The DNR has determined a fish kill on Lake Odessa in Louisa County is due to natural causes.
The DNR investigated the fish kill after a homeowner observed dead fish on the lakeshore Sunday. DNR’s environmental specialists checked the lake Monday, finding dead fish spread out along three miles of shoreline. The water is relatively shallow and water temperatures were high – from 85 to 90 degrees. Oxygen levels were normal, but ammonia and pH were slightly elevated.
The dead fish were mostly gizzard shad, which are very sensitive to changes in temperature, high or low, according to Chad Dolan, DNR fisheries biologist. There were also a few larger dead fish of other species – including one white bass and one channel catfish.
“It’s important to understand that Lake Odessa functions much differently than Iowa’s small inland lakes,” said Dolan. “It’s a river backwater with water levels that fluctuate with the Mississippi River. We manage the area to mimic the natural conditions historically present in backwaters – higher in spring and fall and lower and drier during the summer.”
Recent plankton blooms may have led to low oxygen levels in small areas especially at night when the tiny plants weren’t producing oxygen, but Dolan thinks that’s the temperature rise was the main cause of the fish kill. “There are about 3,000 surface acres here so the wind fetch builds waves and oxygenates the water,” he said. “There may be a few dead fish due to low oxygen in isolated shallow pools.”
“This is a unique and complex area,” said Andy Robbins, the wildlife biologist in charge of managing the water levels to benefit wildlife, primarily migratory birds. The U.S. Army Corps of Engineers owns the complex. The U.S. Fish and Wildlife Service manage the northern third of the complex as a national wildlife refuge. The DNR manages the southern two-thirds as a wildlife management area.”
River backwaters are typically shallow and subject to siltation. Historically, they dry up during the summer, helping the sediments to harden and compact, which gives wetland and moist-soil plants a chance to root and grow. The plant growth further consolidates the sediment, which results in excellent water quality and great fish production. When waters rise in the spring, fish move into the backwaters to breed, moving to the river during dry summer months, and moving into the backwaters again to feed in the fall. The plants provide important food and cover for migratory birds.
“If the lake wasn’t drawn-down in the summer to consolidate sediments and spur plant growth, the lake would eventually fill with soft silt. This material can be resuspended by wind, waves and bottom-feeding fish like common carp leading to poor water clarity,” Dolan said.
“During most years we are not able to draw the water levels down as planned due to high water or flooding on the Mississippi River. So far, this year is really no exception and water levels in Odessa are currently just a few inches below normal,” Robbins said.
When drawdowns are possible, the management really pays off for waterfowl in the fall. The water is raised and the newly established plants are re-flooded, providing an excellent source of food and cover. The Odessa Complex is a popular stop for migrating ducks and the hunters who hunt them. “We focus on migratory birds, but our management benefits all wildlife species,” Robbins said. “This area has the greatest diversity of reptiles and amphibians found anywhere in Iowa.” More than 240 bird species use the area.
Find more information about the water and wildlife management at Lake Odessa at www.iowadnr.gov/Hunting/PlacestoHuntShoot/WildlifeManagementAreas/LakeOdessaWMA.aspx.
For more information about fish kill natural causes, contact Chad Dolan, fisheries, at 319-694-2430 or Chad.Dolan@dnr.iowa.gov. For more information about managing water levels to benefit wildlife contact Andy Robbins, wildlife, at 319-523-8319 or Andy.Robbins@dnr.iowa.gov.