Animal waste and agrochemicals are leading cause of fish kills in Iowa waterways

* In Iowa, agriculture has contributed to 35% of all fish kill events recorded between 1981 and 2022, according to state data.

* The monetary value of the fish lost far outpaces the amount the state has fined perpetrators.

* The state said the way fish kills are monitored — reporting by community members — impairs its ability to hold the responsible parties accountable.

Kolan Clark's lifelong passion for fishing dates back to the age of 5, when he caught his first fish under the watchful eyes of his father and grandfather. Now in his early thirties, he looks forward to continuing the tradition by taking his children to the Des Moines River when he becomes a father.

However, he's concerned about the effect the river's pollution has on the fish population.

"I’ve fished the Des Moines River most of my life. Me, my grandpa and my dad would take home like six stringers full," he said, referencing a rope or a chain used to hold fish in the water. Now, "I might get lucky and get maybe three or four fish in one stringer."

One reason people such as Clark see less fish is mass die-off events, and the leading cause of those is the state's economic lifeblood.

Over the past four decades, 35% of fish kills are related to the state's primary industry — agriculture, according to an Investigate Midwest analysis of state data from 1981 to 2022.

Iowa is one of the top 10 agricultural states in the country. It is the nation's leading producer of corn, a commodity whose productivity depends heavily on fertilizer.

It is also the nation's largest hog producer. Nearly one-third of the nation's hogs are raised in Iowa, with the collateral effect of generating thousands of tons of animal waste or manure.

Jamie Mootz, the water monitoring data analyst at the Iowa Department of Natural Resources who handles the fish kill database, describes these events as unintentional incidents.

"You could call all of them an accident, but some are avoidable," Mootz said.

Despite millions of dollars lost from fish kills over the decades, the state rarely brings the hammer down after fish kills. Overall, the amount of fines handed out is just one-sixtieth of the monetary value lost from fish kill events.

Over four decades, nearly 1,000 fish kill events have been recorded statewide, averaging 23 yearly. Although the number of events has steadily decreased, agricultural activities remain the leading cause.

The decrease in events could mean there are fewer fish to be affected.

"Declines in overall fish kill events likely reflect the overall continuing degradation in Iowa's waterways," said Tarah Heinzen, the legal director of Food & Water Watch, a Washington D.C.-based nonprofit organization that focuses on corporate and government accountability relating to food, water and corporate overreach.

If a stream cannot support aquatic life due to chronic agricultural runoff, Heinzen said, there won't be fish to kill in the event of a manure spill.

But where these events still cause fish kills, she added, it is unsurprising that animal waste continues to be the leading cause.

"Dead fish events just likely can't show the true extent of Iowa's water quality problems or how factory farms are driving them," she said.

The Des Moines River, where Clark enjoys fishing, is part of Iowa's list of impaired lakes, rivers and streams due to "unknown toxicity," according to the Iowa Department of Natural Resources draft 2022 list.

The Des Moines River, Iowa's longest internal river and central Iowa's primary drinking water source, also leads the state's rivers and streams with the most fish kills since 1981, when the state began consistently tracking fish kills.

While the leading cause of fish kills in Iowa's waterways is agriculture, the causes can vary by individual water body. According to the analysis, most of the events in the Des Moines River are related to the environment, accounting for 23% of the events.

Fish kills from environmental factors are a product of disease, spawning stress and temperature-related phenomena such as extreme heat, frost and dissolved oxygen "declines," according to the state's department of natural resources’ website.

Although the Des Moines River stands out in the rankings, other rivers, such as Floyd River — in Woodbury County and more than 5 times smaller than the Des Moines River — top the list of rivers with the most fish kill events caused primarily by pollution from animal waste and agrochemicals.

The fish kill database reflects reports the department of natural resources receives from people who detect dead fish in their communities.

Mootz explained that the department does not do patrol work but instead goes to the scene of fish kills once they are reported. For this reason, he added, the reports may not include cases of fish kills in areas not frequently visited by people, such as the state's more rural communities.

Other factors impair the department's ability to catalog and estimate the economic impact of every fish kill.

"Our counts are conservative on most kill events due to lack of water clarity, scavengers removing fish, and decomposition if the kill is not reported in a timely manner," said Joe G. Larscheid, fisheries bureau chief at the state's department of natural resources.

Over four decades, the cost of fish kills in Iowa amounts to an economic value of $12,778,984, according to the state's database.

The department uses a model from the American Fisheries Society, which has been used to determine restitution in court cases, to calculate the cost of the events. Larscheid said a game species is worth $15 per fish unless the society's value is higher.

However, penalties for causing fish kills amounted to $214,055 over the same four decades.

Larscheid said one of the agency's challenges when determining penalties for fish kills is that reports often come in of a fish kill well after it has happened, making it challenging to identify the source or responsible party.

"Anyone seeing dead fish should report those sightings," he said.

Fish kill events are not the only indication of the impact of agriculture on Iowa's water quality.

An analysis by Investigate Midwest and The Gazette of two decades of beach monitoring data from Iowa's natural resources department showed that E. coli and microcystins — human health risks — surpassed safe standards in recorded water samples.

A fifth of monitoring tests administered since 2002 have exceeded the state threshold of 126 units of bacteria per 100 milliliters of water.

