Finding prey at bottom is an advantage
Written by Detroit Free Press   
Thursday, 12 May 2005 11:56
Lake trout are slower than salmon and usually stay near the bottom, where they are especially vulnerable to lampreys.

The St. Marys River at the northern end of Lake Huron once produced more lampreys than all of the other Great Lakes tributaries combined, putting a serious dent in the lake trout population. But when the state Department of Natural Resources and U.S. Fish and Wildlife Service came up with an effective means of treating the river five years ago, lamprey wounding rates on lake trout dropped from about 30% to 10%.

Now the lakers have a big advantage over salmon, because when the alewife population plummeted, the salmon found less to eat in the upper waters where they cruise. The lake trout still found gobies, crayfish, sculpins and other bottom-dwelling prey.

But zebra and quagga mussels might be reducing prey numbers on the bottom. State fisheries biologist Jim Johnson said juvenile lake trout -- fish smaller than 12 inches -- used to frequent deeper water to avoid their parents, who will happily practice cannibalism. Now the bigger fish seem to be roaming deeper in search of food.

"We used to find that separation, with the bigger fish shallower and the smaller ones deeper," Johnson said. "But now we find the bigger fish in the nets we set at 185 feet, apparently because they have to go that deep to get food. The juveniles can't go deeper than that and get back up. Their swim bladders get too compressed. So apparently they don't have that deep refuge anymore."

Biologists like to stock a lake with eggs taken from trout that have always lived there, figuring they can't do better than nature, which spent thousands of years adapting the fish to the habitat. Lake Superior was restocked with the Marquette strain of lake trout, which were taken into a hatchery just before the lake trout disappeared from that lake. Lake Superior's lake trout now seem to be thriving with good natural reproduction.

Lake Michigan is also getting its own genetic strain again, but the fish aren't from Michigan. They are the descendants of Lake Michigan trout taken from the Beaver Islands in the late 1800s and stocked in Lewis Lake in Yellowstone National Park in Wyoming, where the strain has been kept genetically pure.

But the lakers that live in Lake Huron are not the same as the ones that lived there before the lampreys arrived. After the laker population crashed, Huron was restocked with other strains, including the Seneca from New York. They handle lamprey predation better, probably because they have been preyed on by lampreys much longer.

"No one kept a Lake Huron strain of lake trout in the United States," Johnson said. "But we're working with Canada, and we should be able to start stocking Michigan's waters with Lake Huron strain fish from Parry Sound in Georgian Bay."

 
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