Will picky eating habits 
be the end of prized Chinook salmon in Lake Michigan? 
11 hrs ago Zbigniew Bzdak/Chicago Tribune 
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Chinook salmon, the most coveted catch for anglers in the Great Lakes, may have an uncertain future in Lake Michigan because of its picky eating habits, according to a new study.
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Researchers with the Illinois-Indiana Sea Grant and Purdue University sorted through the stomachs of five species of game fish across Lake Michigan to better understand the diets of salmon and trout species. Chinook salmon, the linchpin of Lake Michigan’s recreational fishing industry, was the most inflexible eater, almost entirely feeding on alewife, a small herring native to the Atlantic coast.
The alewife, which had historically been blocked from entering the Great Lakes because of impassable Niagara Falls, arrived after the construction of Ontario’s Welland Canal and established itself in all five Great Lakes in the 1950s. It was the most abundant fish in Lake Michigan in the 1960s and early 1970s.
But in the past several decades, Lake Michigan’s food web has been dramatically altered by invasive mussels that filtered much of the lake’s plankton — the alewife’s main food source.
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Meanwhile, federal and state agencies collectively continued to stock Lake Michigan with millions of salmon each year, which in turn continued to voraciously eat the alewife.
Since 2012, chinook stocking has been cut by 57 percent in an attempt to balance the number of predators to prey. Still, the alewife numbers hit record lows in 2017, according to estimates from the United States Geological Survey.
Though alewife remains the main prey for all Lake Michigan trout and salmon, evidence suggests many species are adjusting. Lake trout and brown trout are feeding more on round goby, an invasive bottom feeder. Steelhead, which tends to swim closer to the surface, are eating more insects. Even coho salmon are incorporating tiny crustaceans into their regimen.
But Chinook salmon are doubling down on alewife even as their numbers continue to tumble. Alewife made up about 97 percent of the food found in Chinook salmon stomachs, according to a Purdue University-led study.
“It appears just based off stomach contents that lake trout and brown and steelhead have more flexible diets, and they may be better prepared and able to adjust,” said Benjamin Leonhardt, who conducted the research as a master’s student at Purdue. “But chinook salmon, it appears they just continue to consume alewife despite declines, so they may be more vulnerable than the other species.”
The predicament is somewhat ironic considering that Chinook and coho salmon were introduced into Lake Michigan in the late 1960s by fishery managers to keep alewife numbers in check. Data suggest they accomplished that goal.
For 45 years, the U.S. Geological Survey has trawled Lake Michigan to estimate the abundance of prey fish at depths between 5 to 114 meters, which makes up about 60 percent of the lake. In 1973, researchers estimated about 95,000 metric tons of alewife occupied these depths. After last year’s trawl, the Geological Survey estimate dropped to its lowest mark: 90 metric tons of alewife, more than a 1,000-fold decline.
“They did their job very, very well,” said Charles Madenjian, a research fishery biologist with the Geological Survey’s Great Lakes Science Center. “They are an alewife eater extraordinaire.”
The decline in alewife numbers occurred long before invasive mussels sapped the lake of its plankton, the base of the food chain. To Madenjian, this suggests the imbalance of predators and prey was more a factor than food availability.
But this wasn’t solely a result of stocking efforts. Until recently, fishery managers also didn’t realize salmon were spawning at rates higher than anticipated.
In Lake Huron, where alewife numbers are even more dismal, chinook salmon have been migrating to Lake Michigan in search of food.
This created a feeding frenzy in Lake Michigan that led to the alewife’s decline. It’s unclear what may happen if alewife numbers continue to slip.
Alewives never flourished in Lake Superior, which still maintains a chinook salmon fishery. There, salmon feed on a wider variety of prey fish. But Lake Superior salmon are also much smaller.
“A big one in Lake Superior is
6 or 7 pounds,” Madenjian said.
“A big one in Lake Michigan is
over 20 pounds, and some have gotten well over 30.”
In the past 20 years, the catches and weight of salmon have declined, Madenjian said. Other species have fared much better. Catches of coho salmon and steelhead have remained steady. Anglers reeled in more than four times as many lake trout last year compared with 2006 levels.
In the new report, it showed salmon and trout caught in the eastern side of Lake Michigan had much more diverse diets. While alewife made up 90 percent of the brown trout’s diet in western Lake Michigan, it constituted a little more than 20 percent of its diet on the eastern side.
Researchers say this may be due to fewer alewives in the east, in addition to different landscape. Because the western shores are mainly rocky, it’s easier for fish like round gobies to hide. In the east, the sandy shores leave other species of prey fish exposed.
While fewer alewife is bad news for chinook salmon, it does have its upside, according to Tomas Hook, director of the Illinois-Indiana Sea Grant. In addition to eating plankton, alewife are known to feed on the eggs of native fish, Hook said. Alewife also contain an enzyme that causes early mortality rates in the offspring of larger fish that consume them.
In areas like Lake Michigan’s Grand Traverse Bay, scientists say the drop in alewives has allowed for a recovery in cisco, also known as lake herring, a native species that was greatly reduced during the alewife invasion.
Whether salmon might switch to eating a species of cisco remains to be seen.
In the meantime, fishery managers continue to follow the alewife closely as they ponder how the nuisance turned indispensable.
“Some people think that it started at the bottom of the food web, and these changes caused alewives to decline,” Madenjian said. “Some blame it on a bad winter or two. In my own opinion, predation played a major role, and salmon and trout stayed with them until the very end.”
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Topic: L. Mich Picky eating habits (Read 1940 times)