The newsrooms also discovered which state park beaches most often exceeded those standards. High test results can trigger beach advisories encouraging visitors to stay out of contaminated water.

Microcystins are toxins produced by certain types of harmful algal blooms. These blooms depend on warm, stagnant water and nutrients such as nitrogen and phosphorus — key components of fertilizers used on farm fields.

E. coli, the ingestion of which can lead to potentially deadly health problems, originates in the feces of wildlife and humans or from manure runoff from farm fields.

Animal waste has been responsible for over one-fifth of all fish kill events in the state since 1981. It's so prevalent that only one year, 2018, had no reported fish kills due to animal waste.

There are other reasons for fish kill events, but the numbers are relatively small. According to the analysis, 17% of fish deaths have an unknown cause, in some cases because fish decomposition is advanced when the natural resources department conducts the analysis. About 5% of the events are due to natural death of the fish.

In Iowa, the state has recorded an average of six animal waste-related fish kill events annually.

Daniel Andersen, an associate professor of agricultural and biosystems engineering at Iowa State University, explained that when manure enters a body of water, it undergoes decomposition by bacteria.

"During this process, organic matter in the manure consumes dissolved oxygen in the water. As the oxygen levels drop, fish and other aquatic organisms may become stressed or suffocate if the oxygen depletion is severe enough," he said.

Andersen, who specializes in manure management and water quality, pointed out that drier summers — like what many areas are experiencing now — make streams more vulnerable to fish kills. Less water means the toxic manure is more potent. Also, warmer water holds less oxygen, which plays an important role in diluting the manure.

In Iowa, livestock manure has become a popular alternative to fertilizer applied to fields in the fall.

Most manure spills, Andersen added, are related to transportation to application areas.

Farmers use two systems to move manure from storage to land application areas. One is a truck system where individual loads of animal waste are driven from farm to field. ​​

In this case, spills can occur due to equipment failure, where a seal on the tank doesn't work or gets stuck in an open or slightly open position, leading to manure leakage. Additionally, spills can happen if the vehicle carrying the manure collides with another car or if the operator loses control of the vehicle.

The other application system uses an umbilical or hose system where a pump at the farm (and booster pumps along the way) is used to move liquid manure through the hose to the application field. Accidents can occur due to continuous winding and unwinding, similar to a garden hose that undergoes gradual deterioration.

David Osterberg, the former director of Iowa Policy Project, a nonprofit organization dedicated to research and policy analysis, believes the problem with manure management in Iowa is lax legislation that benefits the industry. Osterberg was an Iowa House of Representatives member and chairman of the House Agriculture Committee in the late 1980s.

"Iowa is probably notorious for bending over backward, to use a phrase, to make it easier for the industry to make money. And that's all there is to it," he said.

Agrochemicals are the other agricultural activity that affects aquatic life in the state. Fertilizer, agrochemical and herbicide misapplication are related to 10% of fish killed in the state since 1981.

Mootz, with the department of natural resources, explained that the case studies reveal many of these events occur when fertilizer is sprayed from an airplane near rivers, streams and lakes.

Agricultural runoff is another factor. Excessive use of fertilizers, pesticides and herbicides causes runoff of nutrients and chemicals, particularly nitrogen and phosphorus, that enter waterways.

Nitrogen and phosphorus pollution of Iowa waters from fertilizer applied to Iowa farm fields directly contributes to the Gulf Dead Zone. This ocean region has so little oxygen that almost no life exists beneath the surface waters.

Kolan Clark interview. May 10, 2023

Iowa's department of natural resources fish kill database

USA Economic Research Service FAQs

USA Economic Research Service, Iowa's Rank in United States Agriculture

Iowa State University, United States Pork Production.

Jamie Mootz, a water monitoring data analyst at the Iowa Department of Natural Resources. Interview. May 11, 2023.

Response to Investigative Midwest inquiry of Legal Director Tarah Heinzen, legal director at Food & Water Watch.

Response to Investigative Midwest inquiry of Daniel Andersen, associate professor of manure management and water quality agricultural & biosystems engineering at Iowa State University. May 18, 2023.

Interview Nicholas Paulson, professor in agricultural economics at the University of Illinois at Urbana-Champaign. Dec 12, 2022.

Lance Lillibridge, farmer and member of the board of directors for the Iowa Corn Growers Association. Dec 2022.

Minnesota Department of Natural Resources, Des Moines River State Water Trail.

Des Moines water works, website watershed section.

Iowa Department of Natural Resources, Interior River Search Tool / Floyd River.

Response to Investigative Midwest inquiry of Joe G. Larscheid, fisheries bureau chief at the state's department of natural resources. May 12, 2023

American Fisheries Society "Investigation and Monetary Values of Fish and Mollusk Kills" website.

Interview David Osterberg, former director of the Iowa Policy Project, which later merged with research and policy analysis nonprofit Common Good Iowa. May 19, 2023

EPA, The Sources and Solutions: Agriculture.

Investigative Midwest, Fertilizer washes off Midwest farm fields and taints communities’ drinking water, poisons the Gulf of Mexico

Type of work:

Mónica Cordero is a Report for America corps member and part of the Mississippi River Basin Ag & Water Desk team. Her expertise includes data analysis with Python and SQL, and reporting under the... More by Mónica Cordero, Investigate Midwest/Report for America