Alaska’s waters are sustained by a complex marine food web and support the world’s most important salmon fishery. Scientists are keen to understand how these systems are changing as the climate changes. The challenge is that reliable samples from decades ago are hard to find.
“We really need to open our minds and get creative about what can serve as an ecological data source,” said Natalie Mastic, now a postdoctoral fellow at Yale University’s Peabody Museum of Natural History.
Scientists focus on unusual resource: decades-old salmon
While working on his PhD at the University of Washington in Seattle, Mastic took an unconventional approach. Rather than relying on modern samples, she examined old salmon cans.
These cans contained fillets from four species collected over 42 years in the Gulf of Alaska and Bristol Bay. Mastic and her team opened 178 cans and carefully dissected the preserved fish, counting tiny parasites known as Anisakis embedded in the flesh.
These parasites are killed during the canning process and pose no harm to the consumer, but they contain valuable scientific information.
Why “sushi worms” indicate the health of the ecosystem
At first glance, finding worms in your fish may seem alarming. But scientists say the opposite can also happen.
“Everyone assumes that having bugs on salmon is a sign that things are going wrong,” says Chelsea Wood, a Wisconsin associate professor of fisheries and fisheries science. “But the life cycle of anisakid incorporates many elements of the food web. I think the presence of anisakid indicates that the fish on your table come from a healthy ecosystem.”
Because these parasites rely on multiple species for survival, their presence may reflect the overall strength of the marine ecosystem.
Rising parasite levels reveal long-term ocean trends
The team’s findings, published in the journal Ecology & Evolution, showed that levels of anisakid increased in chum salmon and pink salmon from 1979 to 2021. Parasite levels remained stable in coho and sockeye salmon.
“Anisakid has a complex life cycle that requires many types of hosts,” said Mastic, lead author of the paper. “As with pink salmon and chum salmon, seeing their numbers increase over time indicates that these parasites were able to find all suitable hosts and reproduce. This may indicate that there are enough suitable hosts for anisakid and that the ecosystem is stable or recovering.”
How parasites track the entire marine food web
Anisakid begins its life as a floating creature in the ocean. When small creatures like krill eat them, they enter the food chain. When a predator eats an infected animal, the parasite moves up the food chain.
For example, krill is eaten by small fish, which in turn is eaten by larger fish such as salmon. Eventually, the parasites reach marine mammals, where they reproduce. Their eggs are released into the ocean and the cycle begins again.
“In the absence of a host, for example a marine mammal, the anisakid will not be able to complete its life cycle and its numbers will decline,” said Wood, senior author of the paper.
Are these parasites dangerous to humans?
Humans are not part of the Anisakid life cycle. Since the bugs are already dead, there is little danger from eating properly cooked fish.
However, Anisakis, also known as the “sushi bug” or “sushi parasite,” can cause illness if eaten raw or undercooked seafood live. Symptoms may resemble food poisoning or, in rare cases, a condition known as anisakiasis.
Where did the salmon sample come from?
The canned salmon used in the study was provided by the Seafood Products Association, a Seattle-based industry group. The organization kept these cans for many years for quality control purposes, but they were eventually no longer needed.
Mastic and co-author Rachel Wericki, an assistant professor at Newman College in Pennsylvania, tested different techniques to analyze the samples. The worms are about 1 centimeter (0.4 inch) long and are often curled up inside the fish’s muscles. By gently pulling the fillets apart with forceps and using a dissecting microscope, the researchers were able to accurately count the fillets.
Why are parasite levels increasing?
There are several possible explanations for elevated levels of anisakid in some salmon species.
One important factor may be the Marine Mammal Protection Act of 1972. The law helped populations of seals, sea lions, killer whales, and other marine mammals recover after years of decline.
“Because anisakid can only reproduce in the intestines of marine mammals, this could indicate that levels of anisakid increased during our study period, from 1979 to 2021, due to increased breeding opportunities,” Mustik said.
Other factors include rising ocean temperatures and environmental improvements related to the Clean Water Act.
Why some salmon species don’t show change
The stable parasite levels observed in coho and sockeye salmon are more difficult to interpret.
There are many different species of Anisakid, each relying on a unique combination of hosts. The canning process preserved the external structure of the worms, but destroyed the internal features that scientists needed to identify specific species.
A new way to study the ocean’s past
The researchers believe their method could be applied to other preserved seafood, such as canned sardines, providing a new way to study historic ecosystems.
They also hope their research will encourage scientists to think creatively about untapped data sources.
“This study happened because people heard about our research by word of mouth,” Wood said. “To gain these insights into past ecosystems, we need to build networks and establish connections to discover untapped sources of historical data.”
Funding and contributors
The study included California State University undergraduate student Aspen Catra, as well as Bruce Odegaard and Virginia Ng of the Seafood Products Association. Funding was provided by the National Science Foundation, the Alfred P. Sloan Foundation, the Washington Research Foundation, and the University of Washington.
