Opening 40-year-old canned salmon revealed ocean recovery — what parasites tell us about ecosystem health

A tiny clue inside a salmon can

What would you think if someone told you there are parasitic worms in canned salmon? Most people would cringe. But a research team at the University of Washington saw something entirely different in these worms. A reversal of intuition. They say it is a sign that ocean ecosystems are recovering.

Opening 42 years of canned salmon

The team examined 178 cans of Alaskan salmon produced between 1979 and 2021. They opened each can and dissected the fillets with forceps and a dissecting microscope. What they counted were parasitic worms called anisakids, each roughly one centimetre long.

The results were clear. In chum and pink salmon, anisakid levels had increased over the 42-year period. In coho and sockeye, no change.

Why more parasites can be 'good news'

The anisakid life cycle is complex. The worms first infect krill. From there they move into fish, and ultimately reproduce inside marine mammals such as seals, sea lions, and orcas.

In other words, more parasites means more marine mammals. The effects of the Marine Mammal Protection Act of 1972 may be showing up in the parasite counts.

What the researchers say

Everyone assumes that worms in your salmon is a sign that things have gone awry. I see their presence as a signal that the fish on your plate came from a healthy ecosystem.

— Chelsea Wood (University of Washington, Associate Professor)

Seeing their numbers rise over time indicates that these parasites were able to find all the right hosts and reproduce. That could indicate a stable or recovering ecosystem.

— Natalie Mastick (Yale Peabody Museum, Postdoctoral Researcher)

We have to really open our minds and get creative about what can act as an ecological data source.

— Natalie Mastick

Canned food as a window to the past

Another key contribution of this study lies in its methodology. Archived canned goods can serve as a tool for reading the history of marine ecosystems. Data tracking 42 years of change is not easy to come by. An unlikely source — canned salmon — opened a window to the past.

For more on ocean environmental changes, see 'Ocean warming alters archaeal nitrogen cycling.'

A note from the author: The idea of measuring ecosystem health through canned salmon surprised me. More parasites means a healthier ocean — the opposite of what you would expect.

Opening 42 years' worth of cans one by one and counting worms with forceps: unglamorous work, but without that persistence the data would not exist.

The notion that a tin of salmon can serve as a time capsule was refreshing.