Lake Superior is the largest freshwater lake in the world by surface area, and it has something the other Great Lakes don’t — stable populations of mostly native fish species.
But scientists say a key fish in Superior’s food web is now in trouble because of mild winters and an appetite for caviar in Europe.
A new demand
There wasn’t much demand for lake herring until a few years ago. Craig Hoopman says it used to be fed to mink and used as fertilizer.
“And it was just a, a real cheap product, and you can make a fairly decent living doing it,” Hoopman says.
Hoopman is a commercial fisherman in Wisconsin who fishes around the Apostle Islands. He says around 2007, Scandinavians started buying lake herring eggs, or roe, for caviar.
“Now, the numbers are... we get really good money for our roe. It’s shipped to Estonia or Sweden so it’s kind of really a small niche,” he says.
But what Hoopman calls a “small niche,” scientists now see as a threat to a fish having other problems.
Scientists try to crack the puzzle
Mark Vinson directs research on the R/V Kiyi for the U.S. Geological Survey. The boat is called the Kiyi because that’s the name for a particular kind of herring only found in Lake Superior.
Vinson says this fishery is enormous. And the Apostle Islands are the most prolific breeding grounds. He says in November, a third of the adult herring in the lake will leave the deep water and head to the west end to spawn.
"What's going on right now we don't think could go on for long without seeing some impacts," Margenau says.
“It’s on the order of 60 million fish that are coming into the western arm of Lake Superior in the fall to spawn,” he says. “And 60 million fish is a lot of fish. That’s bigger than the historical runs on the Columbia River, for example.”
Of course, a lake herring is smaller than a Pacific salmon — a big one is 15 or 20 inches long. But these fish, also called cisco, are the key source of food for other fish in Lake Superior, starting with their eggs.
“Those eggs provide food for other fish and then those young fish come off, which are then eaten by other fish,” Vinson says.
The problem is not many young fish are coming off the spawning beds in recent years.
Herring are having trouble making babies
The Kiyi does annual surveys of Lake Superior. They sweep trawl nets all over the lake and count the fish they find. And it’s been a decade since they saw large numbers of young lake herring. This year is decent, but some years they’ve found almost no young fish at all.
Vinson says explaining why lake herring have such trouble reproducing some years is the holy grail of fish science up here.
“There hasn’t been anyone who’s really solved it, like put their finger on it and says ‘this is the reason that recruitment is so variable,” Vinson says.
Water temperature has something to do with it. The fish don’t reproduce well without ice, and there hasn’t been much ice on Lake Superior until the last couple winters.
That’s why researchers are worried about the fishing around the Apostle Islands. Commercial fishers in Wisconsin can catch as many pregnant lake herring as they want.
In April, the Great Lakes Fishery Commission urged the state of Wisconsin, and some Indian tribes there, to reduce the harvest.
Terry Margenau, at the Wisconsin Department of Natural Resources, agrees it’s a problem.
“What’s going on right now we don’t think could go on for long without seeing some impacts,” Margenau says.
One of Margenau’s biologists put it more bluntly at a conference in March. He used the word “collapse” to describe what might happen to the lake herring fishery.
But Margenau says he’s optimistic Wisconsin and the tribes can come up with some new rules, before fish start to spawn in November.
