Climate Change at Isle Royale: Cisco

A school of cisco near an Isle Royale shoreline.

Mariah Reading

Cisco, also known as lake herring, are fish native to the Great Lakes. The species plays a key role in the food chain of Lake Superior and Isle Royale’s inland lakes, particularly as prey to lake trout. However, cisco population in the Great Lakes is dwindling due to heavy commercial fishing, introduction of invasive species, and climate change.

Prehistoric Fish

The cisco that is found in Isle Royale’s inland lakes is special. When glaciers retreated, some of the lakes on the island were separated from Lake Superior forever. Over time, the genetics of cisco have changed in the big lake, but the isolated inland populations have stayed much the same as they were in prehistoric times.

Climate Concerns

Cisco use near-shore freshwater areas for spawning, and Isle Royale’s steep shores and underwater reef topography is ideal for their estuaries. However, cisco are cold water fish, and cannot withstand temperatures warmer than about 67 degrees Fahrenheit. As the climate warms, increasing water temperatures could make Isle Royale’s inland lakes uninhabitable for the fish. In 1995-1997, a cisco survey was conducted at Isle Royale. Small amounts of cisco were located in four inland lakes: Lake Desor, Siskiwit Lake, Lake Richie, and Sargent Lake. In 2013, scientists conducted a follow-up study, and were unable to locate cisco in Lake Richie and Sargent Lake. These findings could be the first example of localized extinction as a direct result of climate change.

Bottom Line

Just a few degrees of warming can be catastrophic to climate-sensitive species like cisco. As the climate continues to change, cisco could disappear from Isle Royale’s inland lakes and Lake Superior entirely, putting underwater ecosystems at risk.

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