Smith: Invasive bloody red shrimp's impact on ecosystem unknown

Eric Geisthardt reached into a bucket of Lake Michigan fish and picked out a 7-inch-long rainbow smelt.

"This (fish) should've been in 300 feet of water," said Geisthardt, a graduate student at the UW-Milwaukee School of Freshwater Sciences. "Very interesting to consider what it was doing close to shore instead."

The fish was captured on a September 2016 fisheries assessment of the Milwaukee harbor.

It, and dozens of other smelt, yellow perch and alewives, were collected in a net set in about 25 feet of water inside the break wall.

The fish all had one thing in common: Their stomachs were full of bloody red shrimp, a relatively new aquatic invasive species in southern Lake Michigan.

Like the vast majority of invasives, the crustacean (also known by its scientific name, Hemimysis anomala) is likely competing with and compromising native species.

But scientists and anglers over the last several years also have noticed the shrimp is serving as a valuable food source for many fish species.

Could the invasive have a positive effect on the lake's degraded ecosystem? Or will it, like so many other non-native species, be a net negative? And what is the potential impact if the bloody red shrimp were to be transferred to inland waters?

"We are seeing Hemimysis all over the place on Lake Michigan," said UWM professor and researcher John Janssen. "It's clear many fish are eating them. But we don't know enough yet."

Experienced sport anglers in southern Lake Michigan also are noticing bloody red shrimp in the stomachs of fish they catch year-round.

"You wouldn't believe how many shrimp I found in cohos (salmon) caught this spring off Milwaukee," said fishing guide Eric Haataja of West Allis. "The fish seem to utilize them for sure."

The Great Lakes ecosystem has been severely damaged by more than 180 invasive and non-native species, according to the National Oceanic and Atmospheric Administration.

Species such as the zebra mussel, quagga mussel, round goby, sea lamprey and alewife reproduce and spread, ultimately degrading habitat, out-competing native species and short-circuiting food webs.

Scientists point to the filter-feeding quagga mussel as a key reason the lake's plankton have been significantly reduced and forage fish numbers have declined to record lows.

Thirty percent of invasive species in the Great Lakes have been introduced through ship ballast water, according to the U.S. Environmental Protection Agency.

That includes the bloody red shrimp, which is native to freshwater margins of the Black Sea, the Sea of Azov and the eastern Ponto-Caspian Sea, and likely arrived in North America courtesy of transoceanic shipping.

Hemimysis anomala was first documented in the Great Lakes in 2006 and has been found in each Great Lake except Lake Superior. The first local record was in September 2007 at Bradford Beach in Milwaukee.

The animal gets its name from its color, which is often ivory-yellow or translucent but peppered with red pigments.

The small shrimp - about a quarter- to half-inch long as adults - form swarms in excess of 1,500 individuals per square yard.

The masses can appear like "blood-stained water."

The bloody red shrimp prefers rocky substrate and generally is found in nearshore waters of about 15 to 35 feet in depth. It often spends daylight hours under and among rocks, then emerges and rises in the water column through the night.

Janssen and his colleagues and students at the UWM School of Freshwater Sciences have been studying the species in waters near Milwaukee for nearly a decade.

The work has been funded in part by the U.S. Army Corps of Engineers to evaluate rocks added to improve fish habitat and shore up the harbor wall. The project is titled "Physical and Biological Evaluation of Milwaukee Harbor Green Breakwall."

The UWM research has included scuba dives.

"Every dive we've done within a couple miles of a harbor mouth, we flip over cobble, and we find Hemimysis," Geisthardt said. “Night dives we tend to see the swarms of them. In fall, it’s just insane how many (Hemimysis) there are."

Lake Michigan has a native mysid, called the opossum shrimp, which is typically found in deep water. It's not clear to researchers what impact the bloody red shrimp has had on numbers of the native crustacean. 

Geisthardt has evaluated the stomach contents of bloody red shrimp and found diatoms, midge larvae, phytoplankton and zooplankton.

"They are omnivores and can take advantage of any benthic production," Geisthardt said. "They are really plastic as what they can eat. In a river mouth scenario, they have quite a bit to choose from."

In a plankton-deprived ecosystem such as modern Lake Michigan, adding another invasive species that consumes phytoplankton and zooplankton is obviously a concern.

UWM senior scientist Russell Cuhel is leading a study on the diet of Hemimysis in Lake Michigan. 

But researchers also are attempting to understand the invasive species role as a prey item in the lake's food web.

The list of species documented to feed on bloody red shrimp spans forage fish to top predators and juveniles to adults.

"Everything eats them from alewife, yellow perch, largemouth bass, rock bass and brown trout, "Geisthardt said. "If there is a swarm, all the fish have to do is open their mouth."

Geisthardt said Hemimysis hatch out at about 1 millimeter in length, small enough for even a young yellow perch to eat.

Researchers generally have documented greater concentrations of bloody red shrimp on the Wisconsin side of Lake Michigan, which has more rocky substrate than the Michigan shore.

As such, the invasive shrimp could have been responsible for increased numbers of alewives observed in 2016 in Wisconsin waters and the generally better trout and salmon fishing on the western side of the lake last year.

Among the concerns for the future: What damage could bloody red shrimp cause to inland waters? A study in a lake in the Netherlands documented a decline in native zooplankton after Hemimysis was introduced.

The jury is still out about the overall impact of the bloody red shrimp in Lake Michigan.

But the crustacean could well be changing behaviors of some fish, such as the smelt caught in the Milwaukee harbor in September 2016 by UWM researchers.

"When we started this research, we knew you could find them in September along the break wall, but we didn’t know anything about their role in the food web," Janssen said. "Now we know they are an important component of the food web in the harbor. Beyond that, it's too early to say."