MINNEAPOLIS (AP) — Don Kermeen grew up along the shores of Lake Superior, and early on he learned a lesson about the mighty lake: “For most of the year, you don’t swim in it unless you’re in a wetsuit.”

Not this summer. Superior and the four other Great Lakes have been at or near record high temperatures for the 30 years such measurements have been taken — and there’s still a month left before the lakes typically hit their warmest levels of the year.

“It’s been awesome,” Kermeen, the co-owner of Superior Shores Resort in Ontonagon, Mich., along Superior’s southern shore, said this week. “I think every single guest I had yesterday was out swimming. I don’t care if they were a kid or in their 60s, they were out in that water.”

A more swimmable Lake Superior may be good for tourism, but the effect on everything from fishing to wildlife habitat to water level may be bad. And, while scientists understand why the lakes are warmer this summer, they don’t know if it’s a blip or the new normal.

“I think in general, we don’t like to see it, because we’re not sure if it’s a good thing for the lakes,” said Tom Gorenflo, tribal fisheries manager for the Chippewa-Ottawa Resource Authority in Sault St. Marie, Mich. “Everybody is worried about it, and wondering if it does a fit a larger pattern of global warming, glaciers melting in Alaska and the like.”

Jay Austin, a physics professor at the Large Lakes Observatory in Duluth, said there’s a simple reason for the warmer waters: There was little to no ice cover on the Great Lakes last winter. Open water absorbs sunlight more effectively than reflective ice and snow.

A buoy in western Lake Superior this week recorded 71 degrees — 1 degree off the record. Lake Superior’s normal temperature this time of year is about 60 degrees.

Two buoys in Lake Michigan were 5 to 6 degrees above normal, with similar levels in Huron and Erie. Ontario hit about 70 degrees earlier in July but has since cooled some — an example of the volatility of lake temperatures, Austin said.

The warmer water could drive down ice levels next winter, Austin said. But an unusually cold winter could mean more normal water temperatures next summer.

“There’s a ton of variability from year to year in these types of system,” Austin said, but he was quick to add that this year is “tremendously anomalous.” In addition, he said scientists have monitored a long-term trend toward less ice on Lake Superior over the last three decades.

Water levels on all five Great Lakes trended downward in the last decade to near-record lows before rebounding some in recent years. That has been attributed to warming trends, and warmer lakes could worsen that problem through greater evaporation.

A big question is what warmer lakes could mean to fish populations and commercial and sport fishing. Scientists and fisheries managers are concerned.

Nancy Schuldt, the water quality coordinator of the Fond du Lac Band of Chippewa Indians in Cloquet, said area rivers and wetlands would suffer greatly without a healthy water flow from Lake Superior — in everything from habitats they rely on for fishing, to ill effects in growing conditions for the culturally important wild rice the band harvests every year.

“Even slight warming in our waters and wetlands and we could see some major upheavals,” Schuldt said.

Gorenflo, the tribal fisheries expert in Sault Ste. Marie, said warmer lake water might heighten the impact of invasive species by promoting their growth.

Invasive species like zebra mussels have found their way into the Great Lakes, but so far haven’t thrived the way they have in smaller, warmer lakes, Gorenflo said.

But at least for this year, some fishermen are welcoming the warmer water. John Steiben, owner of Duluth’s Lake Superior Fishing, a charter fishing business, said it’s been a great summer for salmon and lake trout.