Iowa DNR
Iowa DNR Fish Kill Database

Summary of Fish Kills in Iowa

Database Statistics
Total events
Most fish killed in a single event
Waterbody: Lotts Creek
Date: 12/14/2001
Cause: Ammonia/Fertilizer
Estimated sum of all fish killed

The Fish Kill Database preserves fish kill event information in support of the Iowa Department of Natural Resources Water Monitoring Section and their work with the Integrated Report- the 305(b) Assessment of Iowa's Water Quality. This page attempts to summarize fish kill information and provide a broad overview of general trends.

Users should be aware that fish kill event reporting is most likely biased towards anthropogenic (human-caused) events. People are more likely to report pollution-based fish kills, rather than those occurring "naturally", such as those due to hot weather, fish disease/stress, and dissolved oxygen issues. People are also less likely to report, and IADNR Fisheries and Field Office staff are less likely to investigate, fish kills that occur on private property (such as farm ponds) or locations where natural kills are more likely to occur, like small, shallow waterbodies that are more impacted by weather extremes or dissolved oxygen sags.

The history of fish kills in Iowa and their associated data falls into three "eras". Before 1981, the State of Iowa kept very few records detailing the investigation and impact of reported fish kills. Fish kills were not at the forefront of public attention, and reporting of kills to the Department was virtually non-existent. Very little data exists from this time frame, and its reliability is suspect at best. It is not included in the database.

The second data era is from 1981 though the mid-1990's. Fish kills began to be reported with a greater frequency and rudimentary data was recorded. Quality checks on the data did not exist, resulting in some inaccurate or incomplete records of the events. In previous versions of the Fish Kill Database, these data were referred to as "Legacy Data". The data is included, but the records do not contain the detail and data quality of more recent events.

In the late 1990's, as water quality issues became an area of concern, fish kills were reported more frequently to the IADNR. Staff from Fisheries and the Field Offices were able to conduct more timely investigations, which resulted in improved cause and impact determinations.

Are there more fish kills now than before?

This is a difficult question to answer. It may appear there are more fish kills now than in years past, but it must also be noted that more attention is now being placed on fish kills. This increase in public awareness has resulted in citizens keeping a closer eye on local waters and reporting potential problems when discovered, rather than reporting them long after the event or not reporting them at all. The IADNR has also made a concentrated effort to improve the response time, data collection, and follow-up after a fish kill event, and to make the data collected available to the public.

The events of most concern are those of anthropogenic, or human-caused, origin. These events include fish kills caused by animal waste, ammonia and fertilizers, pesticide spills, and other pollutants. Animal waste has been the cause of the largest number of fish kills since the fish kills have been tracked. Animal waste can reach the stream and cause mortality through several pathways: run-off of field applied manure and open feedlots, confinement waste storage failures, and other accidental releases. There are a large number of unknown facility and animal types for animal waste kills, due largely to a lack of this type of information being provided in early data collection and record keeping. More recent fish kill data collection routinely records this data, when available, allowing the IADNR and other interested parties to determine trends and possible areas of improvement to reduce the number and impact of animal waste kills on waters of the state.

Natural/Environmental fish kills include those caused by disease, spawning stress, and temperature related events, including extreme heat, ice-out conditions, and dissolved oxygen “sags.” Typically, little follow-up is done on natural/environmental fish kills, as there is no responsible party that directly caused the kill. Non-point pollution sources may also present a chronic, poor water-quality condition that can lead to an increased likelihood of a natural/environmental kill, such as large algal blooms. For example, agricultural practices on the surrounding watershed may contribute excess nutrients, which in turn feed the algal bloom. When the algal bloom dies and decomposes, the dissolved oxygen levels drop severely and cause a fish kill.

When no clear cause can be determined by IADNR staff investigating the kill, the cause is considered “unknown.” There are several factors that can make assessing a cause for a fish kill difficult. On-site conditions can impair the ability of investigators to determine a cause, such as flooding or difficult physical access to the location. Contaminants can also travel long distances through tile lines, well away from the source. Delays in reporting a fish kill lessen the likelihood of a cause being found. When a kill is reported several days or more after the actual event, the stream’s flow will continue to dilute and dissipate the responsible pollutant and the resulting dead fish, making the determination of an exact cause, the magnitude of the kill, and a responsible party a difficult or impossible task.

Monthly Event Totals

The time of year has a noticeable influence on the occurrence of fish kill events. In the winter months, precipitation occurs as ice and snow and does not immediately run off the soil and into water bodies. The fields are not being worked, and frozen soil and manure are fairly stable. The first upticks in the number of events start to occur in April and May, when the ground thaws, the ice goes out, rains begin, and field work begins. May is the first month when agricultural fertilizers and pesticides are being applied, increasing the risk of incidents, especially if heavy rains should occur.

With the onset of summer, the weather tends to be drier. Traditionally summer, when streams and fish are most vulnerable, are the big months for fish kills. Even relatively small amounts of pollutants can stress or kill fish and aquatic animals like crawdads due to low stream flows and high temperatures in late July, August and September.

Assessment segments with the highest number of anthropogenic events

Clicking on a segment will navigate to Iowa DNR's ADBNet assessment database

Assessment Segment
# of Events