By Adam Carpenter and Pete Mackin
LAKE SUPERIOR – The Department of Natural Resources research vessel Lake Char is high and dry this time of year, but the data gathered from the past season’s studies continues to make waves.
The Lake Char is the first vessel specifically built for the DNR to study the world’s largest freshwater lake – Lake Superior. The larger and newer vessel means biologists can venture farther out and survey a larger sample of fish. It’s also much safer than the Lake Char’s predecessor, which had sunk at least once in its career on the Great Lakes.
According to fisheries biologist Shawn Sitar, Lake Superior lake trout are a healthy example for the other Great Lakes to follow. Part of the mission for the crew of the Lake Char is determining why trout are healthier in Lake Superior than they are in other lakes, to help those lakes recover their populations.
“This boat is actually twice as fast as the old Research Vessel Judy,” Sitar sad. “We’re able to go offshore quicker. It’s a much larger platform, which is much safer. I guess the primary thing is that it is much safer.”
Biologists are trying to determine why lake trout in Lake Superior do better than in the other Great Lakes. Although there are likely many factors, one possibilities has to do with the food the trout eat.
In Lake Superior, trout feed primarily on smelt. In the other great lakes trout consume a lot more alewives.
“There are very few alewives in this lake and that may be one reason why the other lakes may be messed up in terms of lake trout populations,” Sitar said. “Alewives produce thiaminase, which breaks down B vitamins. So there’s a whole theory thinking in those lakes, eating a lot of alewives basically prevents those populations from recovering.”
In Lake Superior, anglers have long-ago noticed that smelt aren’t running up rivers the way they did back in the 1980’s. But while the smelt are hard to find in rivers, they are easy to find in the bellies of lake trout.
“Lake trout bellies are consistantly full of smelt,” Sitar said. “Lake trout select for smelt. Now if you imagine the smelt populations as a big pie, with the trout and smelt dippers taking a portion of that pie, as smelt numbers decrease and lake trout numbers increase – which is happening – the trout consume a greater portion of that pie.”
Sitar also notes that some research shows that smelt are adapting to breed off-shore versus up river. The smelt that run up river are naturally consumed by smelt dippers and predator fish, while those that breed off the beaches tend to be the smelt that survive in greater numbers, thus passing that trait on to progressively more generations of their offspring.
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