Why do some trout that dine on small invasive fish die? Researchers gaining clues

Apr 24, 2014

NOAA's Great Lakes Environmental Research Labratory scientists examining a catch of alewives in Lake Michigan near Muskegon.
NOAA's Great Lakes Environmental Research Labratory scientists examining a catch of alewives in Lake Michigan near Muskegon.
Credit R/V Laurentian NOAA / Creative Commons

You’ve probably heard about the big bad invasive silver or bighead carp, also known as Asian carp.

But there’s another invasive fish that’s roughly a third the size of the carp that’s already done a lot of damage to Great Lakes fisheries. Alewives have been a particular menace in Lakes Michigan and Huron. The invasive fish cause all kinds of problems for native lake trout.

Alewives scarf down lake trout eggs and very young fish. But even once lake trout grow big enough to turn the tables and eat the alewives, the invasive fish still cause problems.

Researchers are making progress toward understanding why some Great Lakes trout die after eating alewives.

A study released this month by the U.S. Geological Survey says lake trout appear to be experiencing changes in their immune systems, kind of like humans with inflammatory diseases. Scientists think alewives could be to blame.

Alewives contain an enzyme that destroys thiamine. It’s an essential vitamin for the trout.

Lead researcher Chris Ottinger says the immune system changes could make the trout more susceptible to other diseases.

“Fish that are exhibiting this kind of shift in inflammatory response can end up either being more susceptible to certain diseases or they can be more likely to carry the infection. In other words, that infection doesn’t really cause severe disease in that individual, but that individual doesn’t really get rid of the disease,” Ottinger said. “In that case they’re really sustaining that particular disease organism within the population.”

“One question we might ask is – alright let’s see specifically how the thiamine deficiency is impacting lake trout susceptibility and disease development to bacterial kidney disease, which is a major disease in the Great Lakes,” Ottinger said.

Hatchery operators in Michigan trying to restore lake trout populations bathe young trout in thiamine solutions before releasing them into the wild. But many still don't survive after they begin eating alewives.

Ottinger says it’s not clear if those young fish bathed in thiamine have the same immune systems as healthy lake trout that developed without a thiamine deficiency.

The good news is the alewives seem to be dying off, especially in Lake Huron. And that’s allowing lake trout there to make a pretty impressive comeback.