Fishing tales often include exaggerations about the sizes or numbers caught. Rather than embellishment, the story of declining fisheries in the Great Lakes is one about how changes in the shallow, nearshore areas affect the deep, offshore regions and important fish populations.
Nutrition to feed the food web in deep parts of the lakes is getting trapped in shallow zones near the shores, so there are fewer, leaner fish caught in the offshore regions of every Great Lake (except Lake Erie). In a recent report, expert science advisers to the International Joint Commission (IJC) say the evidence suggests invasive mussels are one of the main culprits.
In some regions of the lakes, what looks like a carpet along the lake bottom are actually thumbnail-sized invasive mussels.
“Invasive quagga and zebra mussels are unwelcome aquatic hitchhikers who found their way from Europe and Asia to the Great Lakes by ship,” says Erika Jensen, interim executive director for the Great Lakes Commission.
If you’ve ever encountered these mussels while visiting a lake, you know their sharp shells can cut your feet, but more than that, says Jensen, “they reduce the amount of food available to other aquatic organisms and exacerbate problems like nuisance and harmful algae.”
Lake Erie is well-known for annual harmful algal blooms in its shallow western basin. But in the shallow, warm bays and shorelines of the other four Great Lakes, algal blooms are becoming more common. Examples include stinky, stringy green algae washing up onto Sleeping Bear Dunes National Lake Shore on Lake Michigan, would-be visitors to Lake Ontario’s Sodus Bay canceling their plans because of an algal bloom and visitors to Lake Superior’s remote Apostle Islands reporting oozing, green surface scum.
Lake Erie is on a strict diet to reduce fertilizers, waste and other nutrients that feed its algal blooms.
However, in Lakes Huron, Michigan, Ontario and Superior, a nutrient diet isn’t necessarily the remedy. That’s because the total amount of the nutrient phosphorus measured in these lakes is already relatively low.
“In lakes with low nutrient levels, we’re seeing these benthic algal blooms, such as mats of Cladophora growing on the bottom of the lake beds, because the invasive mussels are trapping the available nutrients in these shallow areas,” says Bob Hecky, member of the IJC Science Advisory Board’s Science Priority Committee member and professor emeritus at the University of Minnesota-Duluth Large Lakes Observatory.
However, this trap is causing a ripple effect that influences the lakes’ delicately balanced ecosystems.
The destructive duo of algae and mussels in the nearshore also spells trouble for the fish in deep parts of the lakes.
"Looking at data from the nearshore and the offshore, as the water gets deeper, we expect to see gradually decreasing measurements of the total amount of phosphorus and the growth rate of the algae and organisms at the bottom of the food chain,” says Joe DePinto, Science Priority Committee member and independent consultant.
“But, instead of a smooth, slow decline in these numbers, we’re seeing high measurements in the nearshore and then a steep drop-off to very low measurements in the offshore.”
With this “trapping” in the nearshore caused by invasive mussels, the nutrients cycle is interrupted.
Like Goldilocks, the lakes need their nutrients supply to be just right, but the precise amount differs from lake to lake. Nutrients, in the right balance, are needed in the deeper, colder parts of the lakes to help algae grow.
“The amount and types of fish at the top of the food chain are affected by available sources from the bottom of the food chain,” says Roger Knight, fishery management specialist with the Great Lakes Fishery Commission.
When the nutritional building blocks of the food chain are cut off, there’s a cascade effect. With fewer nutrients, there are fewer plants and zooplankton for the smaller organisms and fish to eat. This limits the food sources for fish when they’re living out in the deep parts of the lakes, and they’re going hungry with less to eat.
An interactive Story Map depicts the impacts invasive mussels have on the Lake Michigan food web. Credit: Illinois-Indiana Sea Grant
While nutrient trapping by invasive mussels is a big part of the problem, there are also larger, more complex processes that factor into the declining fish populations. Climate change affects factors like water temperature and nutrients. The disappearance of shrimp-like Diporeia that fish feed on may be linked to invasive mussels. Mussels also increase water clarity, which impacts algal growth rates and predator-prey dynamics among fish. And it’s hard to draw a straight line to connect mussels to fish population changes when prey fish, like native bloaters and invasive alewife, are at historic low numbers, and fishery managers stock the lakes with predator fish.
Unlike the water, the solution isn’t very clear. That’s because there are still many gaps in what scientists and managers understand about the links between what’s happening with the trapping of nutrients and mussels near the shore and the size and number of fish they’re catching in the offshore.
The IJC’s Science Advisory Board collaborated with fisheries experts, including those from the Great Lakes Fishery Commission, and water quality experts to better understand the causes of the declines in the lakes’ fishery.
The Science Advisory Board report explains what water quality and fisheries scientists understand and what they still need to find out about the reduction in fish in the deep waters as a function of the nutrient trapping in the nearshore by invasive mussels and algae.
The report recommends creating ecosystem forecasting models that, like a weather forecast, can account for the many factors and predict the impact of changing nutrient loads on fish populations.
In the meantime, fishers may have to adjust their expectations (or exaggerations) about the sizes and numbers of fish caught from some areas of the Great Lakes.
Allison Voglesong Zejnati is public affairs specialist at the IJC’s Great Lakes Regional Office in Windsor, Ontario.