Impact assessment of the pulse trawl fishery (2016-2019)

The main findings of the project are the following:

• The field strength of the gear that is biological relevant is confined to the width of the pulse trawl gear. The field strength outside the pulse trawl gear is below the threshold level that invokes a response in fish (including sharks and rays).  
• Exposure to a pulse stimulus does not lead to additional mortality but may lead to spinal injuries in fish.
• Pulse-induced spinal injuries are low except in cod. Population level effects in cod are negligible in the North Sea stock and small in the southern North Sea stock because of the low exposure probability, and because the injury probability is lower in small cod.
• Electroreceptive fish like elasmobranchs are not specifically sensitive to the high frequency pulses used in the sole fishery
• The effects of pulse exposure, studied in selection of benthic invertebrate species, was found to be non-lethal and temporary.
• Pulse stimuli used in pulse trawling for sole do not affect geochemical processes
• Impact of pulse trawls on the benthic ecosystem is due to mechanical disturbance and not to electrical disturbance.
• The impact of mechanical disturbance of the pulse trawl is less than that of the conventional beam trawl.
• Pulse trawling improves the selectivity of the beam trawl fishery for sole and reduces the bycatch of undersized fish (discards) and benthic invertebrates.
• Survival of pulse trawl discards is estimated to be higher in plaice, turbot and brill, while no significant difference was found for sole and thornback ray.
• Pulse trawling allows fishers to catch their sole quota with a lower spatial footprint and a lower impact on the benthic ecosystem due to a lower penetration depth and sediment resuspension.
• Pulse trawling does not cause a chronic exposure to electric pulses because of the low frequency of exposure above the threshold field strength and low duration of a pulse stimulus.
• It is highly unlikely that pulse trawling will compromise the reproductive capacity of the target species by non-lethal exposure to pulse stimuli.
• It is highly unlikely that a possible adverse effect of pulse exposure of eggs and larvae will lead to adverse population level effects.
• The improved efficiency to catch sole in pulse trawls and the changes in spatial distribution may give rise to competition with other fisheries.
• Pulse trawls reduce the fuel consumption per kg landings by 20% and the fuel consumption per unit of sole quota by 52%.

Implications of assessment results in relation to the legislative framework of the EU on fisheries and the marine environment

The project provides strong support that, in comparison to tickler chains, pulse trawls can be used to sustainably exploit the quota of North Sea sole and at the same time substantially reduce the ecological and environmental cost. Pulse trawling therefore contributes to the objectives of the Common Fisheries Policy for sustainable exploitation. The improved selectivity further contributes to the objectives of the Landing Obligation to reduce the unintended bycatch.

The increased catch efficiency in comparison to tickler chains may lead to competition with other fisheries and may pose a problem for fisheries managers and stakeholders to find solutions to share out fishing opportunities fairly within a given legal framework.

The reduced spatial footprint and impact on the fish community and benthic ecosystem of pulse trawling will reduce the fishing pressure on the diversity, food web and the integrity of the sea floor. The lower footprint and towing speed likely reduce the wear on nets and engine and as a consequence will reduce the contaminants and marine litter. This will contribute to the objectives of the Marine Strategy Framework Directive.

Although no specific research has been carried out to study the impact of pulse trawling on Natura 2000 species and habitats, the available knowledge allows us to assess a possible adverse impact as highly unlikely, because exposure to electrical stimulation does not result in negative effects, probability of exposure is likely to be (very) low and the overall footprint of the pulse fishery (in comparison to tickler chains) has been reduced.

The reduction in fuel consumption in comparison to tickler chains will reduce CO2 emissions and contribute to the objectives of the Paris agreement.

Effect of pulse stimulation on marine organisms

The wire-shaped electrodes of a pulse trawl generate a heterogeneous electric fields with the highest field strength close to the conductors. Field strength dissipate with increasing distance from the conductors. Within the trawl width the field strength ranges between ~ 5 and ~ 300 V.m-1. Outside the trawl, the field strength is less than ~ 5 V.m-1. Field strengths in the water column and in the sediment are similar. The duration of a pulse exposure is 1.5 seconds.

