Recent discoveries have shown a rise in parasite counts in pink and chum salmon that was preserved in cans across four decades.
This stomach-turning discovery provides a possible sign that parts of the ocean food web are reconnecting with one another.
Inside 178 archived cans, researchers found a record that ordinary fish surveys had missed – 372 dead worms preserved in salmon flesh.
By pulling apart the preserved flesh, Natalie Mastick, a parasite ecologist, linked the worms to decades of changing exposure at the University of Washington (UW) in Seattle.
The pattern appeared in two of the four salmon species, while coho and sockeye showed no clear changes.
This strange evidence points toward recovery in some oceanic routes, but not within every salmon pathway.
The discovery revealed anisakid worms, tiny roundworms that use fish and marine mammals as hosts. However, these parasites need more than one host to survive.
They begin in seawater, enter small animals such as krill, and then pass into fish that eat infected prey.
“Anisakids have a complex life cycle that requires many types of hosts,” said Mastick.
When one required animal drops out, the worm loses a link and its numbers can fall.
Healthy marine food webs are a type of feeding relationships among ocean life. They can support parasites because hosts are plentiful enough for the chain to keep moving.
For anisakids, survival depends on infected prey being eaten again and again until a marine mammal completes the cycle.
That same dependence means their numbers tend to rise when enough hosts are present to keep the chain intact.
Canned salmon now adds a local clue to this case, but only for the salmon species whose counts actually climbed.
Canning heat killed the worms in these salmon, so the archived fillets did not show a new risk for those who eat canned fish.
Raw or undercooked fish can still carry live young worms that cause anisakiasis, an illness from worms attaching inside the gut.
Symptoms can include belly pain, nausea, vomiting, diarrhea, mild fever, and rare allergic reactions in some people.
Cooking seafood to 145 degrees Fahrenheit, or properly freezing fish, kills parasites before they can invade tissue.
Chum and pink salmon carried the clearest increase, which means their diets or habitats may expose them differently.
Young pink and chum often feed near shore, where seals, sea lions, and whales can shed parasite eggs into water.
Coho and sockeye stayed steady, possibly because their prey choices or parasite species did not change in the same way.
Marine mammals sit at the end of the anisakid route, because adult worms reproduce in their intestines.
In 1972, the Marine Mammal Protection Act was passed in the United States. This law protected these animals and reduced human interaction on whales, dolphins, seals, and sea lions.
Other hosts give the worms more chances to reproduce, so salmon may pick up more larvae through prey.
Recovery can therefore make seafood look less tidy while revealing a wider living network beneath the catch.
Warmer ocean water may also help some anisakid stages develop faster, though heat can hurt other hosts.
Temperature changes can alter where tiny drifting prey, fish, and marine mammals feed, which decreased the odds of infected prey meeting salmon.
Better chilling on fishing vessels probably slowed worm movement after capture, but the study found no clear chilling effect.
That detail suggests the rising counts were unlikely to be caused by newer handling practices.
Finding reliable old salmon samples are rare because fish usually get eaten, sold, or discarded, rather than being stored for science.
“We have to really open our minds and get creative about what can act as an ecological data source,” said Mastick.
The cans survived because the Seafood Products Association, a Seattle trade group, kept them for quality checks before sharing them with UW scientists.
Archived sardines, tuna, or other seafood could reveal changes that regular field surveys never captured in time.
Old cans cannot answer every question because heat destroyed the inner features that were needed to identify each worm species.
Researchers confirmed 127 of 372 worms as anisakids, then treated the rest as likely anisakids because all identifiable specimens matched.
One chum salmon can from 2019 held 115 worms, a trend that was more sensitive to a single sample.
Even with those limits, the pink salmon trend remained clearer, and the cans still captured change that no routine record kept.
Forty years of canned salmon turned dead worms into evidence of ocean recovery, leaving strange, measurable traces in food.
The next step is to analyze more historical records, allowing scientists to distinguish healthy recovery from harmful stress and reduce risks for salmon, marine mammals, and seafood consumers.
The study is published in the journal Ecology and Evolution.
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Image Credit: University of Washington
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