Flooding in northwest Iowa earlier this summer propelled an invasive species of fish called Asian carp towards the Iowa Great Lakes, but biologists say a barrier seems to be keeping them out.
Over the last three weeks, scientists detected Asian carp in Milford Creek in Dickinson County. High water and flooding tend to drive these fish towards the lakes and the strength of June and July floodwaters allowed them to swim upstream over the top of two dams near the Iowa Great Lakes, said Mike Hawkins a fisheries biologist with the Iowa Department of Natural Resources.
But Hawkins says they seem to be stopping at a $1 million electric barrier officials and community members installed six years ago.
“The electric barrier is the last line of defense and it looks like we’re successfully holding them back right now,” Hawkins said.
Scientists were collecting data on fish in the lakes in August 2011, when they stumbled upon two bighead carp – one species of Asian carp. The fish were between 14 and 16 inches long. Later that fall and in early spring 2012, they also started to see silver carp, as the two often migrate together. Hawkins says silver carp tend to jump out of the water when they are frightened.
“[It] was thought to potentially impact recreation,” Hawkins said. “It really scared people, you know, ‘what if this fish jumped up behind my boat and hit a tuber behind me or a water skier?’”
The DNR worked with folks in the community and Minnesota to bring funding together for a barrier designed to provide strong protection – particularly when water levels rise and these fish have a greater chance of moving upstream towards the lakes. The barrier was installed in 2012.
“As water levels come up, the electric barrier is designed to step its electrical field strength up and create a field that those fish are not able to pass,” Hawkins said.
The National Weather Service has sites in the Iowa Great Lakes where it monitors floodwaters. When flooding happened in June and the first couple of days in July, NWS sites revealed water levels from flooding were the second highest on record since 1933, said Mike Gillispie, a hydrologist with the National Weather Service in Sioux Falls, South Dakota.
West Okoboji reached a peak water level of 6.6 feet above mean sea level. At a monitoring site in Spirit Lake, the water reached 16 feet above sea level.
“The impacts weren’t that great as far as any properties go, but it is a significant water level when you start getting up to the second highest level in 80 years, that’s saying something,” Gillispie said.
Hawkins said during a day where the water is flowing at average speed, nearby dams can stop Asian carp from migrating. But large flooding events like earlier this summer make it easier for the fish to pass.
It is unclear if the barrier is holding back all of the fish, because they aren’t tagged and trackable, he said. But even if some carp swam through, they don’t tend to spawn in lakes.
Michael Weber, an assistant professor at Iowa State University who studies aquatic lakes, said Asian carp eggs need the flow of a river as they develop. The eggs are semi-buoyant, meaning they neither float nor sink.
“If they don’t have that flow, the eggs would settle out to the bottom and get covered up by sediment,” Weber said. “And they tend to suffocate.”
Even if a few fish got through the lake barrier, Hawkins said he is not too worried because of their inability to spawn.
“We just need to keep the masses out,” he said. “A few fish, we could handle. We know we have some in the system already. We just want to make sure there aren’t more.”