Kate Bailey – Catchment Partnership Officer
Electric fishing is a method used to determine fish populations in watercourses, commonly carried out by organisations such as the Environment Agency and the various River Trusts. Sites across the River Esk catchment up in the north of the North York Moors have been monitored using this technique over the past 20+ years as a way of estimating juvenile fish populations. In the last five years an increased number of sites have been surveyed along the Esk and we’ve managed to gain a better understanding of fish population changes.
This summer we took the chance to learn the technique ourselves. Simon Hirst our River Esk Project Officer, six keen volunteers and I went on a fully certified electric fishing course. Our new specialist team is made up of local anglers and existing River Esk Volunteers, all of whom care about the river and want to safeguard the species that live in it.
The two day course was essential to ensure our ‘in house’ monitoring can be carried out safely and to make sure the information we collect is scientifically robust so we can draw conclusions from it.
During an electric fishing survey an electrical current is sent through the water which temporarily stuns the fish enabling them to be easily caught in nets. To produce an electrical current one team member wears the backpack which holds a battery and control box. A cathode trails behind the backpack resting in the water and the operator holds a pole with a metal ring on the end (anode). Once the operator places the anode in the water and turns the system on, the electrical circuit is complete and a small current passes through the water. The equipment has multiple safety features to ensure surveys can be carried out safely for all involved – that’s the fish, the operator and all the other team members in the water and on the bankside.
Two or three team members follow behind the operator with the nets and buckets ready to quickly remove the stunned fish from the river.
Once caught, the stunned fish are transferred into a large holding tank on the bankside. Each fish is measured using a special measuring board – this helps us estimate the relative age of the fish. The size and species is noted down and the fish is then released back into the river as quickly as possible.
Sites are identified – usually an approximately 50 metre stretch of river – and the team enters the water at the downstream end and walks upstream fishing as they go until they reach the top of the site. It is useful to fish up to a feature in the river, for example a natural barrier like a riffle, to ensure the greatest percentage of fish can be caught. If the end of the site was situated in the middle of a pool, for example, the fish would be driven beyond the far end and hence not be recorded.
Once the site has been fished, it also needs to be measured – this allows us to work out species density (i.e. the number of fish per unit area). We measure the channel width at 10 metre intervals and calculate an average width. This average width is multiplied by the length of river surveyed to give us a total area of river fished! Fish densities are usually recorded as the number of fish per 100 metres2
Health & Safety
Fish welfare (and the welfare of our volunteers and us) is extremely important, there is little point in measuring fish populations to help sustain numbers if we end up with dead fish. When electric fishing is carried out correctly this methodology does not harm the fish. As part of the training we learnt how to set the control box to the correct settings to ensure the electrical current will temporarily stun the fish but will not cause any damage. This is determined by the electrical conductivity of the water – a reading is taken before carrying out the surveys. It is important to temporarily keep the fish in a large holding tank in the shade to ensure there is sufficient oxygen for them and that the water temperature does not increase too much, and we release the fish back to the river as soon as possible.
Fish aside, all team members who are in the river must ensure they are fully insulated (e.g. rubber chest waders) and must not put their hands into the water when the backpack is switched on. Prior to undertaking a survey each stretch of watercourse is checked for hazards and a site risk assessment produced. This is to make sure there are no ‘nasty’ surprises (such as deep holes!) when the team enter the water.
Electric fishing is a really good way of estimating fish populations and this is extremely important on the River Esk. Atlantic salmon is a species that is struggling due to a wide variety of issues including water quality and habitat issues, barriers to fish migration and poor survival rates at sea. Monitoring juvenile numbers across the Esk every year will help to highlight areas where these issues are magnified and can therefore help target the conservation work we’re undertaking in the catchment to improve water quality and riparian habitats.
Our electric fishing team were out and about in the catchment in the autumn surveying six specific sites. It proved a success – the surveyed fish were released back and all the volunteers survived. Species recorded included Brown trout (Salmo trutta), Atlantic salmon (Salmo salar), Stone loach (Barbatula barbatula), European eel (Anguilla Anguilla), Bullhead (Cottus gobio) and Brook lamprey (Lampetra planeri). Overall our results showed healthy fish populations at each site, although as expected relatively low numbers of Atlantic salmon were found.
We plan to monitor juvenile fish populations annually in September, to build up our understanding of the local status of the Atlantic salmon and the other species found in the Esk.