Working Group on Electrical Trawling (WGELECTRA; outputs from 2021)

The Working Group on Electrical Trawling creates a platform for supra-national joint research projects on electro-trawling and scientific publications. The group also reviews all relevant studies on marine electrofishing and discusses the ongoing and upcoming research projects in the light of knowledge gaps. Research areas covered by the group included fishing tactics and dynamics, organisms and ecological impacts and selectivity of electro trawling. A study into the exploitation of local fishing grounds revealed that pulse trawlers and conventional tickler chain beam trawlers had similar tactics spending 10% of their tows searching for a fishing ground and spending 90% of their tows exploiting a fishing ground. In-situ field campaigns revealed a lower impact of pulse trawls on biogeochemical parameters compared to traditional beam trawl methods. Laboratory experiments found that while alternating or pulsed bipolar currents readily penetrated the sediment, biogeochemical effects appeared to be inhibited from occurring. The combined results concluded that the environmental impact of electricity from pulse trawls is relatively minor compared to the mechanical disturbances created from the same gears. Behavioral response thresholds for pulsed electric fields were determined in laboratory experiments for electroreceptive as well as non-electroreceptive fish species. Comparison of these thresholds to simulations of electric fields around commercial fishing gears suggest that electrical pulses are unlikely to substantially affect the investigated fish species outside the trawl track. A field study into direct mortality among fish and benthic organisms in the wake of pulse trawlers refuted claims that pulse trawling causes mass mortality among non-target species. A study into the selectivity of shrimp pulse trawling vs. traditional trawling concluded that that shrimp fishing using pulse gear does not result in higher amounts of undesired bycatches of small shrimp, fish and benthos when compared to the traditional shrimp beam trawl fisheries. The outline of a PhD project that started in 2021 into organism and ecological impacts of electrofishing for razor clams in Scottish shallow coastal habitats was presented and preliminary results were shared

Physical and electrical disturbance experiments uncover potential bottom fishing impacts on benthic ecosystem functioning

Both physical and electrical impacts have been linked to North Sea fisheries activity. This study evaluates how these effects can influence marine ecological functioning by assessing their consequences on benthic pelagic coupling. Experiments were conducted on sediment microcosms taken from 9 North Sea and 2 Eastern Scheldt locations. Samples were subjected to physical disturbances by mechanically stirring the sediment surface or electrical stimulation with exposure to high frequency pulsed bipolar or direct currents. Electrical exposure times of 3 and 120-s were used to simulate in situ exposure times related to sole (Solea solea) and razor clam (Ensis spp.) electric fisheries respectively. Water column oxygen rapidly declined after sediment resuspension, inducing an immediate uptake ranging from 0.55 to 22 mmol oxygen per m−2 of sediment disturbed. Mechanical disturbances released the equivalent of up to 94 and 101 h of natural ammonium and silicate effluxes respectively. Fresh organic material significantly predicted the magnitude of mechanical-induced oxygen, ammonium, phosphate and silicate changes. No biogeochemical effects from bipolar (3 s or 120 s) or 3-s direct current exposures were detected. However, significant changes were induced by 120-s exposures to direct currents due to electrolysis and ionic drift. This lowered the water column pH by 1–1.3 units and caused the appearance of iron oxides on the sediment surface, resulting in the equivalent of 25–28 h of sedimentary phosphate removal. Our findings demonstrate that prolonged (+1 min) exposure to high frequency pulsed direct currents can cause electrochemical effects in the marine environment, with implications for phosphorus cycling. Nevertheless, bi-directional pulsed currents used in flatfish pulse trawling and AC waveforms featured in Ensis electrofishing, seem to severely limit these effects. Mechanical disturbance, on the other hand, causes a much greater effect on benthic pelagic coupling, the extent of which depends on sediment grain size, organic matter content, and the time of the year when the impact occurs

Long-term changes in ecosystem functioning of a coastal bay expected from a shifting balance between intertidal and subtidal habitats

Coastal areas are subjected to several anthropogenic stressors with much of the world's intertidal areas receding due to human activities, coastal erosion and sea level rise. The Dutch Eastern Scheldt (ES) has been predicted to lose around 35% of intertidal areas by 2060. This study investigates differences between biogeochemical fluxes of intertidal and subtidal sediments of the ES and assesses how ongoing erosion may modify the sedimentary ecosystem functioning of this coastal bay in the coming decades. Monthly fluxes and porewater concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), silica (DSi), carbon (DIC) and oxygen (O2) as well as organic matter (OM) characteristics were measured from intertidal and subtidal sediments from June 2016–December 2017. Compared to subtidal stations, OM was significantly more reactive in intertidal samples and exhibited 37% higher O2 fluxes, suggesting a strong influence from microphytobenthos. Subtidal sediments exhibited an average efflux of nitrates (0.28 mmol m−2 d−1) and phosphates (0.09 mmol m−2 d−1) into the water column, while intertidal areas displayed average influxes (nitrates = −1.2 mmol m−2 d−1, phosphates = −0.03 mmol m−2 d−1) directed into the sediment. The calculated removal of total DIN and DIP from the water column was 34–38% higher in intertidal compared to subtidal samples suggesting stronger denitrification and phosphorus adsorption to solid particles from intertidal sediments. As an upscaling exercise, we estimate potential erosion induced changes if the ES stations are representative for the system. With this assumption, we estimate 11% and 8% reductions for respective nitrogen and phosphorus removal in the entire ES by 2060. Given the global observations of eroding intertidal areas and rising sea levels, we suggest that the predicted habitat loss may cause significant changes for coastal biogeochemistry and should be investigated further to understand its potential consequences

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