Some, or all of those factors may be contributing to an unsettling scene along the shores of Dickinson County lakes this fall. Bass carcasses can be seen every few steps along the beach at Pikes Point State Park, which is on the northern shore of West Lake Okoboji. The die-off is affecting primarily East and West Lakes Okoboji and the lower chain of lakes.
"It looks like a fairly significant fish kill," said Mike Hawkins of the Iowa DNR. "I wouldn't say it's minor. There's thousands of fish being affected. I don't have a real good handle on whether we're still ongoing or if it's finished up or curtailing. It's kind of hard to tell, but we're continuing to see dead fish on the shoreline."
The fisheries biologist was first called out to the State Pier area near Arnolds Park Amusement Park on Sept. 4.
"That's where we could see some live fish, out away from shore -- but they were acting sick," he said. "There were some dead fish on the shore."
Hawkins then went to Brown's Bay, on the south end of West Lake Okoboji.
"By going down to Brown's Bay, you're looking at a different part of the lake," he said. "It's pretty hard, in a lake that size, to put enough pollutant in to have an effect on fish. Seeing some dead fish down in Brown's Bay is kind of an indicator that it is disease-related."
Live bass samples have been sent to a wildlife service office in LaCrosse, Wis. Staff members at a fish health and disease center there will try to determine whether the bass disease is viral or bacterial.
"It's not unusual for a fish population to go through some of these issues," Hawkins said. "It's certainly not common all of the time on a single body of water, but it does occur. It would be similar to some type of bug coming through the human population."
A bacterial disease can be identified within a week. It may take another month for a viral illness to be identified. Even with test results, Hawkins says, "there's really nothing we can do. It's more of a case of trying to figure out why."
"I know folks who work in the animal disease and fish disease areas also benefit from understanding different die-offs and population-related problems," he said.
Hawkins uses the word "infestation" to describe the arrival of yellow bass in the Iowa Great Lakes. The species was first noticed in 2004 and is not native to the Iowa Great Lakes.
"Since that time, their populations have absolutely exploded in the Lakes ..." Hawkins said. "There are so many out there that the disease is actually pretty easily spread from fish to fish. We'll know more in about a month or so, but this may be related to the infestation of yellow bass in the Lakes here. That does happen to those populations. They can reach a peak, crash and then come back at some moderate level."
He's also seeing something unusual with the disease. The white and yellow bass are considered temperate or "true" bass, according to the fisheries biologist. Largemouth bass are part of the sunfish family. The two varieties typically wouldn't contract the same malady, but the largemouth bass population also has been effected.
Hawkins said bass caught in the Iowa Great Lakes should be safe to eat, but, he adds: "I wouldn't go out and net up sick fish."
"But if a fish is actively biting and coming after your lure, and you're catching it, that means the fish is probably pretty healthy," he said. "I don't know of any fish disease that is transmissible between fish and humans."
The DNR official says the mystery disease hasn't spread to other varieties of fish.
"It needs to work its way through," Hawkins said. "After a disease like this, normally a population is stronger for it. The individuals that are left are going to carry the antibodies for whatever it might have been. You end up with a little bit stronger population. Disease is a natural process."