POPULATIONS OF minnows and alewives have thrived in the warmer temperatures of Lake Michigan this year, making for a great salmon fishing season. Water temperatures in Lake Michigan this year are way above the 20-year average, according to George Leshkevich, a physical research scientist at NOAA/Great Lakes Environmental Research Laboratory (GLERL).

On Monday Lake Michigan was a balmy 73.8 degrees; on the same day last year the lake measured 69 degrees and in 1996 it was a cool 60.4 degrees. Water temps will normally vary from year to year, Leshkevich said, but overall, the Great Lakes are getting warmer.

“That’s the long-term trend that we’ve been seeing — increased water temperatures,” Leshkevich said.

While that may be good for swimmers, and even for some fish populations, the long-term effects may be harmful for Lake Michigan and for all of the Great Lakes.

Warmer lakes have several ramifi- cations, he said. Increased temperatures have contributed to harmful algal blooms (HABs), created by a rapid increase and accumulation in algae populations. HABs can last from mid-July to mid-October, can affect the ecosystem of a lake and can be toxic or non-toxic. HABs can also cause odors and, if left unchecked, may become too difficult to filter out of water supply systems, Leshkevich said.

“It makes a mess,” Leshkevich said. “You can see boat tracks through it and it creates real health problems for water intake stations, which have to filter all that stuff out ... It can affect the population in a variety of different ways.”

HABs can be seen very clearly on satellite images, with shallow Lake Erie being the most affected by them. On Lake Michigan, HABs have been seen in Green Bay, Wis. Algae can also settle to the bottom of the lake, where it will deplete oxygen, or it can percolate to the surface of the water and cause an odor.

LAKE MICHIGAN is much warmer this year than it has been, beating the average temps for the last 20 years by about four degrees. For fish, warmer water temps may actually be a good thing, as salmon fishermen have seen this year on Lake Michigan.

A mild winter and early warm-up can be beneficial to some fish, said Heather Hettinger, a fisheries biologist with the Michigan Department of Natural Resources. In areas near Beaver Island and the Straits, she said, bass, which prefer warmer water, have moved farther out into the lake. For steelhead, which are born in rivers, more fish may survive if the water is warmer, as it will entice them to linger there before venturing into larger waters.

Out in the lake the gradual warmup is beneficial for forage fish such as minnows, shiners and alewives, Hettinger said. That makes for good salmon fishing as salmon feed on forage fish.

“It certainly is benefitting the salmon fishing this year,” Hettinger said of the warm water temps.

For the past few years fisherman have had to change some of their techniques, but in the long term better survival of some species of fish could be seen, Hettinger said.

Significantly warmer temperatures could alter the timing of spawning within a fish population, she said.

“Maybe bass start spawning a little earlier. Maybe salmon start running streams a little earlier.”

In smaller lakes an increase in water temps will cause some species of fish to significantly die off, though to have a species completely disappear almost never happens, she said.

“Certain species of fish and certain sizes of fish don’t respond to increased water temperatures very well,” Hettinger said, though fish in Lake Michigan do not have that response because it is a larger body of water and stays cooler than inland lakes.

“There are always areas where there’s much cooler water,” she said.

The Ann Arbor-based GLERL gathers data and conducts research on the Great Lakes. Water temperatures in Lake Michigan are obtained by satellites and by nine buoys.

Satellites, which take the temperature of the surface or ‘skin’ of the water, have been in use since 1992. Buoys, which measure water temperatures about one-half to one meter below the surface, have been collecting data since about the late 70s. Data from the sources is compared to make sure it is accurate, Leshkevich said.

GLERL also tracks ice formation on the lakes, and hand-in-hand with warmer temps is less ice — dramatically less ice, Leshkevich said. “The trend line is downward for less ice coverage,” he said. “The temperatures are exactly opposite. The temperature trend line is going higher.”

Lack of ice and warmer water temps work together as a sort of self perpetuating cycle, he said. Heat is stored in the lake during a hot summer. So come fall, if temps are not extremely low in November and December, the lake doesn’t lose the heat, he said. And if the lake doesn’t lose the heat, ice won’t form.

Without ice there is no solar radiation, which occurs when the sun’s rays are reflected off the ice and back into space. The sun’s heat is then absorbed by the water and stored, contributing to the increase in water temperatures, especially at the surface.

According to data at GLERL’s Coastwatch website, www.glerl.noaa. gov, at the peak of winter 2008-’09, about 55 percent of Lake Michigan was covered in ice. Compare that to 2011-’12, when just 15 percent of the lake was iced over. While last winter was unusually warm, it’s a trend that has been going on for several years, Leshkevich said.

In 2008-’09, Lake Superior — the deepest and coldest of the Great Lakes — was about 95 percent iced over; by 2011-’12, ice coverage had dropped to less than 10 percent. In Lake Huron ice coverage dropped from 85 percent to less than 25 percent in the same time period, and in Lake Erie — the shallowest of the five lakes — ice coverage dropped from nearly 100 percent coverage to almost none last year.

So what’s the cause of the upward trend in water temperatures?

“It’s probably a combination of natural variability plus maybe some anthropogenic (man-made) factors, which are greenhouse gasses, global warming, climate change, whatever you want to call it,” Leshkevich said.

Compared to eons of time, the 40 years that temperature data has been gathered on the Great Lakes is just a blip, Leshkevich said.

“It doesn’t mean it can’t reverse for some reason and go in the opposite direction,” he said.