Dokken: Mmmm, zebra mussels . . . one more reason to beat the drum for the lowly sheepshead

Anyone who’s spent any time along the Red River this summer, especially near shore, likely has seen the extent of the zebra mussel infestation.

Dried-up zebes by the thousands can be seen stuck to rocks and other hard surfaces as the river continues to drop to its lowest level in years.

The zebra mussels are bad.

The invasive mollusks are here to stay, unfortunately, but now it appears one of the Red River’s underutilized and underappreciated fish species is doing its part to put the hurt on the nasty critters.

Enter the lowly sheepshead, or freshwater drum; the names are used interchangeably.

A few weeks ago, I wrote about some research underway along the Red River to age freshwater drum. Researchers determine the age of the fish by removing inner ear bones called otoliths and slicing the bones into thin cross sections.

From that point, they count the rings under a microscope, similar to the process for aging a tree. What they’ve found is that even midsize sheepshead along the Manitoba portion of the Red River are 50 to 60 years old.

To see if sheepshead on the U.S. portion of the Red River live that long, the Minnesota Department of Natural Resources and Mark Pegg, a fish ecologist and instructor at the University of Nebraska-Lincoln, recruited Grand Forks catfish guide Brad Durick to collect otoliths from the sheepshead his clients catch. He's collected about 40 sets of otoliths so far this summer.

Pegg and UNL students began working on the Manitoba portion of the Red River about a decade ago tagging channel catfish, and the sheepshead-aging work is a spinoff of that research.

Last weekend, Durick was removing the otoliths from a sheepshead near Grand Forks for the research crew. The fish was “super fat,” Durick said, and so he cut open the stomach out of curiosity to see what was inside.

The fish was packed with crushed zebra mussels.

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The photos Durick shared with me are grotesque, to be sure, but the fact that freshwater drum are gorging on zebra mussels makes a strong case for releasing sheepshead back into the river and awarding them some protection.
They’ll never control the population explosion of zebra mussels, but their contribution to the Red River ecosystem can’t be understated.

The species’ appetite for zebra mussel destruction isn’t unprecedented.

According to Nick Kludt, Red River fisheries specialist for the DNR in Detroit Lakes, Minn., the appetite freshwater drum have for zebra mussels has been well documented in the Great Lakes. In one study, Kludt said, 48% of the freshwater drum that were sampled showed a preference for zebra mussels over other food sources.

A similar study on Lake Winnipeg, by comparison, showed that freshwater drum mostly ate aquatic insects and didn’t go for the zebra mussels, Kludt said.

Maybe they’ll change their diets, if the zebra mussel-gorged sheepshead Durick encountered near Drayton is any indication. Other species known to eat zebra mussels include pumpkinseeds, red-ear sunfish (aka “shell cracker”), river redhorse, common carp and lake sturgeon, Kludt says.

“I don’t know that we’ll ever get a bio-control on the mussels via predation, but some native species are using them as a food source,” Kludt said. “The main limiting factor is adult mussel size (and) shell thickness and the ability of the fish to crush those shells.”

Based on the photos Durick shared, the sheepshead he caught crushed ‘em up just fine.

Sheepshead, or freshwater drum, are noisy fish, and males make a kind of grunting sound during the breeding season that is produced by muscles rubbed along their swim bladders, according to information I found on the National Park Service website. The Red River literally vibrated with the sound of grinding sheepshead one night in late June while I was fishing with Durick near Drayton .

The sound has resulted in numerous other names for freshwater drum, including croaker, thunder pumper, grinder and gaspergou, which originates from the French casse-burgeau, which means “to break a clam,” according to the NPS.

I’ve never looked, but freshwater drum apparently have large, molar-like teeth in their throats that are ideal for crushing clam shells.

And zebra mussel shells, too, obviously.

Hooray for the sheepshead!