Forty years ago, the Twin Cities stretch of the Mississippi River was nearly devoid of aquatic life. Today, the river from St. Anthony Falls in Minneapolis to Lock and Dam 2 in Hastings is a haven for native mussels and the site of an incredible conservation success: In August 2012 DNR researchers found evidence that a reintroduced population of federally endangered Higgins eye mussel (Lampsilis higginsii) is naturally reproducing.
Meet DNR malacologist Bernard Sietman and watch the mantle display of a Higgins eye mussel.
"It's a great story, a recovery like this in a big metropolitan river," says DNR malacologist Bernard Sietman. "Reproduction from a reintroduced population is, as far as I know, unprecedented for an endangered mussel."
The Higgins eye reintroduction began in 2000 when the DNR and the U.S. Army Corps of Engineers relocated 100 adult mussels from the Mississippi River in Iowa to Pool 2, which flows from St. Paul to Hastings. Another 270 adult Higgins eye were relocated to Pool 2 in 2001. In subsequent years more than 9,000 cultured juvenile mussels were also placed in Pool 2 near Hidden Falls in St. Paul.
Like most freshwater mussels, the Higgins eye has a parasitic stage of life. Female Higgins eye lure predatory fish with fleshy flaps that look like a small fish. When a fish attacks a mussel's lure, larval mussels, glochidia, are released and attach to the gills of a host fish. In time, these larvae develop into juvenile mussels and fall to the river or lake bottom. Juvenile Higgins eye mussels relocated to Pool 2 were propagated using largemouth bass held in underwater cages.
Adult Higgins eye mussels introduced into Pool 2 were engraved with a number, and juveniles were marked with a dot of black superglue. With the discovery of two unmarked juvenile Higgins eye mussels last year near Hidden Falls, researchers concluded that natural reproduction is occurring.
Considering the relative scarcity of reintroduced Higgins eye, the odds of finding a single naturally produced offspring are pretty long, according to Sietman. "It's not like trying to find a needle in the haystack; it's more like trying to find a needle in the hayfield," says Sietman. "Finding two suggests there are many more in the river."
Historically, more than 40 native mussel species lived in the Mississippi River between St. Anthony Falls and Lake Pepin. However, water quality in this stretch began to decline in the late 1800s due to the discharge of raw sewage into the river. Fish kills became common in the 1930s, and by the 1970s, only a few hardy species of mussels remained.
By the 1990s a decade-long effort to upgrade wastewater treatment and separate sanitary and storm sewers had dramatically improved water quality. Fish returned and native mussels began recolonizing as mussel larvae hitchhiked upstream on the gills of host fish. A 1999 DNR survey turned up 27 native mussel species between the Twin Cities and Lake Pepin. Higgins eye shells, but no live specimens, were found.
The U.S. Fish and Wildlife Service listed the Higgins eye as endangered in 1976. Conservation efforts became more urgent in the 1990s with the spread of invasive zebra mussels via barge traffic. These exotic mussels attach to and suffocate native mussels. Restoration is funded by the U.S. Army Corps of Engineers and is part of a larger effort to restore the Higgins eye and the federally endangered winged mapleleaf mussel (Quadrula fragosa) to the Mississippi River and the upper portions of its tributaries where they were historically common in Wisconsin, Iowa, Illinois, and Missouri.
Ironically, Pool 2, once a dead zone, now offers one of the best refuges for native mussels in the upper Mississippi River. Invasive zebra mussels are abundant downstream of Hastings and are well established farther south in Lake Pepin. The source of zebra mussel larvae at Lake Pepin is so large that it has devastated native mussel colonies for hundreds of miles downstream. Despite a source near Brainerd and repeated introductions from barges, zebra mussels have not become established upstream of the St. Croix River's confluence with the Mississippi in Pool 3. The lack of a large source of larval zebra mussels upstream could prevent these invasives from gaining a foothold in this stretch, according to Sietman.
Sietman says the DNR and its partners, including the U.S. Army Corps of Engineers, the U.S. Fish and Wildlife Service, and the National Park Service, hope that a naturally reproducing population of winged mapleleaf mussels will someday return to Pool 2. Researchers are developing methods for propagating winged mapleleaf mussels.
Michael A. Kallok, online editor