Rodent control measures are contaminating fish

Rodenticide use in sewer systems causes extensive water pollution

ein Rattenschatten an der WandClick to enlarge
The active substances in rodenticides pollute bodies of water through wastewater.
Source: Lugaa /

Rat poison that is used in municipal sewer systems is a source of emissions to surface water where it bioaccumulates in fish liver. These are the findings of a new research project done on behalf of the German Environment Agency (UBA). The active substances used in baits are not degraded completely in wastewater treatment plants and are thus channelled to rivers. There are legally binding regulations that prohibit the bait from coming into contact with wastewater. President Dirk Messner of the German Environment Agency said: "Cities and municipalities must comply with regulations to protect waters when carrying out chemical rat control measures. There are already numerous innovative solutions on how this can be implemented, for example through special bait protection stations. Fish in our rivers may no longer be allowed to become an unwanted target for pest control measures."

Most of Germany's cities and municipalities apply rat control measures in their sewer systems using rodenticide-containing feeding baits. The rodenticides contain anticoagulant active substances and lead to death from internal bleeding within a few days after the bait has been consumed. Bait is deployed in sewer manholes by wire. The research shows that heavy rainfall and backflow incidents can trigger the release of anticoagulant rodenticides from baited sewer systems into wastewater. These anticoagulants are not removed entirely during conventional wastewater treatment and are thus introduced to surface water together with treated water. The current study corroborates these findings based on analysis of anticoagulant rodenticides in the liver samples of carp which were kept for half a year in ponds containing treated municipal effluent.

The study includes the analysis of numerous environmental samples collected in wastewater treatment plants and rivers in the course of municipal rat control measures. At the same time, fish from the sampled streams were analysed for traces of rodenticide. Nearly every one of the fish liver samples analysed in the research project – perch, pikeperch, brown trout, chub, and gudgeon – contained residues of rodenticides, namely of so-called second-generation anticoagulants and chiefly brodifacoum, difenacoum, and bromadiolone.

These active substances are persistent, bioaccumulative and toxic, or in other words, they are not easily degraded in the environment, accumulate in aquatic organisms and have a poisoning effect. A follow-on UBA project by the Federal Institute of Hydrology in collaboration with the Bavarian Environmental Protection Agency investigates what the exact effects of rodenticide residues are on fish health. Health risks for humans from the consumption of anticoagulant-contaminated fish was not the subject of the investigations in the research project.

There are strict and legally binding requirements for the use of rodenticides throughout the EU. The baits must not come into contact with (waste)water in order to prevent active ingredients from being released into the treated effluent or being washed off. This requirement is not consistently implemented in current application practice in many cities and municipalities, according to the nationwide survey on municipal rat control carried out in the research project. The survey revealed that in 2017, bait was mainly hung on wire in the sewer manhole. In view of current research results, the baits must be prevented from coming into contact with (waste)water for the protection of the aquatic environment. There are also practicable alternatives: Various manufacturers of bait protection stations or traps already offer professional and smart solutions for effective, legally compliant and environmentally sound rat control in sewers.

The final report which has now been published is the result of a research project commissioned to the Federal Institute of Hydrology by the German Environment Agency.

Umweltbundesamt Hauptsitz

Wörlitzer Platz 1
06844 Dessau-Roßlau

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 Rodentizide  Rattenbekämpfung  Rattengift  Antikoagulanzien  Fischbelastung  Gewässerbelastung