Quantifying behavioural and muscle activation thresholds of non-target fishes for pulsed electric fields as used in flatfish electrotrawling

Electrical pulse trawling is an alternative to conventional beam trawling for common sole (Solea solea), with substantially less discards, lower fuel consumption, and reduced impact on the benthic ecosystem. Pulsed electric fields between electrode arrays induce a muscle cramp, immobilising the fishes on the sea bottom, making them easier to catch. Concerns exist, however, that the electric fields extend well beyond the netting, potentially affecting fishes outside the trawl track. Here, we address these concerns by measuring amplitude thresholds for involuntary muscle contractions and for behavioural responses, and compare these response thresholds to the field strengths around the fishing gear. For behavioural threshold measurements, both electro-receptive and non-electro-receptive fish were placed in a large circular tank with seven, individually controllable, evenly spaced electrode pairs, spanning the tank’s diameter. Responses were assessed from high-speed camera recordings for different pulse amplitudes and for different positions of the fish relative to the stimulating electrodes. Computer simulations of the electric field, verified with measurements in the experimental setup, were subsequently used to determine the threshold electric field strength at the location of the animal. For measurements of involuntary muscle contractions, fish were anaesthetised and placed in a tank with electrode pairs at different locations along the anteroposterior axis of the fish. Thresholds were established by increasing the pulse amplitude until a visible muscle twitch was observed. Threshold field strengths for both behavioural responses and for involuntary muscle activations were subsequently compared to the electric fields around commercial fishing gears. Preliminary results show that behavioural thresholds were not substantially lower in electro-receptive fish than in non-electro-receptive fish. Moreover, the behavioural field strength thresholds were similar to those for involuntary muscle activations. By modelling the field strengths around commercial electrode arrays, we show that both behavioural and muscle activation responses will be limited to distances less than 1 metre from the fishing gear. These findings suggest that electrical pulses as used in pulse trawling are unlikely to substantially affect the investigated fish species outside the trawl track.