Both the number of crappies and the number of anglers pursuing them are on the rise in northwestern Iowa, and particularly on the Iowa Great Lakes. Photo by Ron Sinfelt.

Something's awry at the Iowa Great Lakes. Old-timer anglers on this collection of northwest Iowa waters have noticed changes in the species and numbers of fish they catch, and long-term creel and netting surveys by the Iowa Department of Natural Resources support the bait shop talk: Fish populations are changing in the Iowa Great Lakes.

In the 1950s and '60s, the Iowa Great Lakes -- collectively, West Okoboji, East Okoboji and Big Spirit lakes, along with a handful of smaller waters -- were predominantly walleye and yellow perch fisheries. Largemouth bass, smallmouth bass, bluegills and crappies were present, but only as secondary species. In the past decade, however, anglers and fisheries biologists have noticed that warm-water species, such as bass, bluegills and crappies, have shown up more and more frequently in surveys.

"On West Okoboji, we've definitely seen a notable shift away from (yellow) perch populations and toward more of a bluegill, bass and crappie fishery," said fisheries management biologist Mike Hawkins. "We've got long-term data from both anglers' creel surveys and from bag seine nets that we've set for decades in the same locations, in the same lakes, at the same times of year. The data indicate that changing conditions may favor warmer-water species."

Hawkins can tick off several possibilities for why fish populations are changing in northwest Iowa. A warming climate is a possible factor, but he also cites work undertaken by municipalities that has improved water quality in the Iowa Great Lakes.

"During the same time that we've recorded warming in the climate, we've also had significant changes in water quality in those lakes," he said. "A sanitary sewer system was installed so houses around the lakes don't (need) septic systems that can leach into the lakes, so we aren't seeing as many algae blooms due to excess nutrients.

There have also been major improvements in farming practices in the watersheds that supply the lakes. The improved water quality from those changes has made life a lot easier for sight feeders like crappies. So to say that a climatic warming trend is the only cause of changes in crappie populations at the Iowa Great Lakes ignores other factors."

Hawkins acknowledged data gathered by state climatologist Elwynn Taylor that indicates a trend toward milder winters in Iowa over the past decades. "(Iowa's winters have been) an average of 4 degrees warmer between 1970 through 2005," said Taylor. "To a climatologist, that's significant. Maybe it's significant to crappies too."

Whatever the reasons, bluegill, bass and crappie populations over the past decade have held steady or increased in the Iowa Great Lakes, while perch and walleye populations have been merely stable.

"Yellow perch and walleyes are cool-water species," said Hawkins. "Largemouth bass and bluegills favor warmer waters. Crappies are sort of in the middle. In the past, the Iowa-Minnesota border was toward the northern end of (the crappie) range, but for the past decade they've been doing pretty well in this area. The slight climatic warming probably isn't the only cause, but it's a factor."