Freshwater ecosystems, essential for biodiversity and human well-being, face numerous threats, including habitat degradation, invasive species, and climate change. Among these challenges, the European eel (Anguilla anguilla) stands out as a critically endangered species.
In her doctoral thesis, Silje Halvorsen delved into recent research employing environmental DNA (eDNA) to investigate the challenges faced by eels and other freshwater species and to explore potential solutions for their conservation.
Cheaper and Better Than Traditional Traps
Traditionally, researchers have used nets, fishing rods, or traps to determine the types of fish inhabiting a water body. This method can be expensive, time-consuming, and harmful to fish populations.
“Collecting a water sample takes just a few minutes and doesn’t disturb the fish. The method is also the most sensitive. It can provide information about which species were present in a lake or stream at the time the sample was taken,” says Halvorsen.
Environmental DNA: A Powerful Conservation Tool
Environmental DNA, the genetic material released by organisms into their surroundings, has revolutionized ecological monitoring. By extracting and analyzing eDNA from water samples, researchers can detect the presence and abundance of species without needing traditional sampling methods.
The use of eDNA in this way is relatively new. While eDNA had been used to detect bacterial communities in soil and sediments, researchers began applying it to water in 2008. A recent development shows it can also be used with air samples.
“Researchers have examined moist air in zoos with tropical climate exhibits where microscopic water droplets are present in the air, and DNA is found in these droplets,” Halvorsen explains.
It might even be possible to estimate the number of animals in an area using this method, although the research is still in its early stages. Halvorsen is among those studying this application, which could be useful for tracking rare species or those establishing themselves in new locations.
“The limitations of eDNA are that it doesn’t necessarily reveal whether the fish detected in a sample are alive or dead. And even if a species doesn’t appear in a sample, it could still be present. However, the more samples you take, the more confident you can be,” she adds.
Using eDNA in Conservation
Estimating Eel Populations
The European eel has experienced a drastic population decline due to various anthropogenic pressures. A major challenge is the presence of migratory barriers, such as hydroelectric dams, that hinder the eel’s life cycle.
By analyzing eDNA from water samples, researchers have confirmed that these barriers significantly limit the eel’s ability to reach upstream habitats.
A study published by Halvorsen demonstrated the potential of using eDNA to estimate the number of European eels in small rivers. By analyzing the D-loop region of mitochondrial DNA, the researcher could count the number of eels in both controlled and natural environments. While this is a promising advancement, further research is needed to refine this method and turn it into a reliable population monitoring tool.
The Impact of Rotenone Treatments
Rotenone, a piscicide, is often used to eradicate invasive fish species. However, the effects of rotenone on non-target species, including endangered ones like the European eel, are not fully understood. Halvorsen’s research aims to observe wildlife before, during, and after treatment.
“We haven’t found other studies that examine the full reaction of a fish community to this type of treatment. Usually, only the target species is studied,” Halvorsen notes.
Her study examined the ecological consequences of rotenone treatment in a lake with an invasive pike population.
The results indicate that native fish species, including the European eel, can successfully recover after rotenone treatment. This finding suggests that rotenone can be a valuable tool for managing invasive species when used carefully and in conjunction with other conservation measures.
“The rotenone treatment worked as expected. Even three years later, we found no traces of pike in the samples,” Halvorsen says.
Some species have reestablished themselves in the water. Trout, eels, and sticklebacks have returned from the sea or upstream streams. However, researchers will continue monitoring the lake to oversee the ecosystem.
Conclusion and Future Directions
Environmental DNA has proven to be an invaluable tool for studying the ecology and conservation of freshwater species, particularly the European eel. By providing a non-invasive and cost-effective method to detect and quantify aquatic organisms, eDNA has the potential to revolutionize freshwater monitoring and management.
According to Halvorsen, future scientific studies should focus on:
- Developing standardized eDNA protocols for various freshwater ecosystems and species.
- Exploring the use of eDNA to detect early signs of population decline and inform adaptive management strategies.
- Integrating eDNA data with other monitoring tools, such as traditional fish surveys and habitat assessments.
- Evaluating the long-term impacts of rotenone treatments on freshwater ecosystems.
Reference (open access)
Halvorsen, S. (2024). Exploring freshwater challenges for conservation efforts: Insights into threatened and invasive fish species using environmental DNA. [Doctoral Dissertation.] University of Agder.
