Endangered Eels Found Using eDNA

Based on DNA shed by organisms into the environment, scientists in Japan have found a way to measure the distribution of endangered eel species in Japan.

AsianScientist (Mar. 13, 2019) – In a study published in the journal Aquatic Conservation: Marine and Freshwater Ecosystems, scientists in Japan relied on environmental DNA (eDNA) to map the distribution of an endangered eel.

Eels are migratory fish that spawn in the ocean and grow up along the coast and in rivers. The Japanese eel Anguilla japonica is found across East Asia and has been an important part of Japanese life as a food source, a subject of traditional poems and art, and sometimes, even as a target of worship. However, eel catches have fallen drastically since the 1970s, and in 2014, the A. japonica was added to the International Union for Conservation of Nature Red List of Threatened Species.

Most river surveys of Japanese eel use electrofishing. However, this method requires a lot of time and resources, and for widely distributed species it may not collect enough data. Surveys are usually carried out in the daytime, while the nocturnal eels hide among vegetation and dirt.

Rapidly-advancing eDNA technology can monitor aquatic lifeforms through extraction and analysis of DNA present in water, without capturing the organisms themselves. In this study, the team led by Dr. Hikaru Itakura of Kobe University, Japan, investigated whether eDNA analysis could be used to show the distribution of Japanese eel.

They collected one-liter samples from 125 locations upstream and downstream in ten rivers in Japan and analyzed the eDNA from these samples using a real-time polymerase chain reaction system. At the same time, the team carried out an electrofishing survey in the same locations, comparing their electrofishing results with that of the eDNA analysis.

Japanese eel eDNA was found in 91.8 percent of the locations where eel had been confirmed using electrofishing (56 of 61 locations). eDNA was also detected in an additional 35 areas (mainly upstream) where eel individuals were not found. This shows that eDNA analysis is more sensitive than conventional surveys for detecting the presence of Japanese eel in rivers. Electrofishing data for eel numbers and biomass also positively correlated with eDNA concentrations, showing that eDNA could help scientists estimate the abundance and biomass of Japanese eel.

This method could potentially survey populations on an even wider scale, said the researchers. It is non-lethal, making it ideal for monitoring endangered species.

Furthermore, the eDNA analysis method is effective in detecting foreign eel species. For 20 years there have been reports of foreign eels (European eels and American eels) being released into Japanese waterways. These species look the same as Japanese eel, making them hard to detect. They are also long-lived so they may impact the ecosystem over long periods of time. By carrying out a wide-ranging investigation using eDNA analysis, the researchers could swiftly identify foreign eel species and their distribution.

“Concentration of eDNA in rivers is influenced by physical properties such as water depth and the speed of the current. Next we must increase the accuracy of eDNA analysis by clarifying the impact of these physical properties on eDNA concentration,” said Itakura.

The article can be found at: Itakura et al. (2019) Environmental DNA Analysis Reveals the Spatial Distribution, Abundance and Biomass of Japanese Eels at the River‐basin Scale.


Source: Kobe University. Photo: Kobe University.
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