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.

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.