Researchers hypothesize that the salmon are feasting too heavily on anchovies, a fish that is now swarming the California coast in record numbers. Unfortunately for the salmon, anchovies carry an enzyme called thiaminase, which breaks down thiamine—a vitamin that is essential to cell function in all living things.
“Vitamin B1, or Thiamine, is an essential nutrient that animals must obtain through their diet,” said Chris Ottinger, a USGS immunologist and lead author of the study. “We found that alewives, one of the main diets of many Great Lakes fish, contains an enzyme called “thiaminase” that destroys the thiamine in fish that consume them.
A similar scenario hit salmon in the Great Lakes.
I can’t remember if this is an anti predator adaptation.
*I know a misdirection for dams as the likely subject.