Old Canned Salmon Reveals Rise in Ocean Parasites

Researchers at the University of Washington opened 178 cans of expired salmon, some over 40 years old, and found a surprise inside: well-preserved parasitic worms that tell a story about Alaska's ocean health. The cans came from the Seafood Products Association in Seattle, stored as part of quality control since 1979. What started as a simple offer of old stock turned into a 42-year record of marine life. Lead researcher Natalie Mastick and senior author Chelsea Wood used these samples to track changes in anisakid nematodes, small worms that live in fish and need a full food chain to thrive.

Background

The cans held salmon caught in the Gulf of Alaska and Bristol Bay between 1979 and 2021. They included four species: 42 cans of chum salmon, 22 of coho, 62 of pink, and 52 of sockeye. These were not random finds from store shelves but part of a long-term archive kept by the Seafood Products Association for checking product quality. When the association needed to clear space, they contacted Wood, who saw potential for studying parasites over time.

Parasite ecologists like Wood and Mastick had long sought ways to look back at marine mammal health through fish parasites. Anisakids spend part of their life in salmon before moving to seals, sea lions, or whales. If those top predators are around, worm numbers stay high. Without them, the cycle breaks, and counts drop. Field studies are hard and costly, but these cans offered a ready-made timeline without needing boats or nets each year.

The team dissected the fish filets, even though canning had softened the worms a bit. They counted anisakids per gram of salmon flesh. This method worked because commercial canning kills the parasites with heat, making them safe for people but leaving their bodies intact for study. No one ate the contents; this was pure science.

Key Details

Results showed clear patterns by salmon type. In chum and pink salmon, worm numbers rose steadily from 1979 to 2021. Chum samples had low counts early on but climbed over decades. Pink salmon followed the same path, with more worms in later years. Coho and sockeye stayed flat, showing no big change.

Worm Counts by Species

• Chum salmon: Increase from near zero to higher levels, signaling more hosts in the food web.
• Pink salmon: Steady rise, matching chum trends.
• Coho salmon: Stable, no up or down shift.
• Sockeye salmon: Level throughout the period.

Anisakids are about 1 centimeter long, thin, and common in wild fish. They start in krill or plankton, move to small fish, then salmon, and end in mammals. Rising numbers mean krill populations support fish, which support salmon, which support seals and whales. A drop would point to missing links, like fewer marine mammals from pollution or hunting.

"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," said Mastick.

The study appeared in the journal Ecology and Evolution in April 2024. Researchers noted the cans' storage in cool, dry conditions kept seals tight, preserving contents without botulism or other issues. They checked 178 cans total, covering a full range of years and locations.

Wood explained the common view of worms in fish as a bad sign misses the point.

"Everyone assumes that worms in your salmon is a sign that things have gone awry," said Wood.

In truth, for ecologists, more worms mean balance. The canning process—high heat and pressure—stops any health risk to humans. Parasites die, but their shapes hold up for counting under microscopes.

What This Means

This find offers a new tool for tracking ocean health. Canned fish archives could become standard for long-term data, cheaper than ocean surveys. It shows Alaska waters, at least in these spots, have steady or improving food webs. More krill means more forage fish, more salmon, more predators—a chain intact despite fishing pressure and warming seas.

For chum and pink salmon, the rise hints at recovery or stability. These species run in huge numbers from Alaska streams, key to commercial catches. Steady coho and sockeye suggest not all salmon face the same conditions, perhaps due to different migration paths or worm types preferring certain hosts.

Scientists now plan to check other archives, maybe tuna or sardines, for global trends. Climate change worries about collapsing fisheries get a counterpoint here: some systems hold strong. Fisheries managers might use worm counts as a health check, alongside fish stock numbers.

Consumers see no change. Canned salmon stays safe past dates if seals look good—check for dents or bulges, smell on opening. The worms add no risk and highlight wild salmon's natural state. This work shows how everyday items like pantry staples can unlock big environmental secrets, giving a window into seas we cannot easily watch.