The Counted Parasite

Researchers opened 178 cans of salmon collected over forty-two years — from 1979 to 2021 — and counted the parasitic worms inside.

Anisakid nematodes embed themselves in salmon flesh during the fish's life. They are killed during the canning process and pose no danger to consumers. But they are preserved along with the fish, frozen in time at the moment the can was sealed. Each can is a snapshot of the parasite load the fish carried when it was caught, and that load carries ecological information.

Anisakid worms have a complex life cycle that passes through multiple hosts. They reproduce only in the intestines of marine mammals — seals, sea lions, orcas. Without marine mammals to complete the cycle, anisakid populations decline. With more marine mammals, anisakid populations grow. The worm count in a salmon is an indirect measurement of the marine mammal population that the salmon's ecosystem supports.

Over the forty-two-year span, anisakid counts in chum and pink salmon rose steadily. Coho and sockeye remained stable. The increase in chum and pink salmon parasites corresponds to the recovery of marine mammal populations following the Marine Mammal Protection Act of 1972 — a law that took decades to produce measurable ecosystem effects, which were then preserved, accidentally, in canned fish on grocery shelves.

The structural reversal is total. Parasites are normally indicators of contamination, disease, system failure. In this case, more parasites mean more marine mammals, which mean a more complete food web. The thing you'd want less of in your food is evidence of health in the ocean that produced it. The measurement inverts the value judgment. The parasite count doesn't measure contamination. It measures recovery.