Poisoned Waters: How rodenticides are quietly contaminating Britain's waterways and wildlife
SGAR poisons are implicated in deaths of fish, otters and birds
This is the third in a series of articles highlighting the need to ban rodenticides in the UK. In previous weeks we have explored how UK councils are failing in their legal obligations to avoid causing unnecessary suffering and how rodenticides are killing countless non-target animals. You can read the articles here and here. In the coming weeks we will expose the shocking lack of regulation in the ‘pest’ control industry, and examine how humane alternatives can be used to control rodent populations. We will be making the case for local councils and private users to radically change their approaches to wildlife control in our communities.
Few people are really aware of the horrific suffering caused to rats and mice who have consumed anticoagulant rodenticides. Fewer still are aware that Second Generation Anticoagulant Rodenticides (SGARs) pollute our seas and rivers, affecting birds, mink, otters and aquatic life too.
Otters, for example, consume SGARs by feeding on poisoned rodents, who typically survive for an agonising 3-10 days before dying from internal bleeding. But this is by no means the only explanation. There is another major source of contamination which is largely overlooked: sewer baiting.
Sewer baiting
Sewer baiting is the practice of laying rodenticide bait within the sewer network, and both councils and privatised water companies are spending significant public money funding it. Yet the consequences extend far beyond sewers. Researchers have indicated that sewer baiting may well be to blame for exposing otters, fish and other creatures to SGARs.
Heavy rainfall can flush these poison pellets directly into rivers. Rodenticides may also survive inadequate treatment at water treatment plants, allowing them to enter waterways and therefore introducing them into the aquatic ecosystem.
The UK Chartered Institute of Environmental Health’s 2021 revised National Sewer Baiting Protocol now requires that “in sewers, baits must be applied in a way that they do not come into contact with water and are not washed away.” But research suggests that it is effectively impossible to prevent baits being washed away at times of high rainfall.
A German study from 2020 investigated how SGARs enter river systems in the first place. The researchers established a direct link between rat poison bait placed in combined sewer systems and the presence of SGAR residues in wastewater treatment plant influent, effluent, and freshwater fish livers. Contamination levels rose significantly during and after heavy rainfall events.
The research confirmed that conventional wastewater treatment does not fully remove these compounds, meaning they pass through treatment plants and enter rivers via discharged effluent and combined sewer overflows. The authors called for further research into ecological risks for aquatic organisms and fish-eating predators from ongoing low-level exposure to SGARs.
Research on rodenticide contamination of UK waters is inadequate
The issue of rodenticide contamination in waterways has yet to be investigated fully in the UK. However, research conducted outside the UK has found that fish such as salmon and mullet are poisoned by directly consuming rodenticide pellets which have been washed into oceans or waterways - or by consuming contaminated algae, invertebrates or other prey. Animals that feed on them are, in turn, poisoned.
The aquatic environment can also be contaminated through pollution from rodenticide manufacturing and inadequate disposal of waste, as well as the carcasses or urine of poisoned animals entering water systems.
The UK Health and Safety Executive’s Wildlife Incident Investigation Scheme (WIIS) carries out tests on animal corpses which have been submitted to them by government bodies, local authorities or members of the public when there is a suspicion of pesticide or rodenticide poisoning. WIIS’s latest data for 2025 reported 18 cases of animal carcasses containing traces of rodenticides being found in or close to water.
Where rodenticides have been used intensively, researchers have found them to be present in soils and sediment. Studies have also found that the presence of rodenticide can inhibit the growth of algae.
Otters
According to a 2025 report by Wildlife Poisoning Research UK (WPRUK), data obtained from WIIS showed that a shocking 90.5% of Eurasian otters tested had been exposed to SGARs.
WPRUK wrote that in nearly half of the otter carcasses tested, a potentially lethal dose of rodenticide was found in their livers. In three cases, rodenticides may have been the cause of death. However, this is likely to be the case for many more of them. Of course, the data only shows results for otters whose bodies were handed to WIIS for testing: the real number of victims is uncountable.
Bromadiolone was the most prevalent poison detected in the otters tested between 2009 and 2024. The poison was found in 81% of the animals, followed by brodifacoum at 76.2% and difenacoum at 61.9%. The proportion of British otters exposed was found to be far higher than in comparable studies from Germany (67.2%), California (31.5%), and Pennsylvania (17.1%).
WPRUK concluded its report by affirming that more research is needed. No published data appears to exist on SGAR levels in UK rivers, and the report calls for urgent monitoring of sewage effluent and river life. It stated:
“If it is proven, as it has been in other countries, that SGAR poisons are entering the aquatic ecosystem, then the regulations permitting the use of these poisons in sewage systems need to be urgently reviewed.”