Muscle activation by electrical pulses is determined by the strength of internal electric fields inside the organism. Internal electric fields differ from the surrounding external fields due to conductivity differences of the body relative to seawater. Because the conductivity in sediment is less than in water, fish that are buried in the sediment experience a lower internal field strength than fish in the water. Internal electric fields in a typical roundfish drop below a value of about 20 V.m-1 at a distance of about 50 cm. This value is only weakly affected by the location between the pair of electrodes, or by the orientation of the fish. Susceptibility to electrical pulse decreases with fish size. At similar heights above the electrodes, field strengths in small fish are lower. In addition, the chance that small fish are exposed to high field strengths close to the electrodes is smaller.

Field strength thresholds were estimated in laboratory experiments for different responses. Thresholds provide information about the surface area where the field strength around a pulse trawl is exceeded and will affect the animals exposed. The threshold for a behavioural response is between 3 and 6 V.m-1 (external field strength) and does not differ between electroreceptive and other fish species. Although electroreceptive fish like elasmobranchs are highly sensitive to low frequency electrical stimuli generated by their prey organisms, they are not specifically sensitive to high frequency pulses used in the pulse trawl fishery for sole. The muscle activation threshold was estimated at 15 V.m-1 (internal field strength). A previous experiment showed that the external field strength threshold for spinal injury in cod was estimated at 37 V.m-1, whereas 50% of the cod developed a spinal fracture when exposed at 80 V.m-1.

Extensive sampling of fish caught by commercial vessels showed spinal injuries in most of the fish species sampled from pulse trawls and tickler chain beam trawls. Comparison of the injury probability of fish caught in a pulse trawl with the injury probability of fish caught in a conventional beam trawl or a pulse trawl where the pulse was switched off, indicated that injuries can be ascribed to mechanical damage inflicted during catching. Pulse-induced spinal injury probability was restricted to cod with an average injury probability of 36%. Spinal injury probability in cod seems to be related to fish size indicating that small cod are less sensitive to pulse exposure. Pulse-induced spinal injury probability was low ( <=1%) in the other 11 fish species studied.

The effects of pulse exposure, studied in selection of benthic invertebrate species, was found to be non-lethal and temporary. Animals either did not respond (sea star, serpent star) or showed a cramp or squirming response (crabs, polychaetes), and showed an avoidance response after the stimulus.

The effect of pulse exposure on the geochemical processes was studied in both laboratory and field experiments. With the pulsed bipolar current (PBC) used in the pulse fishery for sole the potential effect of electrolysis is negligible. The studies carried out did not detect any measurable effect of pulse exposure on the biogeochemistry and the benthic disturbance by pulse trawling therefore will come from mechanical disturbance.

Scaling-up the impact to the level of the fleet and population

The pulse fishery for sole uses a pulsed bipolar current (PBC). The data logger installed, which stores the pulse parameters used during fishing, showed that the amplitude over the electrode ranged between 54 and 58V with lowest values recorded in summer and highest values recorded in winter. Average pulse frequency and pulse width was 89.4 Hz and 239 µs in the Delmeco system, and 60 Hz and 336 µs in the HFK system. The power supplied per meter gear width ranged between 0.5 – 0.6 kW.m-1. Pulse parameters were within the boundaries set in the regulation.

To assess the consequences of a transition from conventional beam trawling to pulse trawling, the effects of mechanical and electrical stimulation during a trawling event needs to be scaled up to the level of the total fleet. The impact of both gears was compared by studying the impact of the Dutch pulse license holders (PLH) before and after the transition to pulse trawling. The PLH can be used as a proxy of the total fleet because they landed 95% of the Dutch sole landings after the transition to pulse trawling. This provides an under-estimate of the consequences of the transition by 23%, because PLH increased their share of the Dutch sole landings from 73%, when fishing with conventional beam trawls, to 95%, when fishing with pulse trawls.

