Carp die, but cause unknown
North Dakota and Minnesota officials are looking into what killed thousands of carp in the Bois de Sioux River south of Wahpeton, N.D.
Dead fish have started to pile up between Wahpeton and Breckenridge, Minn., where the Bois de Sioux joins the Otter Tail River to form the Red River, said Mark Henry, a fisheries specialist with the Minnesota Department of Natural Resources.
Because only carp are affected, Henry said a carp-specific disease is likely a factor.
He said tissue samples have been sent to St. Paul for analysis.
Mike Sauer, senior scientist with the North Dakota Department of Health's water quality division, said the fact only carp are dying points to a fish pathogen, and there is no threat to human health.
A change in dissolved oxygen levels in the river may also have contributed to the problem, according to Mike Ell of the North Dakota Department of Health.
Ell said the U.S. Army Corps of Engineers began cutting water flows out of Lake Traverse on July 1.
He said lower flows combined with hot temperatures this past weekend likely reduced the amount of oxygen in the river.
Diminished oxygen levels may have stressed the carp and made them more susceptible to disease, Henry said.
He added it's unknown when the fish started dying, but he said given the state of decay of many of the fish, the problem likely began several days ago.
The North Dakota Game and Fish Department was notified of the problem Monday.
Ell said fish kills are not unusual and low oxygen levels are often to blame. He said it is unusual, however, to hear of a fish kill in that portion of the Bois de Sioux.
Ell said the dead fish will likely not be around for long as scavengers feed on them. And given the amount of flow in the Otter Tail and Red Rivers, he said there is little chance for water odor or taste problems downstream.
Shawn Kessel, Wahpeton city coordinator, said the city had received no complaints about the fish, but someone did call to suggest the problem was due to recent mosquito spraying.
Kessel said the city has sprayed for mosquitoes many times in the past without incident and he said the likelihood of a connection to the fish kill "is really, really remote."
He said there is no danger to the city's water supply as drinking water is drawn from aquifers.
The sooner officials hear about a fish kill, the better the chances of finding the cause, said Henry, who encouraged the public to report any significant fish problems they observe in a river or lake.
According to a Bois de Sioux Watershed District report listing fish found in the river, varieties include walleye, northern pike, panfish and bullheads.
Carp, which contribute to water cloudiness by stirring sediment and freeing phosphorus that feeds algae, are considered by some to be a rough fish, meaning they are less desirable than other fish and less commonly eaten.
However, some anglers value carp as a game fish, according to a North Dakota Game and Fish Department Web site.
Minnesota considers the common carp a regulated invasive species, meaning it is against the law to introduce it into bodies of water. However, a fish caught while angling can be returned to the same body of water.