Mink
Protect the Wild has analysed the data published by WIIS. The figures show that, of three mink carcasses submitted to WIIS for testing since 2016, two showed traces of rodenticides. Mink live near riverbanks and waterways, are good swimmers and often feed on fish. Obviously the number of mink affected by rodenticide is likely to be far higher.
Birds
WIIS also analysed the corpses of Gulls, Swans, Herons, Geese and Ducks. Over 20 of their bodies were confirmed to contain rodenticide residues.
In 2024, an Osprey, a fish-eating bird of prey, was found dead with brodifacoum in her liver.
Fish
One startling omission in the WIIS figures is the lack of research on fish in the UK. We don’t currently know the number of fish affected by rodenticides, but data from outside the UK suggests that it is likely to be significant.
In 2019, German researchers published a study to provide evidence of anticoagulant rodenticides in freshwater fish. They analysed bream liver samples from 17 sampling locations in 2011 and 18 sampling locations in 2015. Brodifacoum was detected in 88% of samples in 2015, with difenacoum, bromadiolone, flocoumafen and difethialone also present.
German study shows threat to fish-eating birds
A 2024 study examined SGAR contamination in fish-eating birds and other wildlife across several German regions. The researchers analysed liver samples from 96 Great Cormorants, 29 Common Mergansers, various freshwater fish species, and 42 coypu. SGAR residues were found in almost half of the Cormorants and Mergansers, with brodifacoum, difenacoum and bromadiolone compounds detected almost exclusively. By contrast, no SGAR exposure was found in the coypu.
The researchers concluded that because coypu live in the same aquatic environment but eat plants rather than fish, the contamination in Cormorants and Mergansers was via consumption of contaminated fish, rather than through direct contact with water. The study provides further evidence that SGARs released from sewer systems are accumulating progressively up the food chain to fish-eating predators.
Switzerland has regulated SGARs to protect wildlife and water systems
The Swiss authorities banned the use of anticoagulant rodenticides for private individuals in 2025. The move tightened up earlier legislation passed in 2017. Swiss advice states that “the use of rodenticides” is only eligible for authorisation “under exceptional circumstances”.
The legislative move came after the publication of a Swiss study of deaths of non-target wildlife in 2022, which documented the effects on songbirds, foxes, birds of prey, hedgehogs, otters and fish. The report indicated “widespread contamination of the aquatic and terrestrial environment” by rodenticide poisons.
However, banning SGARs for private users isn’t enough to protect our wildlife and rivers. A complete ban is needed.
The UK needs to catch up
Although the data regarding the risk that rodenticides pose to fish and fish-eating predators in the UK is currently inadequate, WIIS has documented that SGARs affect otters, water birds and mink. If we take the evidence from other European countries into account, then it’s fair to assume that British fish are affected too. We need to stop the practice of sewer baiting and ban SGARs. Rodenticides cause unnecessary suffering to both rodents and non-target wildlife. They are a threat to our shores, waterways and aquatic life too.
Our demand: An immediate end to rodenticide use
Protect the Wild is demanding a complete and immediate ban on the use of rodenticide baits.
Rodenticides are a threat to our waters: The effect of poor sewage management on the UK’s shorelines and rivers is well known. But rodenticides pose a serious threat too.
Sewer baiting needs to be banned: The current protocols for sewer baiting are inadequate to prevent our seas and rivers from being polluted with rodenticide. The practice of sewer baiting needs to stop now.
Rodenticides are killing non-target wildlife: Poison bait does not and cannot discriminate between target and non-target species. Killing protected wildlife is a crime.
Rodenticides are causing unnecessary suffering: The slow, agonising death inflicted by SGARs – which cause internal bleeding over many days – may itself constitute an offence under the Animal Welfare Act 2006, which prohibits the causing of unnecessary suffering to animals.
We are already obliged to do better: Existing legislation, as well as the pest control industry’s guidelines, require that preventative methods such as rodent proofing and prevention measures are exhausted before any rodenticide is used. The routine use of poison bait as a first resort by both water companies and local authorities is likely unlawful.
Humane solutions are not complicated and there are companies out there who follow an ethical approach to managing wildlife in public areas, including sewers and drains. Check out Humane Wildlife Solutions, for example.
As animal rights advocates, we urgently need to step up efforts to stop the silent killing of our wildlife by SGARs and other poisons. We need to make our case loudly and set out the alternatives clearly. The routine use of lethal poisons causes immeasurable suffering and it needs to stop.
It's a wonder we have any wildlife left. Mankind has ever more ways to finish it off.
'The UK needs to catch up' as usual., because, as usual, vested interests are pulling the strings. This country needs real change.