Written by: Luciana Bucater, Chris Bice, George Giatas, Brenton Zampatti and Qifeng Ye
Throughout the world electrofishing is commonly used by fisheries researchers to capture freshwater fish. It involves generating a field of electricity in the water which temporarily stuns fish in the close proximity (~5m) so that they can be scooped up with a dip net. The technique offers many advantages including:
- capturing fish from a wide range of sizes (20 millimetres to over 1 metre);
- being an active technique that is not reliant on fish ‘moving’ like many passive netting methods;
- allowing rapid surveys; and
- stunning is temporary and fish can be returned to the river unharmed.
Scientists often use boat electrofishing to collect data and/or fish samples with the aim of understanding the status of fish populations (such as species composition, age distribution, and presence of invasive species) and their response to management. Electrofishing can help in answering questions such as, what is the distribution of different species? How many are there? Are they reproducing, and what are the demographics of the populations? And most importantly for the Flow-MER Program, how do these characteristics change through time as a result of environmental water?
Types of electrofishing?
There are three types of electrofishing units; bank-mounted, boat-mounted and carried by an operator as a backpack. Bank-mounted and backpack units are typically used to capture fish in small wadeable streams, while boat electrofishing is used for sampling lakes and large rivers.
In the case of boat electrofishing, the ‘electrofishing unit’ consists of a vessel-mounted generator, a control box, a cathode (vessel hull) and anodes (booms at front of vessel). The collected fish is kept in a ‘live well’ to recover and be released after measurements and tagging take place.
In general, electrofishing is most effective in fresh waters. In marine waters, electrofishing is less effective as the conductivity of water is higher than that of fish, directing the current around rather than through the fish. Nonetheless, new electrofishing units are currently under development that are effective at capturing fish in brackish estuarine environments.
Is it safe for fish?
When performed correctly, electrofishing is safe for fish and does not cause long-term harm. Fish are temporarily stunned (seconds–a few minutes), dip netted and placed into live wells. Individuals are then processed (e.g. measured for length and weight and sometimes tagged) before being returned to the water once recovered. Tagging studies have indicated high survival rates for Golden perch and Murray cod following electrofishing, with fish displaying normal behaviours.
Is it safe for humans?
Electrofishing may conjure thoughts of danger, but it is perfectly safe when appropriate protocols are followed. These include the use of dip nets made from non-conductive materials, linesmans gloves (1000 volts), lifejackets and rubber soled shoes. Additionally, a system of pedals ensures that the electrical circuit is only completed and a field created, when all staff are ready and operating the pedals of the boat electrofishing unit.
Electrofishing in the Lower Murray River
Electrofishing is a key method used for sampling fish as part of various long-term monitoring programs in the Lower Murray River including the Flow – Monitoring, Evaluation and Research (Flow-MER) Program. In the Flow-MER Program, electrofishing is supporting investigations of changes in fish communities through time, and specifically the abundance, growth and recruitment of Murray cod and Golden perch, in association with environmental flows. In this way, electrofishing is a critical tool in assessing the response of fish to the delivery of water for the environment, in the Lower Murray River.
If you enjoyed reading this update, check out ‘Lower Murray River – Project Updates: July 2020’
This article is reproduced from the Lower Murray Newsletter: Issue 4 (July 2020). Download a copy by clicking the button below.
Murray cod numbers rise in Lower Murray River
An exciting finding from Flow-MER is the increase in number of juvenile Murray cod since 2015. In January 2020, our team collected 31 juvenile Murray cod by electrofishing suitable habitats.