LARAMIE — Fisheries biologists with the Wyoming Game and Fish Department believe an herbicide was responsible for killing brown trout in Laramie’s Spring Creek in late May.

Laramie residents contacted the department on May 26 after discovering several dead brown trout in Spring Creek. Fisheries Biologist Steve Gale conducted an evaluation of Spring Creek from 15th Street downstream to 8th Street and observed numerous dead trout. Multiple size classes of brown trout were affected. No other species of fish were found dead. brown trout are the most abundant fish species in Spring Creek.

Gale tested the water quality, including oxygen levels, pH, salinity levels, and dissolved solids and said all parameters were within normal values.

“Our meter measures the quality at the time of testing, so whatever had happened had already gone through the system,” he said. He notified the City of Laramie and collected more than 20 dead fish and sent them to the Wyoming Game and Fish Department’s Fish Health Laboratory to determine the cause of death.  

“We believe it was caused by an herbicide,” Brandon Taro, Fish Health Program Coordinator, said. “All the fish had enlarged livers, which is consistent with the effects of herbicides on fish.” Gale said algae were dead from where a storm drain empties into the creek below the 15th Street Bridge downstream to the 9th Street Bridge. The algae above the storm drain were still green and unaffected. “We’re not able to pinpoint the exact cause, but everything is consistent with an herbicide poisoning,” Taro said.

If the use of an herbicide on a broadcast basis is necessary, the city of Laramie would like residents to consider the following points:

Choose the correct herbicide: Is an herbicide application really needed? Is the weed problem likely to persist and cause more problems or will it die out after a short time? Should a preemergence (preventive) weed treatment be used or should herbicides be applied after a weed problem develops? Also, consider what weeds have been present in the past, study recommendation books and labels, and seek advice from an expert in the field.

 Apply it properly: Herbicides often are overapplied in urban areas because the amount needed was not calculated or the application equipment was not calibrated. Over-application can cause damage to trees and other desirable plants. Read the label carefully. Make sure all directions and warnings are understood before use, and follow all label directions. Rinse empty herbicide containers at least three times and put the rinsate back into the spray solution. Do not use an herbicide unless it is registered for use on the specific type of turfgrass.

Be sure application equipment is properly calibrated and in good working condition before use. Do not mix or apply more than the amount needed. If possible, leave an untreated strip next to curbs, driveways, and sidewalks and use other weed control methods in these areas. This will keep the herbicide away from areas prone to runoff. Do not apply a preemergence herbicide when heavy rains could cause runoff. Also, do not apply postemergence materials in windy conditions that would cause them to drift onto sensitive plants or runoff areas.

Water after application: The amount of rainfall or irrigation after an herbicide application is important in preventing runoff. Watch the weather. Postemergence herbicide applications should be made at least 24 hours before a rain or irrigation. A slow, gentle one-half inch rainfall after application of a preemergence herbicide is ideal if all the rainwater soaks into the soil. If no rain is in the forecast, water the treated area with one-half inch of slow sprinkler irrigation to dissolve the herbicide. Normal wetting and drying cycles will keep the herbicide in the top one to six inches of the soil profile. Water as infrequently as possible but try to wet the soil to a depth of four to six inches when applying.  

Clean up property and equipment: Do not apply or spill the herbicide material on the driveway, sidewalk, curb or street. If a dry herbicide formulation is spilled, sweep it up immediately. If a liquid herbicide formulation is spilled, first absorb it with soilor other inert absorbent then sweep it up and dispose at an appropriate site. Do not wash spills into the streets and storm drains.

Following an application, it is important to properly clean the equipment. Equipment rinse water can be sprayed over the area just treated. Never empty residues onto bare ground. Never pour rinsate, unused herbicide concentrate, or mixtures into storm drains, sewers, onto the driveway or down the sink. This can contaminate water supplies. Dispose of leftover concentrate and containers according to state and local regulations or at household hazardous waste collection sites.