Electrofishing surveys

Juvenile lamprey surveys using electrofishing

When sampling juvenile lampreys in wadeable stratches of river (i.e. less than 1m deep) we follow the standard methodology given in Natural England’s publication ‘Monitoring the River, Brook and Sea Lamprey, Lampetra fluviatilis, L. planeri and Petromyzon marinus‘ by Harvey & Cowx I (2003). 

Electrical fishing is the preferred technique used for juvenile lamprey sampling in waters less than 1 m in depth. A portable electrical fishing unit with an anode ring is ideal for sampling lampreys. We use Smith‐Root LR‐24 portable electrical fishing units for sampling juvenile lampreys – this machine can automatically adjust voltage and power to the site conditions (i.e. conductivity, water depth). Additionally, settings on this machine can be adjusted for improved ammocoete sampling efficiency. However, there are other suppliers that provide electrofishing units suitable for sampling juvenile lampreys, and we have older electrical fishing units that still work fine and can be used for sampling lampreys.

Juvenile lamprey survey: River Maigue, Co Limerick
Juvenile lamprey survey: River Maigue, Co Limerick

It is noted that we formerly placed a fine mesh on our anodes to capture lampreys; with an assistant also with a dip net. This is apparent from the photos below. Although we have never had issues with this and exercised due care to avoid full electronarcosis of captured lampreys, we no longer use mesh on electrodes as a precautionary approach and to align ourselves with current international best practice. 

According to USDA (2010) electrofishers used for ammocoete sampling should be set with two wave forms, a lower frequency “tickle” wave form to coax ammocoetes out of the substrate and a higher frequency “stun” wave form to immobilize ammocoetes for netting. USDA (2010) electrofishing recommendations for sampling larval pacific lampreys specify using 3 pulses/s (125 V DC) at 25 % duty cycle, with a 3:1 burst pulse train (three pulses on, one pulse off) at 30 pulses/s to remove larvae from the substrate.

APEM (2004) used a method of energising the electrode for 15-seconds bursts with 100 volts of pulsed direct current (DC). A brief 5-second pause was initiated between bursts, allowing lampreys to emerge when the power stopped. Once in the water column, further bursts of electricity immobilised the ammocoetes and allowed them to be netted and removed for subsequent analysis. This approach works fine and and the key to sampling lampreys is the recommendation to first irritate the ammocoetes with electric pulses to encourage them to leave their burrows, without stunning them. You then need to give them a interval to emerge before again applying the electric pulse so they can be caught. This is the same method followed by Niven and McCauley (2013).

Smith-Root LR24 electrofishing unit
Smith-Root LR24 electrofishing unit

The dual wave operation / “on -off ” sequence is used to irritate ammocoetes out of the substrate. While the gear is operated, the anode is slowly pulled backwards in the water to cause lampreys to emerge from burrows as a result of electro-taxis. This procedure is repeated throughout the operation. It is important to avoid exposing ammocoetes to extended periods of electrofishing as it has also been linked to electronarcosis. When necessary, the  higher frequency mode should be activated for capturing emergent ammocoetes. Nets with fine mesh are then used to capture lampreys, and current best practice recommends that mesh is not affixed to the anode frame. By keeping the anode 1-15 cm above the sediment and pulling the anode backwards, the number of lampreys stunned within the substrate is reduced, and by using the 2-stage method described above, the electrofisher should mainly be operated in the  lower frequency output mode to irritate ammocoetes out of the substrate. Captured lampreys are removed quickly using the dip net and placed into a container of river water.

Electrofishing survey for juvenile lamprey - River Mulkear, Co Limerick
Electrofishing survey for juvenile lamprey – River Mulkear, Co Limerick

During lamprey salvage or translocation, or quantitative assessments, multiple electrofishing passes should be made to ensure a more complete removal of ammocoetes and/or provide a minimum density estimate. A fifteen minute break between passes should be taken to reduce the chance of  electronarcosis. During quantitative sampling, a standardised 1m2 section is fished for 2 minutes (or until lampreys stop emerging from their burrows). The 1m2 site can be closed off using a 1m2 box frame with mesh sides or using heavy duty fine mesh stop nets with bottom weights and floats.

On rivers with lower lamprey densities or at sub-optimal habitats we use larger sub-sites; up to 5m in area. It is often difficult to find representative and suitable 1m sub-sections of optimal lamprey habitat and their are practical difficulties with type of survey, on drained rivers in Ireland in particular. Where there is only sub-optimal habitat we do depletion fishing at a selection of the sites as well.

Lampreys from each pass are retained separately for identification, enumeration and measurement of length and weight. Catch details at each sampling point should include the number and length (mm) of River/Brook and Sea Lamprey ammocoetes and number and length (mm) of River/Brook and Sea lamprey transformers. Lamprey removal for translocation follows the same routine as for quantitative sampling but additional passes may be required if efficiency is low, to ensure effective removal. The efficiency of electrical fishing for Lamprey ammocoetes is reduced with increased depth, low light, vegetative cover and water colouration.


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