Heavy Metal Trout in Hard Rock Rivers
11 May, 2026
Daniel Osmond examines how trout have adapted to living in some of our most polluted post-industrial landscapes.
Dr Daniel Osmond is a senior fisheries scientist for Westcountry Rivers Trust and research associate with Jamie Stevens’ Molecular Ecology group at the University of Exeter. He is an enthusiast of wild trout and the places where they live.
This article is reproduced with kind permission from the Wild Trout Trust, and first appeared in the Wild Trout Trust’s Salmo Trutta 2025.
What springs to mind when the countryside of the British Isles and brown trout are mentioned? Green and pleasant, with genteel fish at home in only the cleanest and most lordly of chalk-filtered waters?
Now, contrast this image with the words of Dr Kathleen Carpenter, almost 100 years ago, which she wrote after observing her local Welsh rivers, the Ystwyth and Rheidol:
“In the very heart of the ancient hills themselves, the rushing brooks run foul with mineral waste and treatment oils and acids, and their flow is choked with heaps of stones and rubble.”
Despite the manicured impression that we might often hold of our landscape and its rivers, they have been at the heart of industrial development, including the extraction of metal ores, for millennia.
During the 19th century, over half the world’s copper came from Cornwall alone. Even earlier than these industrial-era workings, throughout the Roman and Medieval periods, many upland watercourses were dammed and diverted before being released in powerful surges to wash away soils and expose valuable ores below – a process known as ‘hushing’ or ‘streaming’.
Beyond the now-abandoned engine houses and spoil heaps, our intensively industrial past has also left another legacy: that of toxic minewater pollution that continues to affect freshwater ecology in many regions.
A wild brown trout living in a metal-polluted river in west Cornwall.
Dissolved metals, such as arsenic, cadmium and copper, are particularly nasty for fish, binding to gills and causing suffocation, or causing oxidative damage after more prolonged exposure. Despite this, brown trout (Salmo trutta) have long been observed to persist in these metal-impacted systems as apex predators where many other more sensitive species are absent. Previous studies have shown a small number of metal-impacted trout populations in western Cornwall to be highly genetically divergent and extremely tolerant of this pollution (Paris et al., 2024, 2015). Yet this left several questions unanswered:
(1) Are the metal-impacted trout of western Cornwall a unique evolutionary oddity, or do we see repeated patterns of genetic divergence across regions of the UK with a shared history of exposure to metal pollution? (2) Are the patterns of genetic divergence the result of long periods of reproductive isolation and random drift, or are these trout populations adapting to survive in metal-polluted environments? And (3) Given a different starting genetic point (as a consequence of local adaptation and different recolonisation routes after the last glacial maximum), do we see trout populations respond with unique or shared genetic changes?
These are the questions that I sought to understand over the duration of my PhD studies – the results of which I’ll explore below.
Sample collection and genetic analysis
During the summers of 2020 and 2021, with the support of Game & Wildlife Conservation Trust (GWCT) and Westcountry Rivers Trust, we collected genetic samples from trout in four regions of the British Isles with a shared history of mining industry: Cornwall, south east Ireland, north-east England and western Wales.
These were collected from sections of river directly downstream of point sources of metal pollution, and we also sought out control samples from relatively unimpacted tributaries in the same catchment, or nearby chemically-similar catchments. At a number of these sites, we were amazed to turn on the electrofishing anode and see fish appear, despite previous reports from the Afon Gogh in Anglesey and Avoca in Ireland suggesting that fish were absent from the sampled reaches.
Electrofishing at Cwmystwyth to collect tissue samples from trout in post-industrial rivers, assisted by Dylan Roberts of Game & Wildlife Conservation Trust (GWCT).
After collecting the samples, we used a combination of different genetic methods to understand these populations. Firstly, we screened over 1,200 fish from across 68 individual sampled sites using a panel of 95 neutral genetic point mutations (SNP-assay: see Osmond et al 2023). The results of this informed our closer look at 149 of these fish from some of the most highly-divergent populations using whole genome sequencing, enabling us to look at changes to all of the genetic information, including those parts responsible for adaptation. We also produced a new genome assembly of a trout from the highly metal-polluted River Hayle in Cornwall, allowing us to compare the larger genomic changes within this unique population.
