Who or what is killing the bass in Green Lake?

Fishing guide Jason Dudek pulled a dead bass with what looked like several knife wounds out of Green Lake in Spicer, Minn. Several more keeper-sized smallmouths were found floating in the lake over the past month, all with the same strange wounds.

On any other lake, in any other part of the country, a handful of dead fish probably wouldn't raise eyebrows. But because it was this lake, Dudek and other anglers immediately feared that a long-simmering war was back.

"I'm not sure why it's come back," said Dudek, who posted his suspicions in a video on Facebook. "But someone is slicing and killing bass again. It's such a waste."

But the state Department of Natural Resources (DNR) doesn't believe that Green Lake's bass were intentionally killed this time around, as they were in the early 2000s by walleye fishermen upset over the prevalence of bass.

The fish may have been killed by something else, something that could be lurking in the live wells of the bass fishermen's own boats.

"I don't know if it makes me happy or sad," said Dave Coahran, the DNR's area supervisor. "But I'm glad in one sense: that the old war of bass vs. walleye wasn't resurrecting itself again like it did 15 years ago."

Scientist have been dissecting fish in a lab to find out how they died, as anglers adjust to the subtle and not-so-subtle ways Minnesota's lakes are changing.

Green Lake — about 110 miles west of Minneapolis — had always been a good walleye fishing lake. But by the late 1990s, it was an excellent one. The water temperature and clarity were in a sweet spot for walleye then, staying just cold and murky enough to make it ideal, Coahran said. There was also plenty of their favorite prey, perch that grew up to 6 or 7 inches long. There were relatively few smallmouth bass, one of the walleye's top competitors.

Just as walleye fishing was at its best, however, there were signs that the lake was going to change.

First, the crayfish population ballooned, Coahran said. And smallmouth bass love crayfish.

The crayfish were so prevalent in 1997 that many kids — and some adults — would refuse to swim in the lake because the entire bottom would be crawling with the creatures, Coahran said.

The water was warming, too. Water temperatures have been steadily increasing for decades around the state, giving new advantages to bass, crappie and sunfish in waters where walleye once dominated. Years of restoration work in the area had also made Green Lake cleaner and clearer, allowing light to penetrate farther and warm the water further, Coahran said.

Finally, in 1997, the DNR changed its bass regulations on the lake in an effort to get the fish to grow larger.

The bass thrived. They got fat on crayfish. When the bass ate all the crayfish, they started relying on the walleye's perch for meals, Coahran said.

The only perch in Green Lake now put all of their energy into laying eggs as soon as possible, growing to just a few inches instead of the 6 to 7 inches walleye prefer, he said.

With the sizable perch gone, bigger walleye and bass started to eat the next easiest thing to catch: young fingerling walleye. By the early 2000s, the walleye population crashed. Bass fishermen took note of the monstrous smallmouth that now ruled the lake. The once-revered walleye lake started hosting several bass fishing tournaments every year.

"You could imagine the hatred by people who had just experienced the best 15 years of walleye fishing you could have," Coahran said. "Suddenly, it's not so easy anymore to catch walleye. A few individuals felt like killing bass was the appropriate response, and they weren't afraid to tell people they did it."

In 2005 and 2006, people started finding knifed bass floating in the water or washed on shore.

The DNR started to artificially prop up the lake's walleye by releasing tens of thousands of fingerlings every year. The stocking efforts have had mixed results.

All was quiet until Dudek pulled what looked like a knifed bass out of the lake this fall.

"Without a doubt this was intentional," he said. The bass fishing goes through its own peaks and valleys on Green Lake, he said. After the heavy pressure in the early 2000s, the quality and size of the bass catches dropped off for nearly a decade. Over the past couple years, it's been excellent again, he said.

Bass tournaments are still held in the summer and fall.

After Dudek and others reported seeing scattered dead bass, the DNR was able to net some as they were dying on the lake. They sent them to a lab in St. Paul to be analyzed. What looked like knife wounds on the fish is more likely the sign of a nasty virus, said Isaiah Tolo, a fish health supervisor for the DNR.

"The confusion may be due to the lesions caused by the virus and secondary bacterial infections," Tolo said.

The pathogen can eat away the skin in a way that looks like a knife wound.

The DNR confirmed Wednesday that largemouth bass virus has been affecting fish in the lake. The virus has only been found in big, keeper-sized fish — the kind that would be caught and put in a live well during bass tournaments. The virus rarely spreads in the open waters of a natural lake, but it spreads quickly inside a live well, when stressed fish are kept close together.

Coahran said it doesn't seem to be much of a threat to the bass in Green Lake as a whole.

The virus "doesn't spread very well in a lake, when the fish are not bunched together," he said. "We've only been seeing two or three dead fish here and there, not hundreds of dead fish all over the place."