-level identification. That gap leaves some questions open, even as the broader trend holds.

Marine mammal protections as a possible driver

The timing of the parasite increase offers a suggestive clue.

The Marine Mammal Protection Act of 1972 marked a turning point for seals, sea lions, orcas, and other marine mammals that had declined sharply in prior decades. As those populations recovered, so did the reproductive opportunities for anisakids — parasites that can only complete their life cycle inside a marine mammal host.

“Anisakids can only reproduce in the intestines of a marine mammal, so this could be a sign that, over our study period — from 1979 to 2021 — anisakid levels were rising because of more opportunities to reproduce,” Mastick said.

Other factors may also be at play. Warming ocean temperatures could be shifting host distributions and abundance, and environmental improvements linked to the Clean Water Act may have contributed to healthier coastal ecosystems. Researchers are careful not to attribute the trend to any single cause, given how deeply interconnected these variables are.

Canned fish as a new frontier for ecological research

One of the study’s quieter contributions may be methodological.

Archived seafood represents an underexplored category of biological data. The team believes the approach could extend to other preserved products — canned sardines, for instance — offering a window into ecosystems from eras when systematic ecological monitoring was sparse or simply didn’t exist. That’s a persistent challenge in marine science: the further back you look, the thinner the reliable record gets.

The study, published in Ecology & Evolution and funded by the National Science Foundation, the Alfred P. Sloan Foundation, the Washington Research Foundation, and the University of Washington, also underscores something harder to formalize: the value of scientific networking. The connection to the Seafood Products Association’s archive came through informal channels, not a structured data repository.

“We can only get these insights into ecosystems of the past by networking and making the connections to discover untapped sources of historical data,” Wood said.

The question now is what other overlooked archives — in warehouses, museum collections, or institutional freezers — might hold similarly unexpected records of how the ocean has changed. The answers, it turns out, may already be sitting on a shelf somewhere.