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The wetland detectives How new technology is helping us survey aquatic life
MEET THE EEL DETECTIVES
Ingenious technology that enables detectives to catch criminals is set to revolutionise our work to protect elusive endangered species. Derek Niemann finds out how it all began with a fish as ‘slippery as an eel’…
ast summer, volunteer citizen
Lscientists were scouring waterways and ponds to help a special fish that is critically endangered. But they weren’t actually looking for eels – they were testing the water.
Nobody is in any doubt that European eels are in deep trouble. WWT’s Principal Research Officer, Laura Weldon, has spent the past five years studying them: “Numbers have plummeted in my lifetime. There are now so few, and yet they are still so understudied because nobody was particularly interested in them before – they were everywhere.” And eels are a valuable guide to the health of a waterbody, since they are both predator and prey.
THE QUEST FOR KNOWLEDGE
Laura is grappling with understanding the causes of their decline. “We are keen to start mapping eels to find out exactly where they are. Part of the reason for eel declines is that there is insufficient good-quality habitat in which they can grow and stay healthy. We’re interested in learning where they are and why. Are they making it upstream to mature? Which barriers, such as weirs, can they get past and which are impassable, and what are they doing when they get through? Traditionally, it was only the older females that were found far inland and their numbers have just dropped and dropped. Exactly how far are they getting?”
Eels are notoriously tricky to monitor. Despite their silvery bodies, they are often hard to see, slinking between waterweeds, or obscured from view by the sun’s reflection on the surface. Given their scarcity over such a huge network of watery possibilities, they are difficult and expensive to catch with conventional fishing nets. Hauling them out of the water is an invasive way to count such a rare species too. Could there be a more benign method of detecting their presence?
One answer lies in a branch of science that has expanded exponentially, ever since police detectives discovered that criminals do not simply vanish without trace from the scene of their crime. Invisible and irrefutable evidence remains; the unique DNA that every human leaves in their blood, hair and even flakes of skin. Within the past few years, conservationists have begun to apply the same forensic techniques to wildlife, seeking environmental DNA (eDNA). And such techniques appear to work particularly well with creatures that live in water. Simply collect the water and, if an animal is present, strands of its DNA, its genetic signature, will be there.
Eel image: Neil Aldridge/WWT
We used to have to catch eels in order to confirm their presence in the lakes around our reserves. Now we can just sample the water for their DNA
EEL DETECTORS
Does it work for eels? Laura began a PhD on eDNA, trialling live eels in a Bristol Zoo aquarium, then tested Slimbridge’s restored Rushy Lake and gained her first positive results. The lake contained eels, and the eDNA testing proved it.
Since then, more trials in Ireland have confirmed eDNA’s applicability to eels in a variety of locations. Not only that, when set against monitoring using conventional nets, eDNA registered eels in places where nets had come up empty at the first time of dipping. Testing could give a reliable indication of presence or absence, while the amount of DNA gave a rough idea of whether the animals were plentiful or rare.
Generous funding from the Green Recovery Challenge Fund then enabled Laura – by now working for WWT – to furnish Slimbridge’s Wetlands Science Laboratory with equipment for DNA testing. Through last summer, volunteers in a pilot project sampled watercourses and ponds near the River Severn.
Laura says: “One of our eDNA days was at Bridgwater Meads, a restoration site in
How to study eDNA
1PICK YOUR TIME
Many of the aquatic creatures in temperate rivers, lakes and ponds, such as eels and fish, are largely inactive during the winter months, so they will be producing very little eDNA material. Testing waits until spring when the water temperature rises above 8°C. By then, the fish are shedding lots of eDNA, in the form of mucus, scales and faeces.
4POST AND UNPACK
The filter gauzes are sent to the WWT Wetlands Science Laboratory for analysis. The lab relies on a sizeable number of samples from a number of locations to remove bias from the results. For example, fish may shoal in one spot and be absent from another only a few metres away. The DNA is extracted from the buffer solution.
2COLLECT THE SAMPLES
Wearing gloves to prevent cross-contamination, volunteers scoop up a container of water from a number of locations on the river, stream or pool. Only water at the surface is collected since eDNA degrades quickly, so it’s best to catch it before it settles at the bottom. Surface water gives an indication of the recent presence of the target species.
5TAKE A PCR TEST
Using the same chemistry that has been adapted for Covid testing, a machine takes a polymerase chain reaction (PCR) test – a molecular examination of genetic material. It is able to recognise short sequences in the DNA – the signature barcode of a species – and then copy them repeatedly so that the sample is big enough to analyse.
3FILTER THE SAMPLE
It’s not the water itself that’s being tested for eDNA but microscopic material suspended in it. The water is poured through a single-use filter, a round disk with a fine mesh gauze that traps all of the important debris. A buffer solution is added as a fixative to preserve the cellular content, because eDNA degrades quickly.
6IDENTIFY THE SPECIES
To identify the species in a sample, we check the patterns of eDNA against a reference library that has different sections of code relating to individual species. As every species has a unique barcode signature, it relies on matching codes to samples. It’s not yet a comprehensive library – only about 66% of Cambodian fish can be identified.
The Lower Mekong region in Cambodia is home to some of the world’s most important wetlands. We’re working with local partners to protect them
AN eDNA REVOLUTION
In the UK, single species eDNA monitoring is being carried out on other threatened species to assess their presence, including great crested newts and rare fish such as allis shad and twaite shad. Other species could follow, such as the lamprey, a fish whose larvae burrow deep and so is phenomenally difficult to survey by traditional means.
WWT is also using metabarcoding – a technique to identify plants and animals using DNA and rapid DNA sequencing – to try to understand whether natural flood management increases the number and range of invertebrates in a body of water.
From the water samples we took last year we identified about 400 species, providing a snapshot of what was around at the time.
Aquatic eDNA is becoming an established tool for monitoring fish, amphibians and other wildlife
Somerset. We were able to demonstrate that there were lots of eels using the waterways all around, but few actually on the site itself. Once the restoration is complete, we can go back and sample the Meads. If the eels love it, their DNA will be all over the place.”
MEASURING CHANGE
Plans are under way to extend testing, both in the UK and overseas, starting in the Mekong Delta of Cambodia, where WWT has been working for more than a decade. Research Officer Jon Greenslade explains: “We are targeting 80 square km of wetlands, a landscape that is incredibly important for sarus cranes, but one that is going through massive land-use changes with the introduction of rice and shrimp farming. One way of measuring the effects of agriculture on wildlife is to study the impacts of these changes on fish populations.
“Sampling using traditional methods with nets will only get you the fish that net selects for. For example, big mesh nets are designed to catch bigger fish and will let little fish slip through. But eDNA testing can tell you everything that’s in the waterbody. Local people will sample wetlands for freshwater fish, and then, in the lab, we’ll use eDNA metabarcoding. It’s like reading a full orchestral score, where we can identify a host of different fish species in a single sample of water.”
Home and abroad, eDNA monitoring is adding a whole new dimension to our understanding of tricky wetland wildlife. Its use is set to spread yet wider. n
EXPLORE OUR WORLD
Discover more about European eels and our work to protect them on our website: wwt.org.uk/severn-vale
