20 years after reaching the Great Lakes, the invasive bloody red shrimp colonizes Lake Superior
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Bloody red shrimp, a tiny invasive crustacean native to Europe and Asia, have established a population in Lake Superior.
Researchers found consistent evidence of the critters that first hitched a ride to the lower Great Lakes 20 years ago at two sites in the Duluth-Superior Harbor in 2025. They found them again shortly after ice-out this year.
“Those data really, I would say, absolutely confirm that they've overwintered here in the harbor,” said University of Minnesota Duluth biology professor Donn Branstrator, lead author of a recently published study in the Journal of Great Lakes Research documenting the findings.
“We found enough individuals at these two sites in all the life stages, juveniles as well as adult males and females, to make the conclusion that there's a self-sustaining population now in the Duluth-Superior Harbor,” Branstrator added, marking the first location in all of Lake Superior where there's evidence of the shrimp reproducing.
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Bloody red shrimp look like miniature shrimps, ranging in size from a quarter- to a half-inch. Their name comes from the red pigment spots on their translucent yellow-white bodies.
They can sometimes be seen when they come out to feed at night in large, reddish swarms along shore near breakwaters and piers.
They’re native to the Caspian and Black seas. They hitched a ride to the Great Lakes in 2006 in the ballast water of cargo ships.
An individual shrimp was found in the Duluth-Superior harbor in 2017. Researchers located a couple more the following year, but didn’t see them again until last year.
Branstator said the shrimp likely traveled to Lake Superior in the ballast holds of domestic ships known as “lakers,” up to 1,000 feet long, that haul iron ore pellets from around Lake Superior to steel plants in the lower lakes. Ships fill those tanks with water to maintain their balance when they’re unloaded, then release the water as they take on heavy cargo.
The shrimp live in the nooks and crevices of dock pilings, breakwalls and rocks along the lakeshore. They hide during daylight hours, so researchers sample for the shrimp after dark, using illuminated traps and specially-designed nets.
Even though the shrimp have established a foothold in Lake Superior, it’s still too early to know what kind of ecological impact they will have.
Bloody red shrimp feed on plankton and algae, the tiny critters at the base of the aquatic food chain. They have the potential to compete with small fish and other native organisms important to Lake Superior’s food web near the shore.
But because the shrimp are in the upper-size range of zooplankton, they represent a potential new target and food nutritional resource for fish.
“That could potentially end up being a positive outcome of their new presence in the food web,” Branstrator said. “So, in the balance, I don't think we really know yet.”
He said it’s also still unclear how well bloody red shrimp will grow and reproduce in the frigid Lake Superior waters.
Researchers are now expanding the area where they’re searching for the invasive species. They’ve added sampling sites around the Duluth-Superior Harbor, as well as along the North Shore of Lake Superior, and one location on Wisconsin’s South Shore.
Branstrator said their discovery demonstrates that despite a concerted emphasis on stopping the spread of aquatic invasive species, they continue to expand.
“Don't stop what we're doing,” Branstrator said. “But realize that this is a marker of the fact that, despite our best efforts, organisms are still on the move.”
