Evaluating fish kills on Iowa streams

By James Wahl
Iowa Dept. of Natural Resources

Fish kills have received a lot of media attention recently due, in part, to the size and magnitude of several kills that occurred this past winter. Although these kills were large, there have been many more small, less publicized kills in the past. The varied causes include livestock manure, commercial fertilizer spills, wastewater discharge, and natural causes such as winterkill. But all have one thing in common: loss of fish.

Whenever a pollutant finds its way to an Iowa lake or stream, there's a good chance it will kill some fish. In fact, the observation of dead fish often is the first report our department receives. Both the environmental protection division and the fish and wildlife division in our department investigate fish kills. Environmental protection staff try to determine the cause of the kill and locate the person responsible for the spill. They also work closely with the responsible party to correct the situation and prevent further damage to the environment.

My job as a fisheries biologist is to determine the distance of the kill and estimate the number of fish lost as a result of the pollutant. My first task is to determine the starting and ending points of the kill. I do this by checking for dead fish at all bridges downstream from the source until I encounter no dead fish.

Next, I estimate number of fish that have been lost. Because the length of a kill often exceeds several miles, it is impossible to count every fish. A sampling scheme is set up that accurately reflects how many fish have died in the section of stream being analyzed. We use a systematic sample with a random start, described in guidelines set by the American Fisheries Society (AFS). These guidelines for investigating fish kills are used by fisheries agencies throughout the United States. Subsample counts are made along 100-yard transects near bridges and also away from bridges. Several counts are made within every mile, and then expanded over the entire reach of stream impacted by the pollutant.

The monetary value set for fish is based on their replacement cost, also determined by the American Fisheries Society. AFS values represent the average cost of hatchery-raised fish. Values are organized by region, species and size, and are derived from a survey of public, private and tribal fish hatcheries in the United States. Values vary, with small baitfish (minnows) bringing the least at 8 cents per fish, to more expensive predatory fish (game fish) that are valued at $4 to $8 per pound.

Because all fish found in public waters belong to the state, the Department of Natural Resources seeks restitution from the responsible party for damages in an amount equal to the cost of the fish. This is not a fine, however, an administrative penalty up to $10,000 also can be assessed. The environmental protection division controls these penalties, which are determined by several criteria. They include degree of responsibility, such as whether the spill was from negligence or accidental; economic savings (was there a benefit to cut corners?); gravity of the violation (how extreme and severe were the losses?); and mitigating circumstances such as extent of cooperation. All these things are considered when determining what administrative penalty should be assessed.

At this time, lost recreational value and investigation and administration costs are not being charged to the responsible party. These areas are, however, under consideration. Lost recreation would be the number of fishing trips lost to that stream as a direct result of the kill. Each trip would have a monetary value, and that figure multiplied by the estimated number of trips lost. Field investigation costs would include salaries, meals, lodging and mileage of those investigating the kill and any associated administrative costs.

We often are asked how much time fish need to repopulate a stream that has had a kill. That's a difficult question and one we don't fully understand, but we do know recovery depends on the extent of the kill and the species and size of fish involved. In general, kills that are partial (some live fish observed) and involve small, short-lived species (minnows, shiners) have a fairly short recovery time. These streams may be repopulated within one year after a kill. On the other hand, complete kills that involve large, long-lived species (northern pike, smallmouth bass, suckers and carp) typically have a long recovery time. If adults of these species are killed, recovery could be from three to seven years, and even longer for the few fish that have reached trophy size.

A new program recently was established that should help streams impacted by fish kills. An agreement between the IDNR and the Iowa Department of Agriculture and Land Stewardship uses money collected for fish kills for environmental improvements on those streams such as installation of bank hides and other features for fish habitat. Over the past three years, nearly $215,000 was collected from fish kills and used for stream improvements in 25 counties. Hopefully, this program will help offset some of the damage to the local environment when fish kills occur.