During the transition period between 2009 and 2017, the PLH maintained their fishing effort (hours at sea) when fishing for sole with an 80mm mesh size in the sole fishing area (SFA), but reduced the surface area swept by the gear by 28%. The lower area impacted is due to the combined effect of a lower towing speed (-10% small vessels, -23% large vessels) and improved catch efficiency and selectivity for the target species sole. Pulse trawls have a 17% (95% confidence limits: 14%-20%) higher catch rate (kg/hour) of marketable sole and a 21% (19%-23%) lower catch rate of other flatfish and 35% (33%-38%) lower catch rate of marketable plaice.

Due to the improved selectivity, pulse trawls caught 27% (17%-36%) less discards (all fish) than conventional beam trawls. The catch rate of plaice discards was reduced by 30% (19%-40%), but the catch rate of sole discards was increased by 65% (16% - 137%). The reduction in discarding is supported by modelling the fishing mortality of the discard size classes imposed by the PLH. The partial fishing mortality decreased in flatfish except sole (33%), gadoids (16%), gurnards (10%) and other fish (16%), and increased for discard size classes of rays (44%) and sole (29%). The lower towing speed and lower catch volume in the pulse fishery resulted in a higher discard survival of plaice, turbot and brill, although no difference was found for sole and thornback ray.

The impact of pulse exposures by PLH was assessed by estimating the exposure frequency of a population to the lowest field strength where fish showed a behavioural response corresponding to the width of the pulse trawl. This is a precautionary assumption because the field strength threshold for injuries as observed in cod is substantially higher and occurs in only part of the trawl width. Based on the VMS (Vessel Monitoring System) recordings of the PLHs at a resolution of 1 minute latitude x 1 minute longitude grid cells (about 2 km2), the exposure frequency was estimated for a population that is randomly distributed over the trawled grid cells: 21% of the population was exposed to a pulse stimulus 1 time year-1; 6.6% was trawled 2 times year-1; 2.4% was trawled 3 times year-1 , 0.3% was trawled 4 or more times year-1, and 70% of the population was not exposed. This low exposure frequency and the short duration of a pulse exposure (1.5 sec) indicates that pulse trawling for sole does not cause a chronic exposure of marine organisms or the benthic ecosystem to pulse stimuli.

The population level consequences of potential pulse-induced mortality among cod that are too small to be retained in a pulse trawl, is estimated to be small (<2%) for cod in the southern North Sea and negligible (<0.5%) for the total North Sea. Adverse consequences of nonlethal exposure of sole on the reproductive output of the population is highly unlikely due to the short exposure duration (1.5 sec) and the low exposure probability of sole during the maturation year. A population level impact on the egg and larval stages of sole is highly unlikely due to the low exposure probability, the high natural rate of mortality of these stages and the density-dependent mortality that will occur later in life. The same conclusion applies to other fish with pelagic or demersal eggs.

The transition to pulse trawling reduced the impact on the seafloor and benthic ecosystem due to a reduction in the footprint (23%) and reduction in sediment resuspension (39%). The reduction in the depth of sediment disturbance will reduce the direct mortality of benthos. Indicators of the benthic impact of PLH decreased between 20% and 61%. Long-term geochemical effects of pulse trawling is reduced due to the lower mechanical disturbance. No additional effect of the pulse exposure is found. Although benthic invertebrates may respond to a pulse exposure by temporary slowing down their normal activities, the duration of this effect is short and will unlikely affect the macro-invertebrate food web.

The local increase in fishing effort of pulse trawlers in combination with the higher catch efficiency for sole may have resulted in increased competition with other fisheries. Competition between the pulse trawl fleet and the Belgium beam trawl fleet was found in the southwestern North Sea.

Pulse trawls allow fishers to tow their gears at a lower speed over the seafloor and reduce their fuel consumption and CO2 emissions.

Documents

Rijnsdorp AD, Boute P, Tiano J, Lankheet M, Soetaert K, Beier U, de Borger E, Hintzen NT, Molenaar P, Polet H, Poos JJ, Schram E, Soetaert M, van Overzee H, van de Wolfshaar K, van Kooten T. 2020. The implications of a transition from tickler chain beam trawl to electric pulse trawl on the sustainability and ecosystem effects of the fishery for North Sea sole: an impact assessment. Wageningen Marine Research report C037/18. 108 pp