Large, lake-wide study of the diet of salmon and trout in Lake Michigan shows some of the changes; more knowledge will come.

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Sometimes you have to empty it in college.

Take the study of the contents of 3,000 stomachs of Lake Michigan trout and salmon.

“I roped up a dozen undergraduates to help,” said Austin Happel. “They each wrote a little report. It was a fun teaching experience at the University of Illinois.

Looks like I work in the agate office of the Chicago Sun-Times sports department.

Half of the stomachs were analyzed by students in Illinois, where Happel, a research biologist at Shedd Aquarium, started the project while working on his doctorate. The other half was done by Ben Leonhardt as part of his masters at Purdue University.

The stomachs were part of a large study on the evolution of the diet of salmon and trout on Lake Michigan. The study delivers some things fishermen already know and some interesting information.

Leonhardt, now with the US Fish and Wildlife Service, was the lead author of “The Complexity of the Lake Michigan Salmonid Diet: 2015-2016” on part of the study. The article is online in the Journal of Great Lakes Research and in its August print edition. The study was funded by the Illinois-Indiana Sea Grant, the Great Lakes Fishery Trust, and the Great Lakes Restoration Initiative. The scientists teamed up with the USFWS and the natural resource departments of neighboring states.

Further results will come from the study, which looked at eating in three ways: analyzing stomach contents for immediate dieting in 2015 and 2016, examining fatty acids to see what they ate over the weeks and, finally, analyzed stable isotope ratios versus months of study. diet.

“It was a huge effort to accomplish something like this, so we needed help from almost everyone around the lake,” Leonhardt said in the IISG preview. “The fishermen have been the biggest contributor. “

Fishermen from Lake Michigan and the Odawa Indian bands from Little Traverse Bay provided the fish for the study.

Coho and chinook salmon were stocked in Lake Michigan starting in the 1960s, as part of a bold plan by Howard Tanner of Michigan to control alewife and start a sport fishery.

Alewives are the main food sought after by Chinooks on Lake Michigan.
Dale bowman

For over half a century, alewives have been the staple food of salmon and trout. But that’s changing (well, not so much for the Chinook, for which alewife makes up over 90 percent of their diet) as alewives have declined while rounded gobies, zebra mussels, and quagga mussels have arrived.

One of the main conclusions is that “because of their dependence on alewife, it is likely that chinook salmon will be more affected than other salmonid species if the abundance of alewife continues to decline in Lake Michigan”.

Lake trout, brown trout, coho, and steelhead have adapted in various ways to the decline of alewife.

One thing the study showed was that variations were occurring even among species changing their diets. On the Michigan side, brown trout and lake trout ate more black-spotted gobies than alewives, but on the west side it was the other way around. Happel said their theory is that the rockier west shore gives gobies more places to hide while “on the east side it’s more sandy and it’s like a snack in an open field.”

“Larger individuals eat more gobies than other browns and lakers,” Happel said. “As they get older, they change their swimming habits. The larger lakers are closer to the bottom, more exposed to the round gobies at the bottom. “

Native species, such as the puffy shiner and sculpin, even in years when their populations are on the rise, have not had as much of an impact on the diets of invasive gobies and alewives. Happel speculated that the alewife flash may be one of their calls. There are good reasons anglers use flashing lures and attractors when chasing salmon and trout.

Another quirk was that in the spring, only in the southeastern part of Lake Michigan, the coho were full of mysis, a tiny crustacean known as the opossum shrimp.

The summary noted that rainbow trout add terrestrial invertebrates.

“It was unexpected for me to get into this project,” Happel said. “They were often full of bedbugs, ladybugs or ants. The best explanation I could find were the scum lines. All [the] the fish came from the anglers, so maybe the fishermen were targeting around the scum line. I think they find them at the scum line, and they are very abundant. You also find tiny alewives around the scum lines.

This sighting could mean something to the fishery.

Lake Michigan continues to evolve, as do smart anglers.

The IISG has a good overview of the layman at iiseagrant.org/lake-michigan-chinook-salmon-stick-with-declining-alewife-as-their-main-meal/.

Chinook, relentlessly sticking to a main diet of alewife, is one of the main draws for anglers on Lake Michigan.  Credit: Dale Bowman

Chinook, relentlessly sticking to a main diet of alewife, is one of the main draws for anglers on Lake Michigan.
Dale bowman

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