Invasive bloody red shrimp now established in Lake Superior off Duluth

A tiny invasive shrimp once seen as an occasional stray in Duluth has now taken hold in Lake Superior. Scientists have confirmed a permanent, self-sustaining population of bloody red shrimp in the Duluth-Superior Harbor, with 81 specimens collected last summer and more found again after ice-out this year.

The finding matters far beyond an identification list. Lake Superior is the final Great Lake where researchers have now confirmed an established population, ending a two-decade march that began in 2006 in waters connected to Lake Michigan and Lake Ontario. For Duluth, Superior and the North Shore, the harbor is not just a shipping corridor but a gateway to the lake economy, and the presence of a reproducing invader raises fresh concern for the food web that supports fish, tourism and the working waterfront.

The new study in the Journal of Great Lakes Research found juveniles, adult males and gravid females at Wisconsin Point and the Montreal Pier, clear evidence that the species is breeding locally rather than simply washing in from elsewhere. Researchers did not find the shrimp at Barkers Island, suggesting the population may still be concentrated in parts of the harbor. A single specimen turned up in Allouez Bay in 2017, a few more followed in 2018, and this latest sampling showed the species had moved from rare sighting to established resident.

What scientists know now is that the shrimp, a 6 to 15 millimeter mysid crustacean native to the Caspian Sea and Black Sea region, can survive year-round in the harbor and overwinter there. What remains less certain is how much damage it will do in Lake Superior. Great Lakes researchers and NOAA materials say the shrimp prefers hard structures and rocky bottoms, avoids direct sunlight, and can gather in reddish swarms near piers and breakwalls at night, which helps explain why Duluth-Superior is such a plausible foothold.

The broader worry is ecological. European studies cited in the research have linked bloody red shrimp invasions to major drops in zooplankton biomass and diversity, and scientists here say the species could change how energy, nutrients and contaminants move through nearshore food webs. Donn Branstrator and other researchers suspect ballast water from cargo ships, possibly including domestic laker traffic, brought the shrimp into the system, and Branstrator said the team is continuing biweekly monitoring at Montreal Pier and other harbor sites to see how far the population reaches.

Lake Superior’s delayed establishment may reflect colder temperatures and geographic isolation, but the harbor now shows that even the coldest Great Lake is not immune. For Duluth and St. Louis County, the lesson is plain: the harbor remains an entry point, and the cost of inaction is measured not only in biology but in the long-term health of the lake economy.