In 1979, old salmon cans were stored in an Alaskan cannery, and decades later, when scientists cracked them open, a rising worm count revealed something unexpected about changing ocean food webs

No one likes finding worms in their food. But when scientists at the University of Washington began to dissect 40-year-old cans of Alaskan salmon, the parasitic worms they found told a truly remarkable story about the state of our oceans.

A new study published in Ecology and Evolution looked at 178 cans of salmon, some of which had been stored since 1979, donated by the Seafood Products Association, a Seattle-based industry group that had kept them to study how cans break down over time. The cans contained four species of salmon caught in the Gulf of Alaska and Bristol Bay: chum, pink, coho, and sockeye. Scientists pried open each tin carefully, used forceps to separate the preserved fillets, and counted tiny parasitic roundworms, known as anisakids, embedded in the flesh. Mastick et al. (2024) found that worm counts in chum and pink salmon increased significantly over the 42-year study period, whereas coho and sockeye did not change meaningfully. This matters beyond the lab. Alaskan salmon had an ex-vessel commercial value of $786 million in 2021, so if there’s a shift in their health, it's a question with real economic stakes.

What even are anisakids?
Anisakids (“sushi worms”) are parasitic roundworms that sometimes live in the fish flesh. They cannot complete their life cycle in humans, but if someone eats raw or undercooked infected fish, the larvae can burrow into the stomach lining and cause a condition called anisakidosis. Symptoms usually include sharp abdominal pain, nausea and vomiting, often within hours of eating. Anisakidosis symptoms can resemble several serious conditions and have been mistaken for appendicitis, Crohn’s disease, and even cancer, leading to frequent misdiagnosis, according to a 2023 review in Parasitology Research.


There is also a more subtle health issue beyond live infections. Even when fish is properly cooked or canned (which kills any live larvae), people who have been previously exposed to anisakids can develop sensitivities to their proteins. Anisakid proteins may trigger allergic reactions in sensitized individuals, ranging from hives and skin rashes to anaphylaxis in severe cases. This means the public health conversation about these parasites doesn’t end with “just cook your fish thoroughly.”

Why are the worms increasing?
The main explanation is the recovery of populations of marine mammals. Marine mammals, such as seals, sea lions, and whales, host anisakids in their intestines, where the worms reproduce and release eggs back into the ocean. Then krill eat the eggs, fish eat the krill and that’s pretty much the life cycle. More marine mammals means more parasites in the food web.

Since the passage of the Marine Mammal Protection Act in 1972, marine mammal populations in the United States have been steadily recovering. The number of humpback whales, harbor seals and killer whales in Alaskan waters is increasing, according to NOAA Fisheries. As these animals recovered from decades of commercial whaling and hunting, anisakids had more reproductive opportunities and their numbers grew.

Climate change could be to blame, too. Increased ocean temperatures have been shown to accelerate the development of anisakid eggs and with rising sea surface temperatures in the northern latitudes, the range of anisakid hosts is predicted to shift northward, potentially bringing more parasites into Alaskan waters over time.

This result fits into a larger global picture. A landmark meta-analysis by Fiorenza et al. in Global Change Biology examined data across multiple fish species and found a 283-fold increase in Anisakis spp. abundance during 1978-2015. Alaska is not an aberration, but part of a pattern being seen throughout the world’s oceans.

Why only some salmon species?
The differences in increases between chum and pink salmon, and coho and sockeye, likely boil down to two things: where each species lives as juveniles and what they eat. Chum and pink spend months in nearshore coastal and estuarine areas before moving out to sea, ; the same areas that can overlap with marine mammal activity. This brings them closer to areas infested with parasites during a crucial phase of their development.

Diet counts too. Coho salmon are piscivorous, meaning that they eat other fish. Unlike chum, pink and sockeye mostly eat zooplankton, such as krill. Anisakids are passed up the food chain by infected prey, so the specific prey species and their habitat can have a big effect on the number of parasites that a salmon accumulates during its lifetime.

Worms: a health check for the ocean
Here’s the part that turns the intuitive response on its head. More worms in salmon isn’t all bad news. Anisakids need a functioning, intact food web to live, with healthy populations of krill, fish and marine mammals all working in order. If any link in the cycle is broken, the parasite cannot complete its cycle.

The presence and increase of these parasites may be a potential signal of ecosystem recovery, said lead researcher Natalie Mastick, a postdoctoral researcher at Yale University’s Peabody Museum. The earlier collapse of marine mammal populations from commercial whaling and the fur trade likely bottlenecked anisakid populations for decades. Their appearance now might just be a return to the situation before Alaska’s waters were industrially exploited.

But the researchers are cautious about describing this as a purely positive outcome. Salmon are already stressed by warming streams, vanishing habitat and invasive species. On top of those stressors, an increase in parasite burden could compound the challenges facing already vulnerable populations.

What this means for seafood lovers
Americans eating canned salmon have nothing to fear; the canning process kills any parasites that may be present. US FDA recommendations for cooking or freezing fish also controls the risk of live parasite infection. Raw or lightly cured fish, such as sushi, ceviche, gravlax, etc., require some consumer thoughtfulness. And for anyone who has had a reaction in the past after eating fish, it is worth knowing that anisakid allergens can survive even in cooked or canned products.

But the bigger takeaway from this study is a creative one. Researchers transformed dusty old cans of salmon, much like the ones you may have buried in the back of your pantry, into an ecological record spanning 42 years. Such scientific resourcefulness may be just what it takes to understand how our oceans are changing, one tin at a time.