Electro-fishing surveys

Electrofishing uses electricity to stun fish before they are caught. Electrofishing is a common scientific survey method used to sample fish populations to determine abundance, density, and species composition. When performed correctly, electrofishing results in no permanent harm to fish, which return to their natural state in as little as 2 minutes after being stunned.

The current runs from the anode to the cathode creating a high-voltage potential. When a fish encounters a large enough potential gradient, it becomes affected by the electricity. Usually direct DC current is applied, which causes firstly, electrotaxis, which forces the fish to become attracted towards the anode and then electronarcosis which stuns the fish. Galvanotaxis is the uncontrolled muscular convulsion that results in the fish swimming toward the anode. At least two people are required for an effective electrofishing crew using the back pack equipment one to operate the anode, and the other to catch the stunned fish with a dipnet whilst 3 are required when the bank side kit is used.

Pulsed current is sometimes used when capturing adult fish but this must be used with care as it can cause damage to the fish by paralysis.

Once caught, the fish are anaesthetised before being measured and counted. Scales are taken to determine the age classes present within the survey site.

The electrofishing data is used to monitor the success of habitat restoration projects and to see if fish passes in man-made barriers are working properly. Where we have removed barriers and planted hatchery reared fry to 'kick start' the re-population process we will survey these areas to assess the survival rates of the joung fish. The Trust carries out annual surveys of many sites around the catchment in order to monitor the population status especially where water quality has been compromised by agricultural practices such as silage 'brie' that has leaked into a water course and where obstacles such as weirs are thought to be inhibiting migration and spawning. Other reasons for annual surveying is to assess the status of water courses prior to engineering works or proposed wind farms such as the planned Clashindarroch Forest and the Doronell wind farms.  

Many of our sites are monitored annually to simply determine whether sufficient adults are returning to spawn.