A point source of minewater pollution from an adit at Cwmystwyth mine into the River Ystwyth.
Adapting to metal pollution
Across the various different regions of the British Isles which we examined, our SNP assay revealed repeated patterns of genetic isolation of the most highly metal impacted trout populations using. Indeed, among all of the populations compared across UK and Ireland, the most major level of genetic differentiation observed was driven by a split from trout found in the highly metal-impacted River Hayle (Figure 1). We then observed nested genetic structure: the first level of genetic differentiation was between geographic regions, and within these regions, we then observed strong levels of genetic differentiation within populations of trout that were in highly metal-impacted regions and those isolated by barriers from the control populations.
Figure 1. Sampling sites for brown trout (Salmo trutta L.) across four regions of the British Isles with a history of metal-extraction and ongoing legacy mine water pollution. Sampled catchments are labelled in their respective inset maps for each of the four major regions and the relative metal-impact at each sampled site is indicated by the colour of the point.
This was especially notable, for example, in north-east England, where we saw little genetic difference at these neutral markers between the control individuals in the Rivers Tyne and Wear from one another, but saw very low genetic diversity and high genetic isolation in each of the three metal-impacted populations (Figure 2). Using demographic history modelling, simulating changes to populations through time, we saw that divergence and population contractions, which happened in relatively recent times and coincided with periods of peak metal extraction, most credibly explained the variation we observe in trout populations today.
Figure 2. A genetic structure plot (DAPC), showing the first three major axes of variation between sampled trout (Salmo trutta) populations at the 95 genotyped genetic markers. Each point represents an individual brown trout, with the colour representing population of origin. The 'genetic average' of each sampled location is labelled within a box. The figures give the first three discriminant functions of the observed genetic variation, with the amount of variation explained by each axis labelled accordingly. (a,b) Analysis of all 1139 genotyped trout from across Wales, Ireland, north-east England and Cornwall. (c,d) Analysis of 435 individuals from across Wales, excluding populations with physical barriers to migration. (e,f) Analysis of 187 individual trout from across northeast England, representing 11 individual sampling locations from the Tyne and Wear.
This study has highlighted the remarkable adaptive potential of brown trout populations to survive even within acutely toxic polluted environments.
Looking more closely at the genomes of some of these divergent populations, we observed regions of repeated genetic change between metal-impacted and their paired control populations. These genetic changes were involved in pathways such as ion transport, regulating oxidative damage and hormone signalling pathways – all of which are known to be impacted by metal exposure.
Interestingly, trout have a number of duplicate copies of genes, a characteristic which originated from a whole genome doubling event approximately 80 million years ago. With double copies of these genes, evolution is less constrained, as the original function of any given gene can still be maintained even if the duplicated version mutates. In this study, we identified a region of DNA under selection in all of our populations which appears to have had not just point changes of an individual nucleotide here and there, but large duplications and insertions of additional coding sequence associated with this adaptation. Crucially, such adaptations seem to be more common and shared in populations within geographic regions, rather than being driven by the impacts of similar metal pollutions.
Underneath the scales of the humble trout lie astonishing evolutionary stories. Genome duplicating events 80 million years ago have left these fish with extra copies of genes: this redundancy allows for new adaptive functions to arise.
In rivers where concentrations of metals would be acutely toxic to trout from relatively clean rivers, and we see trout populations which have adapted to these toxins but are still isolated by historic chemical barriers, cleaning up our ancestors’ legacies of mining waste will allow these populations to interact with other trout again, and be ‘genetically rescued’ for the future by restoring their full potential ‘toolkit’ of genetic diversity.
