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Otter Project

Chemical contamination of freshwaters

Otters are top predators in river ecosystems, making them excellent indicators of environmental health.

Our research

Pollutants accumulate in their bodies over time, so by analysing otter tissues, we can track chemical pollution in the environment. Using archived samples, we examine how contamination has changed over time, assess the success of environmental policies, and identify where more action is needed.

Once widely used, PCBs and OCPs (particularly DDT and dieldrin) caused major declines in otter populations during the mid-20th century. Though now banned, their environmental persistence means they continue to accumulate in wildlife, and can reach high concentrations in top predators.

Our research found concentrations of PCBs and OCPs in otters generally declined between 1992 and 2009 (Kean et al. 2021). However, more recent data on PCB concentrations from otters collected across Wales between 2010 and 2019 show concentrations have been rising again since 2010 (O’Rourke et al. 2026). Some otters had PCB concentrations above a toxic threshold known to affect reproduction in American mink (Kean et al. 2021; O’Rourke et al. 2026).

We’ve also seen links between these pollutants and signs of poor health, such as changes in organ mass and reproductive indicators, though results were variable between individual chemicals (Kean et al. 2013).

PBDEs were widely used as flame retardants in furniture and electronics from the 1970s to the 2000s. Analysis of otters found 1995-2006 found concentrations similar to those seen in marine mammals, with higher concentrations in more urbanised areas (Pountney et al. 2015). Declines in concentrations in otters found between 2010 and 2019 (O’Rourke et al. 2025), suggest that regulations have been effective at reducing environmental concentrations.

PFASs are found in a wide range of products, including waterproof clothing, nonstick pans and food packaging. In Northern Europe, higher PFAS concentrations were observed in otters than in marine mammals or birds of prey, suggesting PFASs predominantly enter the environment from human activities via freshwater pathways (Androulakakis et al. 2021). In England and Wales, the highest concentrations were found in otters from areas with large wastewater treatment plants, arable areas (implicating sewage sludge application), and near a factory that used PFOA in PTFE manufacture (O’Rourke et al. 2022). High PFOA concentrations were still detected in otters collected near the factory even after it had ceased using the chemical, demonstrating the persistence of these chemicals (O’Rourke et al. 2024a). We have also found replacement PFASs in otters (substitutes for regulated compounds, like PFOS and PFOA), raising urgent questions about the risk of these unregulated alternatives (O’Rourke et al. 2024a).

While metals occur naturally, human activities like mining and industry have greatly increased environmental concentrations. Metals have been detected in otters, with mercury and cadmium concentrations shown to increase with age, reflecting their bioaccumulative nature (Walker et al. 2010, Walker et al. 2011). Zinc concentrations have shown a negative correlation with otter body condition, suggesting that zinc pollution of aquatic systems is impacting otter health (O’Rourke et al. 2024b). Following the introduction of regulations on lead in petrol from the 1980s, analysis of lead in otter bone revealed a decline in concentrations between 1992 and 2004, mirroring reductions in emissions (Chadwick et al. 2011). Declines in chromium and nickel concentrations in otter liver have also been observed, likely reflecting reduced industrial use (Brand et al. 2020). Current research is exploring higher concentrations in areas where metal mining has taken place.

Informing policy and driving change

Our research helps shape national policy. Our data feeds into the UK government’s H4 Indicator Programme, part of the 25 Year Environment Plan. This project tracks chemical pollution in abiotic and biotic samples from across terrestrial, freshwater, and marine environments. We’ve contributed samples, analysis, and advice for the latest H4 Indicator Report.

Our research has been cited in publications by Natural Resources Wales, Environment Agency and Government reports. Findings have also been used by environmental charities and featured in national media, raising public awareness of chemical threats. This influence demonstrates the value of otters as sentinels, and how biomonitoring can guide decision-making to reduce pollution and protect ecosystems.

Looking ahead: emerging contaminants

We recently participated in the LIFE APEX Project, which aimed to harmonise methods across Europe to demonstrate the value of top predators in chemical monitoring. Using advanced target and non-target screening, the project assessed the presence of over 65,000 chemicals. This wide-scope analysis detected a broad range of substances, including pesticides, pharmaceuticals, and PFASs, highlighting emerging threats to wildlife, ecosystems, and human health (Gkotsis et al. 2022).

We continue to collaborate with government agencies and analytical laboratories to identify pressing policy questions and emerging contaminants of concern. By expanding the range of pollutants we can analyse in otters, we will continue to provide critical data to inform environmental policy and regulatory decisions.

Relevant publications: