Ice Cover, Food, and Hypoxia: How is Climate Change Affecting Great Lakes Fish?

The changing climate around the Great Lakes will exacerbate existing challenges to aquatic species – such as competing with invasive species and finding food – and present some new ones as temperatures rise, according to researchers in the United States and Canada.

The Great Lakes have experienced an array of issues that put stress on fish, plankton, and other aquatic populations: habitat loss, excessive nutrients, oxygen-poor areas known as hypoxic zones, and invasive species like quagga mussels. Climate change is interacting with these existing problems in complex ways, according to Illinois-Indiana Sea Grant Great Lakes Ecosystem Specialist Paris Collingsworth, who helped author a 2017 research paper on those interactions. Collingsworth also is an assistant research professor with Purdue Forestry and Natural Resources.

Climate change is expected to lead to warmer air and surface water temperatures, Collingsworth said, and there’s already evidence of warming in the waters. If faced with just that aspect, and if these fish have the space to move around, fish would be able to adapt – they regulate their body temperature by moving into whatever that species’ optimal temperature range is, a process called thermoregulation. This has allowed some species of fish in parts of Ontario to move northward into historically cooler waters as they warm up, said Scott Parker, a climate change ecologist with Parks Canada.

“In my own experience I’ve seen white bass moving northward,” Parker said. “It’s more a temperate species that’s coming into our waters.”

Water temperatures also can get uncomfortably warm for fish species like walleye that prefer cool waters and lake trout that prefer cold waters, Parker said. Though the Great Lakes are large enough and cool enough that this hasn’t been much of a concern, he said it’s becoming more of an issue in smaller inland lakes, which warm up faster.

But changing water temperature also is impacting food availability and the frequency of ice cover, two factors that have a dramatic impact on how fish reproduce and survive. Collingsworth said the lake whitefish, for example, spawns in shallow waters in late fall each year.

“They’re counting on ice cover to provide a stable environment for their eggs,” Collingsworth said. “If they don’t have ice cover, then those shallow environments can be turbulent due to wave action and storms, and directly dislodge the eggs.” This essentially kills the larval fish before they can hatch. Other fish, such as lake sturgeon, also spawn in running water in the springtime and could be at risk from heavy spring storms, he added. While the extent of the maximum ice cover and how long it’s on the water varies from year to year, ice coverage has declined by 71 percent between 1973 to 2010 on all five Great Lakes and Lake St. Clair.

Short, warm winters could also cause fish to hatch before they normally should, which leaves those fish ill-equipped to survive. For example, yellow perch hatch based on water temperature, Collingsworth said.

There has been a shift in what kind of plankton species are found in the lakes, said Parker. A recent paper indicates that shift is affecting smaller individual plankton, as well as what plankton species are in these communities; what this could mean for the food web of the Great Lakes is still uncertain.

Warming water temperatures and other issues with invasive species and the nutrient cycle also could lead to a disconnect of when predators and prey are historically active, something seen in marine environments that could translate to the Great Lakes. A fish species may start hatching before – or after – their ideal food source is readily available, forcing the fish to make do with what’s out there or simply die, according to Collingsworth’s report. It’s possible this has caused problems for yellow perch and walleye to successfully reproduce in Lake Erie.

“From studies of larval fish in Lake Michigan, we know that alewives’ growth during their larval phase is about half now what it was in the early ‘00s or late ‘90s,” Collingsworth said. “They’re growing slower, and the presumed mechanism is that they aren’t getting as much food.” He added it’s possible that the growth rate average would be even lower if researchers could include the dead alewife young, too.

An increase in the severity and number of spring storms also could wash more nutrients into lake systems, exacerbating problems seen in western Lake Erie, Saginaw Bay, Green Bay and elsewhere: algal blooms and hypoxia. Hypoxia can pose a significant problem for species of fish that prefer the same temperature ranges that hypoxic zones tend to occupy, forcing fish like yellow perch to stick to other parts of the water column that aren’t as healthy for them just to breathe, Collingsworth said. Water that has stratified into particular layers of warmth and oxygen content – which happens during the summer months and can be interrupted by storms – can make that problem worse, and climate change could extend the amountof time water is stratified.

And more harmful algal blooms can pose human health risks and get into water intakes. The United States and Canada have recently unveiled plans to try and reduce nutrient pollution into Lake Erie, recognizing that the plans may be adapted as time goes on to reflect climate change and changing conditions.

Water levels in the lakes could affect wetlands and other coastal ecosystems if they see prolonged periods of low or high water levels – in turn affecting fish that use those as habitat – but so far there’s a lot of uncertainty how climate change could impact those, Collingsworth said.

Existing stressors like invasive mussels and habitat loss have already fundamentally changed the identity and character of the Great Lakes, Parker said. With climate change added in the mix, fisheries managers will have to adapt stocking and catch limits to a “novel ecosystem.”

“It’s not simply a new ecosystem to our area, it’s a novel one,” Parker said. “We haven’t seen these systems before – anywhere."