Reefs in the Great Lakes

November 2, 2015


The Tangled Food Web

To understand Great Lakes reefs, you first have to take a look at the recent history of fisheries and food webs on Lake Michigan.

Once, the lake teemed with lake trout, lake herring and lake whitefish, supporting viable commercial fisheries well into the mid-1900s. It all seemed inexhaustible – but, of course, that was an illusion.

 Lake trout fish underwater close up head Image originally from Public domain images obtained through Wikimedia Commons
Lake trout fish underwater close up head Image originally from Public domain images obtained through Wikimedia Commons

Around 1960, the lake herring population crashed – the victim of the triple whammy of overfishing, habitat degradation and invasive species.

Lake herring were not just an important commercial species, they also fed lake trout. The lake trout switched to non-native alewives. It was a poor substitute, kind of like replacing your vegetable intake with potato chips.

Lake trout need thiamine to produce viable eggs. Alewives have an enzyme called thaiminase that causes thiamine to break down, so when lake trout eat alewives, their thiamine levels drop so low they can no longer produce viable eggs.

And so, as went the lake herring so went the lake trout. In fact, the lake trout have not had meaningful levels of reproduction in Lake Michigan since 1960.

Instead, the U.S. Fish and Wildlife Service stocks just under 3 million lake trout fingerlings annually.

In 1999, Randy Claramunt was a fisheries biologist for the Little Traverse Bay Bands, a tribal government. Tribal members wondered why lake trout weren’t spawning, and why eggs weren’t surviving. Claramunt spent three years trying to find out.

“During that time period, I feel like I spent more time underwater than I did above water,” says Claramunt, now a biologist with the Michigan Department of Natural Resources. “We went to every conceivable spawning site we could find.”

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Rocks are prepared for reef restoration. Photo: © Matthew Dae Smith/Big Foot Media

This meant finding reefs – raised, rocky structures. That’s where the fish were. They found reefs where lake trout would spawn, reefs where whitefish would spawn, and some where everything would spawn.

“We started thinking: what if we could find degraded or marginal habitat, and restore it so that it is similar to the reefs that are best for spawning fish,” says Claramunt. “If we were successful, one small reef could produce as many or more fish than the state hatchery.”

When looking at reefs, one on Grand Traverse Bay near Traverse City, Michigan, stood out. Nearby was degraded habitat where a new reef could be constructed. It took years of research to figure out how to best restore a Great Lakes reef. Now it’s time to actually put rocks in the water.

I’m joining Herbert, Claramunt and Central Michigan University graduate student Eric Calabro to visit the site of the new reef.

Not Just Any Rocks

Dark clouds loom but the water is still eerily calm as I don a wetsuit and snorkel. The boat is anchored in Grand Traverse Bay; Claramunt is already in the water and he’s motioning for me to jump in.

No sooner am I in the crystal-clear water (invasive zebra mussels have given the water a Caribbean clarity), than Claramunt is motioning me to look down.

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Reef restoration means a lot of time underwater. Photo: © Matthew Dae Smith/Big Foot Media

“You can see rocks everywhere, right?” he asks. “People know fish need rocks to spawn. So they’ll say, ‘There’s plenty of habitat here.’ Wrong.”

Fish need more than rocks. Claramunt starts swimming and motions for me to join him. Soon we come to a mound of rocks with a gentle slope: a reef. And no, it does not look like coral. But for the future of the Great Lakes, this modest-looking pile should probably have – pardon the pun – rock star status.

“From researching these reefs, we know the characteristics that make it awesome for fish,” says Claramunt. “You need rocks that are baseball to softball size. There needs to be some variation in size, of course. They need to be piled 6 to 10 feet high, with a recognizable slope. And you need currents that flow over the reefs to keep the eggs oxygenated.”

In September, a contractor delivered local rocks to create a reef with these exact characteristics, an effort led by the reef restoration team. The rocks were delivered to the bay in time for spawning fish to find them this fall.

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A barge of rocks is transported to the reef restoration site. Photo: © Matthew Dae Smith/Big Foot Media

It isn’t new habitat: fish have spawned here in the past, but in low number and with poor success. The fish will find much improved habitat when they begin showing up – perhaps as early as when you’re reading this.

Invasive alewife populations have recently crashed, so the reef construction could be ideal for restoring lake herring.

“Lake herring have held on for 50 years as a small remnant population,” says Herbert. “The conditions are right to build that population up and have them function as a meaningful part of the ecosystem. More herring means more lake trout and other top predators.”

But even with reefs, there are two big challenges that come in small packages. As is so often the case in the Great Lakes, these challenges are invasive species: the round goby and rusty crayfish.

Invasion of the Egg Eaters

The round goby, a small fish with a big head, and the rusty crayfish look quite different. But they have two things in common: they’re everywhere in Lake Michigan. And they eat fish eggs. Lots and lots of fish eggs. These are two highly effective invasive species.

The gobies are well camouflaged on rocks. But even so, as I swam around, I saw them frequently. Ten feet below me I could see 20 or 30 of them clustered around a rock.

For the reef construction to be successful, these invasives have to be addressed. But they’re not easy to control. Even seismic blasts that bring other fish to the surface barely perturb gobies. Within twenty minutes of being blasted, the gobies were back to eating.

To control them, conservationists have to understand them – and how they’re using the reef.

“We would study reefs and how these species behave, but we would just be observing how they act on the surface,” says Claramunt. “Gobies and crayfish don’t interact with a reef in two dimensions. They interact with it on three dimensions.”

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Conservation scientists monitor for invasive crayfish. Photo: © Matthew Dae Smith/Big Foot Media

As part of the reef construction, baskets were placed throughout the reef. These baskets were filled with rocks and function as part of the reef. Eric Calabro’s graduate research will involve pulling those baskets and identifying how gobies and crayfish utilize the lower layers of the reef. This in turn will help conservationists develop localized control strategies.

Lake Michigan’s food webs have been constantly changing for decades due to changes brought on by people, from overharvesting fish to introducing wave after wave of non-native species. A human-engineered reef coupled with substantial research holds the best promise for restoring native species. This change brings Lake Michigan closer to being a healthy ecosystem and one that also benefits both commercial and sport fisheries.

“Millions of dollars are spent annually to raise lake trout in hatcheries. And they have to keep doing that every year,” says Herbert. “If we can get to the point where lake trout are self-sustaining, that also frees up money that could be used for more habitat restoration, including restoring reefs. That’s an exciting possibility. We have not had lake trout reproducing here in more than 50 years. This reef we’re about to construct could be the first step in reversing that.”

Matt Miller

Matt Miller is a senior science writer for the Conservancy. He writes features and blogs about the conservation research being conducted by the Conservancy’s 550 scientists. He has served on the national board of directors of the Outdoor Writers Association of America, and has published widely on conservation, nature and outdoor sports. More from Matt

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