Carp in Albert Lea Lake and along the Shell Rock River turned up dead earlier this week, and the Minnesota Department of Natural Resources is on the case.

DNR Area Fisheries Supervisor Hugh Valiant said he received reports of hundreds of dead carp, but Shell Rock River Watershed District Conservationist Andy Henschel estimates maybe 100 dead in the entire area.

The first report came Monday from someone who observed dead fish along the lakeshore over the weekend, according to Valiant. Over the course of the week he received five or six calls from government entities alerting him to the dead carp and wondering why.

“The more that die, the merrier,” Henschel said. “Because all we’re seeing is carp, I’m not too concerned at this point.”

A fish kill situation, he said, would be all species. The current kill is confined to carp, according to Valiant, and other species don’t seem to be affected. In fact, Valiant heard there is good walleye fishing in the area.

Carp is an invasive species, and the controlled fish kills done by the DNR have an aim to eradicate carp and bullhead, Henschel said.

The probable explanation, Valiant said, is a sudden oxygen depletion. During the summer when the air and water temperatures warm up, the oxygen concentration of local waters — particularly in southern Minnesota’s fertile waters — can become quite dramatic, he said.

The oxygen concentration in water can go up due to photosynthesis — plants giving off oxygen. The oxygen in water can then be depleted quite dramatically, Valiant said, when the weather becomes overcast or calm, or if plants stop giving off as much oxygen.

In shallow or unstable lakes, the oxygen concentration can become lethally low for fish, according to Valiant. At this time of year, he said, it is common to get reports of fish kills.

A combination of environmental stresses more than likely killed the carp in the area, he said. The heat, low oxygen levels and post-spawning stress probably contributed to the death.

Henschel attributes the dead fish to the time of year. Carp have just finished spawning, he said, and are stressed making them more sensitive. Or, the dead fish could be due to fisherman catching carp, cutting their gills and throwing them back into the water, he said.

However, Valiant seems to think the mortality rate of carp is more widespread than normal, though he hasn’t seen the dead fish first-hand. He has heard reports of dead fish scattered along the entire Albert Lea Lake and along the Shell Rock River to the Iowa border.

“Given that extent of widespread mortality, there could be possibly some other cause,” he said.

For the past few days, Valiant said, DNR crews have been attempting to get a sample of sufficiently fresh dead carp for the pathology lab in St. Paul. A small sample was obtained, he said, but it is smaller and older than he would have liked.

The pathology lab should be able to determine the cause of death within the next two to three weeks, Valiant said.

It sounds to Valiant, he said, that the fish kill was of a limited duration and probably occurred between July 3rd and 5th. The kill doesn’t seem to be ongoing, he said.