Replacing mechanical stimulation by electrical stimulation reduces environmental impact of the flatfish beam trawl fishery

Beam trawls are widely used in European waters to target flatfish, in particular sole (Solea solea) and produce a substantial amount of discards. Tickler chains used to chase flatfish from the seabed have considerable adverse impacts on the ecosystem. Since 2010, innovative electrified beam trawls – pulse trawls – have been used in the North Sea to study whether the ecological impact can be reduced. Pulse trawls use pulsed bipolar current with a peak amplitude of 60 V, frequency of 30 Hz, and pulse width of 340 µm. The electrical stimulus elicits a muscle cramp response, immobilising the fish in front of the trawl. Exposure duration is ~1.5 s and the electric field strength quickly decays with increasing distance from the electrodes to values below fish response thresholds within the trawl width. We describe the innovative pulse trawl and present results of an impact assessment. Pulse trawls are 30% more efficient to catch sole but catch less undersized fish and benthos (57 and 80 % respectively). Lower towing speed reduces the trawling footprint by 23% and CO₂ emissions by 30%. Abandoning tickler chains reduces mechanical disturbance of the seabed, penetration depth, and benthic impact by 20-60%. Pulse exposures on six invertebrate species showed varying acute responses but no lasting signs on locomotor activity and no increased mortality. Bio-geochemical processes were not affected by pulse exposure. The induced muscle cramp may result in spinal injuries and haemorrhages in fish. Extensive sampling of commercial pulse trawl trips (>15,103 fish, 14 species) showed an elevated injury probability in Atlantic cod, but not in other fish species, compared to samples from tickler-chain beam trawls. The population effect of pulse exposure on cod is negligible due to limited overlap between the fishery and population distribution. Although our study showed that pulse trawls can be safely used to exploit the sole quota with a substantial reduction in ecosystem and environmental impacts, societal concerns about the use of electricity fueled by a campaign of an environmental NGO against pulse fishing resulted in a European Union decision to maintain the prohibition of pulse trawls.