Implications for conservation and management
The results of this study have highlighted the remarkable adaptive potential of brown trout populations to survive even within acutely toxic polluted environments. The repeated trend of genetic isolation and reduced diversity within these impacted populations however suggests that these populations are vulnerable to future stressors, with low diversity limiting their adaptive potential. The shared adaptations, particularly within populations within the same geographic region, suggests that standing genetic variation is very important to enable effective adaptation of populations. This also highlights the importance of reducing barriers to fish to migration within and between rivers, allowing adaptive variation to be shared between populations which would otherwise be completely isolated.
Previously, it has been considered that different mixtures of metals (such as copper, lead, and cadmium) have different toxicity pathways and therefore should produce different physiological responses, observed through selection at different parts of the genome.
Our identification of a highly differentiated genomic region associated with metal-adaptation across populations exposed to different mixtures of metals, however, suggests that there are common adaption mechanisms, through hormone-signalling pathways and ion transporters.
And finally, given the remarkable ability of trout to adapt to such hostile environments, the question has been asked: why do we need to bother cleaning up pollution at all?
The answer seems to be: in rivers where concentrations of metals would be acutely toxic to trout from relatively clean rivers, and we see trout populations which have adapted to these toxins but are still isolated by historic chemical barriers, cleaning up our ancestors’ legacies of mining waste will allow these populations to interact with other trout again, and be ‘genetically rescued’ for the future by restoring their full potential ‘toolkit’ of genetic diversity.
Interested in finding out more?
For more information about this research, read Daniel's full paper which was published in Diversity and Distributions in 2024.
You can also listen to Daniel’s guest episode, Heavy Metal Trout in Hard Rock Rivers, in season 2 of Will and Ben the Wildlife Men, a podcast series hosted by naturalists Ben Porter and Will Hawkes, available via Apple Music and Spotify (below).
With such a rockin' title, our curiosity got the better of us and we had to ask Dan if he is a fan of heavy metal... turns out his musical preference is rather less intense, with The Lowest Pair playing presently, and melodically, on his jukebox.
On another fish-inspired musical note, Dan also provided an honourable mention for the Eurofishion Song Contest by the World Fish Migration Foundation, with his favourite entry being 'Migration Vibes', about the New Zealand long-finned eels. Enjoy!
Thanks
Many thanks to Dr Dan Osmond and Wild Trout Trust for allowing us to reproduce this article.
This research was funded by a GW4 NERC FRESH Studentship, with support from Game and Wildlife Conservation Trust and Westcountry Rivers Trust, as well as the EU Interreg France-England Channel project: The SAlmonid MAngement Round the CHannel (SAMARCH).
References
Osmond, D.R.; King, R.A.; Russo, I.M.; Bruford, M.W.; Stevens, J.R. 2024 Living in a post‐industrial landscape: repeated patterns of genetic divergence in brown trout (Salmo trutta L.) across the British Isles. Divers. Distrib.30, e13854. https://doi.org/10.1111/ddi.13854
Osmond, D.R.; King, R.A.; Stockley, B.; Launey, S., Stevens, J.R. 2022 A low‐density single nucleotide polymorphism panel for brown trout (Salmo trutta L.) suitable for exploring genetic diversity at a range of spatial scales. J. Fish Biol. jfb.15258. https://doi.org/10.1111/jfb.15258
Paris, J.R.; King, R.A.; Shaw, S.; Lange, A.; Bourret, V.; Ferrer Obiol, J.; Hamilton, P.B.; Rowe, D.; Laing, L.V.; Farbos, A.; Moore, K.A.; Urbina, M.A.; Catchen, J.M.; Wilson, R.W.; Bury, N.R.; Santos, E.M.; Stevens, J.R. 2024 Combining population genomics and transcriptomics to identify signatures of metal tolerance in brown trout inhabiting metal-polluted rivers. https://doi.org/10.1101/2024.05.30.595956
Paris, J.R.; King, R.A.; Stevens, J.R. 2015 Human mining activity across the ages determines the genetic structure of modern brown trout (Salmo trutta L.) populations. Evol. Appl. 8, 573–585. https://doi.org/10.1111/eva.12266
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