For researchers with a keen eye, rising temperatures, invasive species and a changing fish population all tell the tale of a system in peril

John Casselman doesn’t need to consult 70-plus years of climate data to know the Great Lakes are undergoing some dramatic and troubling changes.

He just follows the fish.

Casselman, a biologist at Queen’s University, has been studying climate change in the Great Lakes Basin longer than almost anyone else in his field, tracking the connection between water temperature and fish populations.

Fish have long been biologists’ best indicators of the Great Lakes’ health, through decades of battles against pollution. Now, Casselman says, they’re telling us a concerning story about climate change, too.

“The fish are telling us that these changes are real. If we don’t pay attention to this, it’s at our peril,” Casselman, former scientist for Ontario’s Ministry of Natural Resources, told The Narwhal.

The Great Lakes are 1.6 degrees Celsius warmer than they were in the 1940s, according to daily temperature readings from the City of Belleville’s pumping station that draws water from the Bay of Quinte.

For fish, that’s a significant change that affects all aspects of their physiology, from spawning rates to growth to feeding patterns. Casselman’s research shows rising temperatures have caused a two-and-a-half fold decrease in the population of cold water fish such as northern pike or trout, while fueling a population boom for warm water fish such as bass.

“We’re seeing significantly more warm water fish, and for cool water fish, like pike or walleye, it’s becoming precarious,” he said. “In many places, lake fish and trout in inland lakes have simply disappeared.”

Now there’s new evidence the pace of the warming may be quickening.

This summer, researchers in a laboratory on the edge of the Detroit River began seeing an alarming spike in the readings coming back from a network a buoys spread across the lakes.

The report, from the Great Lakes Institute for Environmental Research at the University of Windsor, showed increases of 3 degrees Celsius above the long-term average for surface water temperatures in some parts of the Great Lakes. Increases like that, if sustained across the system, would be devastating to cold-water fish populations.

It’s caught the attention of climatologists who worry about the implications on the entire Great Lakes ecosystem — from impacts on fish stocks and toxic algae blooms to shrinking ice cover and more aggressive invasive species.

For those who earn a living on the Great Lakes, the warming trend is one of their most pressing issues.

“It’s quite concerning,” said Kevin Reid, a biologist with the Ontario Commercial Fisheries’ Association. “Although we expect the industry to be viable going forward, we just don’t know what that’s going to be like. There’s so much uncertainty because of all the change that’s occurring.”

In some areas of the Great Lakes, commercial fishers are already starting to change the way they harvest fish because of climate change. Along the western shore of Lake Erie, fishing crews have switched to setting their nets only in the early morning, because as the water warms through the day they’re reporting smaller and smaller catches.

“The quality of the fish is degraded, because the water is so warm,” Reid said.

In parts of Lake Michigan, cold water species such as ciscoes have all but vanished. Larger algae blooms, fueled by warmer water, are also becoming a more pressing issue.

Although the toxins from algae haven’t been shown to be harmful to fish, they cause problems for the gill nets fishers use. In the colder waters of Lake Superior, where there hasn’t been a history of algae blooms, scientists are reporting increasing outbreaks.

Biologists also worry what the warming Great Lakes mean for invasive species, from Asian Carp to zebra mussels, which along with overfishing and pollution have already threatened dozens of native fish species.

One of the biggest threats is sea lampreys, which entered Lake Ontario through shipping canals in the 19th century and almost caused the collapse of the lower Great Lakes fishery by the late 1950s.

In warmer water, the lampreys transform into destructive adults much more quickly, leading some to wonder if current lamprey control programs should be dramatically expanded.

“The Great Lakes are quite susceptible to invasive species. As the lakes become warmer and winter becomes warmer, those species could find the lakes even more hospitable and spread even more rapidly throughout the system,” said Marc Gaden, an adjunct assistant professor at the University of Michigan who is also communications director of the Great Lakes Fishery Commission.

“Then there’s the distinct possibly that the Great Lakes could become more hospitable to invaders from places that we hadn’t considered. Those would be really lethal, because the lakes could have no defense. It would be a one-two punch for native species.”

The lakes’ natural defense against some invasive species — harsh, cold winters — is also weakening.

Ice coverage on the Great Lakes has declined an average of 71 per cent over the past 40 years, according to the American Meteorological Society.

That’s caused increased evaporation and more frequent and intense storms that batter shorelines and fish habitat.

Combine all this with shifts in the lower-end of the food chain, with shrinking populations of forage-based fish such as alewife that feed larger species, and you’ve got an ecosystem under immense stress. Whether people realize or not, the changes happening in the Great Lakes will eventually reach their communities, Gaden said.

For fisheries worth millions to local economies, there’s a lot on the line.

“Changes in the fish community do affect people. They affect recreational fishers, subsistence fishers and commercial fishers. Losses to any of those communities have severe economic consequences,” Gaden said. “There are concerns that these kind of wholesale changes in the environment would have repercussions that folks don’t always think about.”

Casselman, meanwhile, has been warning for years about the impact of climate change on the Great Lakes’ fish. He just hopes the broader public is finally ready to take that warning seriously.

“The fish are trying to tell us something,” he said. “They were the indicators that told us about pollution. They were the indicators that told us about acid rain. Fish can be the indicators that tell us climate change is real.”

Lake Erie. Photo: NASA Earth Observatory