Old Canned Salmon Reveals Rise in Ocean Parasites

Researchers at the University of Washington opened cans of salmon that had sat expired for decades and found parasitic worms inside. The cans came from Alaska's fishing grounds and dated back to the 1970s. The team wanted to track changes in ocean life over time. What they saw showed a rise in certain parasites, pointing to a working food chain in the sea.

Background

The cans started out as part of a quality check by the Seafood Products Association in Seattle. Fishermen and processors set them aside to test for problems. Over years, the collection grew into a stack of dusty boxes. Some cans were from fish caught in 1979, others from 2021. They sat in storage, forgotten until ecologists needed old samples.

Natalie Mastick and Chelsea Wood, parasite experts from the University of Washington, got a call from the association. They took the cans for their work on how parasites affect sea mammals in the Pacific Northwest. Parasites like anisakid nematodes live in fish and move up the food chain to seals, sea lions, and whales. Tracking them helps show if the ocean's links from tiny krill to big predators still hold.

The researchers focused on salmon from Alaska's Gulf and Bristol Bay. These areas supply much of the canned salmon sold around the world. The cans held four types: chum, coho, pink, and sockeye. Each type came from commercial catches processed the same way. Heat from canning kills germs but keeps some worm parts intact enough to count.

This setup gave a rare look at the past. Field studies change year to year, but these cans stayed sealed. They offered a steady record of what swam in those waters decades ago. The team published their work in April 2024 in the journal Ecology and Evolution.

Key Details

The study looked at 178 cans in total. They broke down like this:

• Chum salmon: 42 cans
• Coho salmon: 22 cans
• Pink salmon: 62 cans
• Sockeye salmon: 52 cans

Scientists cut open the fish filets and counted worms per gram. The canning process softened the parasites, but they could still measure them. In chum and pink salmon, worm numbers went up over time. From the 1970s to recent years, the count climbed steadily.

Coho and sockeye showed no big change. Their parasite levels stayed about the same across the decades. This pattern suggests some salmon carry more of these worms based on their habits or the prey they eat.

How They Did the Work

The team weighed each filet and picked out the worms by hand. They noted size, shape, and position in the fish. No fancy machines were needed at first—just careful eyes and scales. Later checks confirmed the worms were anisakids, common in wild salmon.

The cans ranged from one year past their date to over 40 years old. All came from the same processing standards. That meant fair comparisons. No one ate the fish; it was all for science.

"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 Natalie Mastick, the lead author.

Mastick pointed out that rising worms mean the food web works. Parasites need krill, then small fish, then salmon, then bigger hunters. If numbers grow, those steps stay connected.

Safety came up too. Canning heat destroys live parasites, so old cans pose no health risk if seals hold. Experts still say check for dents or bulges before opening any can.

What This Means

The findings point to steady or improving ocean health in parts of Alaska. More parasites in chum and pink salmon suggest plenty of hosts at every level. That matches signs of krill booms and whale comebacks in the north Pacific. Stable levels in coho and sockeye might mean different worm types or local factors at work.

For fishing, it shows wild salmon runs hold up. Processors keep pulling healthy fish from these bays. The archive proves commercial canning can double as a science tool. Future studies might check canned tuna or sardines for similar clues.

This could change how scientists watch seas. Instead of boats and nets every season, they might tap food company storerooms. Long-term data helps spot shifts from warming waters or pollution. Parasites act as signals when fish counts alone don't tell the full story.

Fishermen in Alaska say stocks feel solid this year. Warmer seas stress some runs, but chum and pink hold strong. The worm rise backs that up—no big breaks in the chain.

Canners keep the practice going. New quality cans join the old ones. Researchers hope to check them in 20 years. For now, the dusty boxes rewrote what old salmon cans mean: not trash, but time capsules of the sea.