Blue Suckers may be Rebounding in Minnesota

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Fish and Wildlife Today - Fall 1999

A comeback seems possible for one of Minnesota's rarest fish species.

Are blue suckers repopulating their historic range in Minnesota? New evidence indicates that the elegantly proportioned, blue-toned fish is showing up more frequently in the Mississippi and Minnesota rivers, giving biologists hope that at least a limited natural restoration is underway for one of the state's rarest fish species.

Some of the most encouraging news is coming from the Mississippi River near Red Wing. There, biologists for Northern States Power Company (NSP) have been conducting yearly fish population surveys since 1973. The surveyors use trap nets and electrofishing equipment, which temporarily stuns fish for capture. According to Ken Mueller, NSP environmental biologist at the Prairie Island Nuclear Power Plant, surveyors first saw a blue sucker, just a single fish, in 1982. The species showed up intermittently for the next decade, but since 1991 biologists have found blue suckers every year. Mueller says that the number collected has risen from just a few in the early 1990s to 15 in 1996, and 37 in 1998.

"We're definitely seeing a gradual but steady increase," he says.

DNR biologists have also sampled blue suckers at many spots in recent years. Mark Stopyro, a biologist with the Section of Ecological Services at Lake City, conducted a nongame fish survey of the Mississippi River from the Twin Cities to Iowa in 1995. Using electrofishing gear mounted on a boat, he collected blues with regularity off small underwater wing dams and closing dams, where currents temporarily quicken and scour rocky substrates downstream of the structures. Stopyro reported finding blue suckers upstream as far as Minneapolis, where they had not been reported since 1880.

Jack Enblom, DNR River Surveys Project leader, has surveyed the Mississippi River in the metro area and the Minnesota River since the 1970s and never collected a blue until the mid-1990s. Since then, he has seen increasing numbers of blues in Pool 2 of the Mississippi (Hastings to St. Paul) and in the Minnesota River upstream to Henderson.

The blue sucker is found primarily in southern rivers from Mexico east to Georgia. But its historic range went as far north as western Montana in the Missouri River and up to St. Anthony Falls on the Mississippi River. Early reports from anglers indicated that until the 1920s, the Mississippi River had heavy runs of blue suckers that made a large part of commercial catches. Known as the "sweet sucker," blues are considered one of the tastiest of the sucker species.

Commercial overfishing and pollution are blamed for the species's decline over the past 75 years, but even more harmful were large locks and dams built on the Mississippi River and its tributaries during the early part of the 20th century. Blue suckers spawn over gravel that must be free of silt for the eggs to survive. On the Mississippi, the large dams flooded gravel bars and slowed currents that once flushed silt from spawning areas. They also blocked suckers from reaching spawning habitat on tributaries.

What accounts for the apparent resurgence of blue suckers in southern Minnesota? Dr. James Underhill, curator emeritus of the James Ford Bell Museum fish collection, suspects it could be due in part to cleaner water in the Mississippi and Minnesota rivers as a result of the federal Clean Water Act of 1972. Levels of dissolved oxygen, an indicator of water quality, have increased over the past 20 years. So has the diversity of fish and aquatic insect species, Underhill says.

Another factor, says Stopyro, could be the species's ability to adapt to the Mississippi's changing environment. Beginning in the late 1800s, thousands of underwater wing dams and closing dams were built in the upper Mississippi. Their purpose was to divert water flow to scour the central channel deep enough for barge transport. After locks and dams transformed much of the Mississippi into a series of lakes, the smaller structures may have created enough quick current to scour some gravel spots. There blue suckers can spawn successfully. Says Stopyro, "The Mississippi River's fish population would have been very different and much less diverse today without these structures."

Enblom says that despite cleaner water in the Mississippi, uncommon species such as the paddlefish, lake sturgeon, and skipjack herring still suffer from habitat loss due to locks and dams. But perhaps, he speculates, the combination of less pollution and the micro-habitats created by wing dams has been enough to allow the blue sucker to gain a finhold and begin repopulating the rivers.

"As long as the lock and dams remain, I can't ever see us having the numbers of blue suckers that were reportedly here before European settlement," Enblom says. "But with the cleaner water, the blues might be making a partial recovery."

--Tom Dickson, DNR Division of Fish and Wildlife staff writer