Guts

Forecasting the future of our fisheries from the bellies of Great Lakes predator fish

By Craig Manning | July 3, 2021

Next time you catch a fish, save the stomach: It might just hold the key to saving the Great Lakes fishery.

That’s the message that researchers at Michigan State University (MSU), along with a cadre of other collaborators, have been trying to relay to recreational anglers throughout Michigan and Wisconsin since 2017. The research, dubbed the “Huron-Michigan Predator Diet Study,” is analyzing the stomachs of predatory fish caught in Lake Michigan or Lake Huron, with the goal of gaining a better understanding of how fish diets are changing — and what those changes might mean for the future of the Lake Michigan and Huron fisheries.

Brian Roth, Ph. D., is an associate professor with the Department of Fisheries and Wildlife at MSU, and a fish ecologist whose areas of expertise include “aquatic food web ecology” and “trophic interactions between native and invasive species.”

Since 2017, Roth has been leading a team of graduate and undergraduate students at MSU who work together to track and analyze stomach contents of predator fish collected from Lake Huron and Lake Michigan. While the research project includes funding or assistance from a slew of different entities — including the Michigan Sea Grant, the Michigan Department of Natural Resources (DNR), the Wisconsin DNR, the United States Geological Survey, the U.S. Fish & Wildlife Service, and the Little Traverse Bay Bands of Odawa Indians — Roth says most of the actual analyzing of fish stomachs is happening in labs at MSU.

“The overarching impetus for the study has to do with the fact that, over the past several years, there's been some concern about prey levels in Lake Michigan,” Roth explains. “There's some concern that the predator and prey may be out of balance [for fish populations]. That may or may not be the case, but it was recognized that, in order to determine their balance, we needed to look at what those predator fish are eating.”

The best way to see what predator fish are eating? Take a look inside their stomachs. For the past four years, MSU and its research partners have been urging recreational anglers in Michigan and Wisconsin to save the stomachs of the predator fish they catch. Species of interest include lake trout, brown trout, steelhead trout, chinook salmon, Atlantic salmon, Coho salmon, pink salmon, and walleye. Looking at the stomachs of these fish can provide useful information about their diets, which could in turn offer early clues about future population fluctuations.

For instance, Roth notes that population numbers of alewife — a baitfish he describes as “the most favorite prey of most of these predators” — “crashed” in Lake Huron in the early 2000s, with the lake’s chinook salmon population following suit soon after. While chinooks remain low in abundance in Lake Huron, they’re also still a huge part of the commercial fishery for Lake Michigan. By studying predator fish diets in both lakes, researchers could potentially spot early signs that Lake Michigan is following the arc of Lake Huron.

“The purpose of making sure to include Lake Huron is that it offers a really stark contrast to Lake Michigan,” Roth said. “So [the study] is kind of a twofold thing. First, we need to make sure that predator and prey are in balance in Lake Michigan. And the other thing is to learn about how predators support themselves in the absence of alewife — or the near absence of alewife — as they do in Lake Huron.”

So far, Roth and his team have amassed a database of over 14,000 diet samples. Analyzing those fish stomachs has yielded a few notable takeaways.

First, Roth says alewife “still dominate predator diets” in Lake Michigan – good news for the lake’s populations of chinook, coho, and other species that depend on a strong base of alewife to eat.

Second, fish diets often follow a “seasonal progression,” indicating fluctuations throughout the year in the types of prey that is available, or that predator fish go for. For instance, Roth notes that lake trout samples collected earlier in the season indicate a diet heavy in goby. That trend held true for both Lake Michigan and Huron, but diet patterns diverged from there depending on the lake. In Lake Michigan, lake trout collected later in the season showed signs of a more alewife-dominated diet. In Lake Huron, where alewife numbers are low, lake trout have shifted their later-season habits to span “a whole diverse range of fish species.” Roth says rainbow smelt appear to be the preference for Huron lake trout in the latter parts of the season, but that “otherwise, they’ll eat kind of whatever’s in front of their face.”

“That seasonal progression was somewhat of a surprise to us,” Roth said. “But it's really consistent. We see the same things, more or less, year after year. And that’s an interesting contrast to chinook salmon, which are kind of eating one thing — alewife — at all times, no matter where they are.”

Some of the Great Lakes fish species, in other words, might be more immune to the loss of alewife (or other specific baitfish) than others.

Beyond providing brand-new information about fish diets, Roth says his team’s study should help the Michigan DNR make important decisions on fish stocking programs. Each year, the DNR rears fish like walleye, trout, salmon, pike, and sturgeon and releases them into water bodies in and around the state. That stocking process is complex and variable, with the DNR looking at a slew of different factors to determine which fish to stock, where, and in what amounts.

One key part of the fish stocking equation is balance. The DNR will try to avoid overstocking fish in certain water bodies so as not to bring about too much pressure on the food chain. The Huron-Michigan Predator Diet Study, by providing clearer trend data on predator fish, should act as a useful tool for maintaining this balance. Roth believes that, once the study is completed, it will have “really important management implications” for the DNR’s fish stocking program.

If there’s bad news for the project, it’s that it’s running out of time. The research is funded by a five-year grant from the Michigan Sea Grant. Since that grant period began in 2017, the Huron-Michigan Predator Diet Study is – barring additional funding – in the midst of its final research year. That this final year comes on the tails of 2020 is especially unfortunate, Roth says, given the many ways that COVID-19 stalled the project last year.

“We were unable to access our lab, where we actually analyze the stomach contents, for about six months,” Roth explained. “Even after that, we could only have a certain number of people in the lab at a given time. And we weren't able to go out and actually collect stomachs, either. One of our main mechanisms to collect them is to attend fishing tournaments because it represents a pretty substantial concentration of efficient fishermen. We were unable to attend tournaments up until about August, when usually we'd be out in April.”

The result, Roth says, is that the project doesn’t have nearly as much data to work with as he would have hoped.

“[Our 14,000 samples], that number really seems bigger than it is,” Roth continued. “That’s 14,000 over five years, and once you start splitting that — both among years and within our sampling framework — it’s not a lot. We’re supposed to get 30 individual samples per species, per month, per statistical district. Statistical districts are essentially small divisions of space on each lake. And we rarely hit our targets. So when you start looking at these trends on finer scales, they start to break down. For example, I can't tell you what happens for Atlantic salmon in Saginaw Bay in April, because we just don't have those samples. Maybe people caught that Atlantic salmon, but we didn't get those stomach samples, and so we can't tell.”

Luckily, if you fish, you can help. The Huron-Michigan Predator Diet Study will gladly accept fish stomachs for the aforementioned predator species, so long as they were caught in Lake Michigan or Lake Huron. Anglers interested in helping the cause can learn more at michiganseagrant.org/diet.