April 12, 2024

Decades-old cans of salmon reveal changes in ocean health

Decades-old cans of salmon reveal changes in ocean health

Researchers used canned fish to reconstruct parasite population changes, giving new meaning to the phrase ‘opening a can of worms’

Reinis Bigacs/Alamy Stock Photo

Chelsea Wood, a parasite ecologist at the University of Washington, and her then-student Natalie Mastick had spent months thinking about how to reconstruct fluctuations over time in the risk that parasites pose to marine mammals in the Pacific Northwest. Long-term data sets don’t exist for the vast majority of parasitic species, so Wood and Mastick knew they had to get creative. One idea they had was to use salmon’s parasitic loads as a proxy for marine mammal infections.

Orcas, seals and belugas prey on salmon, which is an intermediate host for various nematodes that complete their life cycle in several steps in these predators. The parasites cannot reproduce and enter the environment without marine mammals, so the level of infection in salmon is closely linked to that of their predators.

Although the logic behind the researchers’ idea to use salmon parasites to understand marine mammal parasites made sense, there was still one problem: Where were they going to get a bunch of old salmon that they could dissect to look for parasites?

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They seemed to be out of options when “out of the blue,” Wood says, they got a call from the Seafood Products Association, a trade group in Seattle. The group told her they were cleaning out the basement and throwing away piles of dusty boxes of long-life canned salmon. Before the cans were thrown away, the association wondered if Wood might actually want them.

“We were like, ‘There could be worms in there!’” Wood recalls. “This is how we started this research: [the Seafood Products Association] asking if we wanted this waste out of their basement and we said, ‘Absolutely, yes.’

Wood and Mastick’s suspicion that canned salmon could serve as a time capsule for worms turned out to be correct: the canned fish contained a lot of of parasites. As the researchers reported this week Ecology and evolution, they were able to use the recovered parasites to reconstruct how the burden of infection has changed over 42 years in four salmon species and found that it increased in two salmon species. While this may sound like a bad thing, the researchers suspect the increase in worm numbers is a sign of “a conservation success story” for marine mammals, Wood says. “It’s possible that now that the marine mammals have come back, their parasites have come back too.”

Canned salmon is cooked while it is sealed, so when they began the study, the researchers weren’t sure whether they would be able to detect anisakids, the type of parasitic nematodes that infect the muscles of Alaskan salmon. The worms, however, were “very striking, at least to us,” Wood says. “We were really surprised.”

Anisakids find their way to salmon via smaller intermediate hosts such as krill or fish. After the salmon eat these smaller hosts, the parasites burrow into the salmon’s muscles, creating “a little pocket,” Wood says. She and her colleagues carefully pierced the salmon muscle tissue with tweezers, and when they opened one of the bags, the worms tended to “pop out,” Wood says. “They were very visible.” (These worms are found in many types of seafood, she adds, and because they are killed during the cooking process, they pose no danger to humans unless the food is undercooked or someone has a particular allergy to them.)

The cellar stock that made the findings possible consisted of 502 cans of chum, coho, pink and sockeye salmon, mostly from Alaska. In total, the researchers analyzed 178 cans, processed between 1979 and 2019 and ranging from 22 to 62 cans per type. Half of the cans contained nematodes and a total of 372 worms were collected. They found that the number of worms per gram of tissue increased significantly over time in chum and pink salmon, but did not change in sockeye or coho salmon.

“It’s important that we understand how parasites and diseases in general affect ecosystems, because the effects can be profound,” said Ryan Carnegie, a biologist at the Virginia Institute of Marine Science, who was not involved in the study. “The idea to use this archive of canned samples to address the issue of long-term nematode infection in salmon was simply inspired, and importantly, it was followed by robust and very careful approaches to generate solid quantitative data to support the hypothesis of test change. ”

The authors of the new study cannot say with certainty which factors are causing the increase in the number of parasites in chum and pink salmon. But the increase does coincide with the introduction of the Marine Mammal Protection Act, which came into effect in 1972. The populations of several marine mammals have increased since then, so Wood says the number of parasites has also increased. Other studies have revealed similar correlations linked to conservation actions, including an increase in parasites in Baltic cod as gray seal populations recovered. As Wood notes, “We know this because fishermen were angry because they kept landing all those cod full of worms.”

Wood and her colleagues also don’t know why sockeye salmon or coho salmon didn’t experience an increase in parasite burden. They hope to find some ecological clues to this answer through a genetic analysis they are currently working on to try to identify the specific parasitic species recovered from the canned salmon.

The new research is already “providing a look at some of the most difficult species to study and providing insight into the nuanced way ecosystems recover,” says Loren McClenachan, an ocean historian at the University of Victoria in British Columbia, who was not involved at the time. involved in the research.

“Using canned salmon as a window into past ecosystems is a remarkably creative approach to uncovering otherwise invisible changes,” she says. “The finding that in some salmon species the number of parasites has increased as the number of marine mammals has increased is remarkable and shows that the basic conditions for healthy ocean ecosystems may be more wormy than people might think.”

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