To test how eDNA compares with traditional fish surveys, we compared fish communities from our monthly eDNA samples on the River Thames at Sunbury Lock with Environment Agency electrofishing records from nearby stretches of the river.

eDNA detected more species per unit of effort and gave consistent data on several important species that electrofishing routinely under-samples.

Electrofishing is the main method for surveying fish in rivers and is used routinely in the Environment Agency’s monitoring. But it needs expensive equipment and trained operators, which limits how often and how widely surveys can be carried out.

It’s also invasive: fish are temporarily stunned and float to the surface to be counted and identified. Although they recover, the process inevitably causes them stress. And some species are less affected by the electric current, so they tend to be missed. This applies particularly to bottom-dwelling species such as loaches, sticklebacks and bullhead, which are partly shielded among the stones on the riverbed.

Our team collects one eDNA sample each month from our long-term monitoring site on the River Thames at Sunbury Lock, analysing each with our Aquatic eDNA Fish assay.

For comparison, we downloaded Environment Agency electrofishing data for the closest locations on the river: 37 surveys from 9 sampling locations, carried out between 1995 and 2013.

• The 8 eDNA samples detected a total of 19 fish species, with an average of 13.9 species per sample.
• The 37 electrofishing surveys detected a total of 17 species (plus a hybrid), with an average of 8.5 species per survey.
• Every species consistently detected by electrofishing was also consistently detected by eDNA, including European eel (Anguilla anguilla).
• Both methods showed roach and perch to be the most dominant species, by relative abundance of sequence counts (eDNA) and individuals (electrofishing).
• Several species rarely or never detected by electrofishing appeared in multiple eDNA samples: European bullhead, lamprey, stone loach, ruffe, three-spined stickleback and minnow. This may be partly due to changes in the fish community over time (the most recent electrofishing survey is from 2013), but several of these species are small and live on the riverbed, making them less susceptible to electrofishing.
• The only (non-hybrid) species recorded by electrofishing in the previous 25 years but absent from the eDNA data was Atlantic salmon (Salmo salar): two individuals in 2002 and one in 2008.

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eDNA proved to be a powerful, non-invasive method for fish surveys. It performed at least as well as electrofishing for community assessment and matched its main conclusions on species abundance. Although the surveys were carried out at different times, our data suggests eDNA is more effective at detecting species such as stone loach, sticklebacks and bullhead, a pattern we’ve also seen in other datasets.

These results are consistent with published research showing that eDNA outperforms electrofishing for fish surveys in streams and rivers (e.g. Goutte et al., 2020; McColl-Gausden et al., 2021). Together, they strengthen the case for eDNA as a cost-effective alternative in routine river monitoring, delivering richer data with less resource and no harm to fish.

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References
Goutte, A., Molbert, N., Guerin, S., Richoux, R. & Rocher, V. (2020). Monitoring freshwater fish communities in large rivers using environmental DNA metabarcoding and a long-term electrofishing survey. Journal of Fish Biology, 97(2): 444–452.
McColl-Gausden, E.F., Weeks, A.R., Coleman, R.A., Robinson, K.L., Song, S., Raadik, T.A. & Tingley, R. (2021). Multispecies models reveal that eDNA metabarcoding is more sensitive than backpack electrofishing for conducting fish surveys in freshwater streams. Molecular Ecology, 30(13): 3111–3126.