Using a mix of computer modeling and temperature measurements, Wisconsin Sea Grant-funded researchers have found that water temperature changes over the past 27 years have made conditions more favorable for Chinook salmon, walleye and lean lake trout and less favorable for siscowet lake trout, which prefer colder water and have lost about 20 percent of their historical habitat. The results were recently published in PLOS One, a journal of the Public Library of Science.
The project builds on research by the University of Minnesota Duluth’s Large Lakes Observatory, which found that Lake Superior is warming at a relatively fast rate. Surface water temperatures increased 2.5 degrees Celsius between 1979 and 2006.
“People should be paying attention to Lake Superior. It’s warming at one of the fastest rates of any other lake observed on Earth,” said Timothy Cline, lead author of the University of Wisconsin-Madison study and now a graduate student in the School of Aquatic and Fishery Sciences at the University of Washington. “With this project, we wanted to investigate how that temperature change can alter the distribution of fish species in the lake.”
The researchers picked lake trout and siscowet trout (Salvelinus namaycush), salmon (Oncorhynchus tshawytscha) and walleye (Sander viterus) for their importance to the economy, recreation and environment. They used a three-dimensional hydrodynamic computer model modified by co-author Val Bennington, based at UW-Madison, to map changes in Lake Superior water temperatures and match that to the temperatures preferred by the four fish species.
“We found that the number of days with preferred temperatures and the amount of water available within the preferred temperature range has increased significantly for lean lake trout, salmon, and walleye,” said Cline. “The number of days and amount of preferred thermal habitat for siscowet is shrinking slightly, forcing them to move farther from the coast. We also found that the eastern side of the lake is warming faster than the western side.”
Between 1979 and 2006, the number of days with available preferred temperatures increased lake wide at a mean rate of seven days per decade for salmon, six days per decade for lean lake trout and five days per decade for walleye. Siscowet lake trout lost three days per decade. Along with a longer growing season, the extent of preferred habitat for lean lake trout and salmon increased by more than 30,000 km2, nearly doubling over the study period. Walleye habitat increased more dramatically: by more than 50,000 km2, a five-fold change from historical conditions. The cold-loving siscowet lost 10,000 km2, or about 20 percent of its historical habitat.
Does this mean fewer siscowet will live in Lake Superior in the future? Not necessarily, said Cline. “We can’t say that this will change fish production,” he said. “But this study is a good start to be able to answer that question. Data is being synthesized in a collaborative effort by a dozen agencies that could provide a better picture of how fish react to climate change. It depends on whether the productivity of the fishes’s food source is affected. But I do think it’s safe to say that we can certainly expect to see changes in the dominant players in the food web as climate change proceeds.”
Cline says the findings have implications for fisheries managers. “If you’re trying to restore walleye lakewide, some of your effort is better directed in areas that have more habitat available now for walleye. Managers may find that managing for coldwater fish in the future may be more difficult as the temperature stresses start affecting reproductive cycles or their ability to forage. It’s important to think about adapting management strategies with this in mind. Also, more effort should be placed on monitoring the eastern waters of the lake where rapid change is most apparent.”
“Lake Superior has more fish than ever before,” said James Kitchell, retired UW-Madison professor and study lead investigator. “And siscowet trout outnumber lean lake trout five to one. This could be due, in part, to a 50 percent greater spatial extent and longer durations of coldwater habitat in the past. Warming is changing fish habitats, and the extent of habitat for lean lake trout is increasing while that of siscowets is declining.”
