Thousands — possibly more — of the silvery-green fish that had gone belly-up are washing up along the banks of the Rouge River this week, and it's an unsettling sight for some Detroit residents.

But after inspecting photos of the fish, Michigan Department of Natural Resources said Friday that while it looks awful, there's nothing to be alarmed about. It's common for the fish – identified as gizzard shad — to die off like that this time of year.

"It's not an issue," said fish expert Jim Francis, with the Michigan DNR.

Still, don't expect the state to clean up the stinky mess.

Unless there was specific evidence that pollution caused the fish to die, he added, it's most likely that this is the result of what biologists call a spring fish kill, a natural phenomenon when "fish survive the winter but die as the water warms rapidly."

Francis said residents — not the state — are responsible for getting rid of the rotting debris. Or folks can just leave it alone and wait for it to slip beneath the water's surface and decay naturally.

Brooke Burns — who messaged the Free Press about the problem near her home — pointed out there also were dead frogs. She also was concerned because her house is near the Marathon refinery.

"We were worried," she said. "It's been like this for a week or two."

Burns, 39, of Detroit said that the rancid fish stink and are attracting seagulls.

"Fish," according to the DNR, "come through the winter in a weakened condition because they've been eating at a reduced rate. As the water warms, their metabolism increases and they divert much energy to strenuous spawning activities."

Last weekend, the DNR concluded that fish died on Belleville Lake in Van Buren Township as a result of warming weather and not enough food during the winter months.

Fish kills also happen in the summer and winter.

Mass fish kills, the state said, in lakes and streams occur during extremely hot weather. The high temps and low dissolved oxygen in the water put stress on the fish.

The fish most likely to be affected by this include pike, perch, suckers, bass, and bluegill living in shallow lakes or bays with a lot of algae or rooted aquatic vegetation. The plants use up the oxygen at night, and there isn't enough just before dawn.

The fish, however, don't sense the danger and don't swim to other areas.

In the winter — especially long, harsh ones — distressed fish gasp for air at holes in the ice. Some may die but aren't noticed until the ice melts, because they are out of sight and the very cold water slows their natural decaying process.

Some winter fish kills can be prevented by aerating the water with an outboard motor.

If you see dead fish, whether it appears to be a fish kill or not, the DNR requests you report it to a customer service center at www.michigan.gov/dnr so it can be checked out and tracked. For the metro Detroit area, call 313-396-6890.

Contact Frank Witsil: 313-222-5022 or fwitsil@freepress.com.