September 8, 2023 at 5:45 a.m.

The Lake Where You Live

Zebras and fishing

By Ted Rulseh, Columnist

Zebra mussels are an invasive nuisance, and in no circumstance would you want them in your lake. They can form a crust on any hard surface. They can encrust pier posts, buoys and any other solid object. The can invade outboard motors, disrupting the cooling systems.

They filter nutrients out of the water, making it appealingly clear but also lower in nutrients, which the mussels hog for themselves. The shells of dead mussels can wash up and collect on beaches and are sharp enough to cut swimmers’ feet. The actual effects on fisheries, though, are not entirely clear.

These small mollusks with striped shells are easy to detect if attached to a boat, an anchor, or other piece of equipment. Much harder to detect are the tiny larval forms, called veligers, that come out of the eggs that the adult mussels lay. The Minnesota DNR says a female zebra mussel can produce 100,000 to half a million eggs in a year. 

One lake’s experience with zebras is instructive. On Minnesota’s Mille Lacs Lake, a noted walleye fishery, zebra mussels were first detected in 2005. By 2009 DNR divers could no longer count all the mussels. In 2012 their average density topped out at 1,269 per square foot. By 2019 the density had dropped to a still significant 574 per square foot. 

The mussels changed the way nutrients are processed in the lake. Material they filter out is either ingested or expelled as feces or mucus-covered pseudofeces, tending to concentrate nutrients at the lake bottom. As a result, there is more slimy green filamentous algae in the water. Algae that grows in the shallows can die, wash up and rot in mats on shorelines. 

The pseudofeces are utilized by crayfish and bottom-dwelling insects, which are prime food sources for smallmouth bass, which in Mille Lacs increased in numbers. Meanwhile yellow perch, the primary forage fish for walleyes, declined. It isn’t clear to what extent zebra mussels were responsible for the changes. The lake ecosystem is complex and has seen various stresses in recent decades. 

For one thing, the lake was also invaded by spiny water fleas. Forage fish for walleyes had already declined by the time the zebras and spinies arrived. And over the years the walleyes became thinner. DNR scientists say the invasives are probably making things worse, although they can’t quantify the extent. In any case, the mussels annoy anglers: A lure or jig that touches the bottom is likely to hook and pull up a clump of three or four zebras. 

The lake saw strong walleye reproduction in 2007-08, but from 2009-12, low numbers of newly hatched walleyes reached maturity. Walleye fishing was excellent in 2012; the DNR believes during that time anglers likely over-harvested fish from the strong 2008 year class, while relatively few young fish were there to replace them. 

Measuring the impact of an invasive species like zebra mussels is complicated. It’s not reasonable to assume that zebras will thrive in any lake they invade. Some lakes may have only sparse populations, while in others the mussels may have massive impacts on the ecosystem. That makes a compelling case for taking aggressive measures to prevent their spread.

Ted Rulseh is a writer, author and lake advocate who lives on Birch Lake in Oneida County. His new book, “Ripple Effects,” has been released by UW Press. You can learn about it by visiting his website at https://thelakeguy.net.


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