Report of the Working Group on Electric Trawling (WGELECTRA)
WGELECTRA chaired by Adriaan Rijnsdorp (the Netherlands) and Maarten Soetaert (Belgium) met from 17-19 April 2018 at Wageningen Marine Research, Haringkade 1, IJmuiden, the Netherlands. The working group was attended by 17 participants from five countries to address the request for advice from the Netherlands to compare the ecological and environmental effects of using traditional beam trawls or pulse trawls when exploiting the TAC of North Sea sole, on (i) the sustainable exploitation of the target species (species and size selectivity); (ii) target and non-target species that are exposed to the gear but are not retained (injuries and mortality); (iii) the mechanical disturbance of the seabed; (iv) the structure and functioning of the benthic ecosystem; and to assess (v) the impact of repetitive exposure to the two gear types on marine organisms. This report does not consider the pulse fisheries on shrimp or on razorclam.
In order to provide advice, WGELECTRA developed an assessment framework to evaluate the ecological and environmental effects of traditional beam trawls and of pulse trawls. The assessment is based on (i) a description of the changes in the beam trawl fleet targeting sole and plaice in the North Sea during the introduction of pulse trawls; (ii) a review of the scientific information on the effects of electrical stimulation on marine organisms; (iii) results of on-going research projects. In preparation for the working group meeting, the chairs circulated a work plan to the participants, including a draft table of content of this report and an outline of the assessment framework. The bulk of the information included in this report was made available to the participants prior to the meeting. The working group meeting was focussed on an in-depth discussion of the scientific evidence and the assessment. As several research projects are still on-going, part of the evidence being used in the assessment is in the preparation phase and has not yet been peer-reviewed.
At present about 89 mainly Dutch owned vessels operate under an exemption from the EU-legislation to catch sole using pulse trawls in the North Sea. In addition, 7 vessels deploy pulse trawls to catch brown shrimp during part of the year. In Scotland, 26 vessels have been granted licences to deploy an electrotrawl to catch razorclams as part of a trial fishery. The stimulus in the razorclam fishery is very different from that in the sole fishery. The current report is focussed on the pulse trawl fishery on sole. Unless specifically stated, where ”typical or commercial” stimulus is stated in this document it refers to the sole pulse.
Pulse trawls for sole were introduced in the Dutch flatfish fishery to reduce the high fuel cost and substantial environmental damage of the traditional beam trawl fishery with tickler chains. The fleet of today’s pulse licence holders land about 95% of the Dutch landings of sole. The fleet comprises two vessel types. The smaller Euro cutters (<= 221 kW) alternate pulse trawling for sole with the fishery for brown shrimps and the otter (twin) trawl fishery for other demersal fish or Nephrops. The larger vessels (>221 kW) use the pulse trawl to fish for sole throughout the year. Some vessels alternate pulse fishing for sole with traditional beam trawl fishing for plaice.
The total fleet directed sole fishing effort of today’s pulse licence holders (beam trawl and pulse trawl) has slightly decreased during the transition to pulse trawling between 2009 – 2017 while their contribution to the Dutch sole landings increased by 20% (from 75% to 95%). During the transition phase, pulse trawlers have shifted their distribution pattern in the southern North Sea. On local fishing grounds off the Thames and along the Belgian coast, fishing effort has increased. In other areas, fishing effort was either stable or has decreased.
Pulse trawls are more selective than traditional beam trawls when catching sole. The landing efficiency estimated from catch and effort data of the Dutch beam and pulse trawl fleet is 30% higher for sole and 40% lower for plaice. The improved species selectivity is also reflected in the 16% (small vessels) and 24% (large vessels) lower catch rate of discarded fish in the pulse trawl as observed in the discard monitoring programme. It is uncertain whether the pulse trawl has improved the size selectivity, e.g. catching fewer undersized fish relative to larger sized classes of the same species. Pulse trawls are deployed at a lower towing speed than traditional beam trawls. Average towing speed is reduced by 22% from 6.3 to 4.9 knots in large vessels and by 15% from 5.4 to 4.6 in small vessels. The replacement of mechanical stimulation by electrical stimulation has reduced the physical disturbance of the seafloor. The average disturbance depth of an experimentally trawled study site was reduced from 4.0 cm with the traditional beam trawl to 1.8 cm in the pulse trawl (-55%). The lower towing speed and cleaner catch are expected to improve the survival of discarded flatfish.
The available literature on the potential negative effects of electrical stimulation of pulse trawling was reviewed. The impact of exposure to electrical pulses is determined by the frequency of exposure and the interval between successive exposures, as well as the sensitivity of the animal. Due to the reduced towing speed and slight reduction in fishing effort in the pulse fishery for sole, the overall exposure probability is reduced. Due to the heterogeneity of trawling, only 17% of the grid cells (1x1 minute) trawled have a trawling intensity of more than one time per year.
A number of laboratory experiments were carried out in which a selection of fish species were exposed to electrical stimuli to study possible adverse effects. These studies indicate that pulse stimulation used in the fishery for sole did not cause direct mortality during exposure but may cause spinal fractures and associated haemorrhages in gadoid round fish species (in particular cod), but not in flatfish species (sole, plaice, dab) or seabass. Preliminary results from an on-going project showed that 18% of 362 cod sampled from nine fishing trips of six pulse vessels showed a spinal fracture and/or full dislocation, while 24% showed smaller spinal abnormalities. Results suggest that the sensitivity is size dependent with lower incidence rate in small (65 cm) cod. Further studies are required to study the relationship between spinal fractures and body size and determine the differences in sensitivity towards spinal injuries across fish species. Data on sub-lethal effects and/or long-term effects are scarce and inconclusive. Small-spotted catshark Scyliorhinus canicula were still able to detect the bioelectric field of a prey following exposure.
Preliminary experiments with a range of benthic invertebrates generated variable results due to the low number of animals tested. More elaborate experiments with brown shrimp and ragworms did not find evidence for increased mortality when exposed to pulses similar to those used in the sole fisheries. However, when exposed 20 times during a 4-day period, an increased mortality was noted for brown shrimp compared to one of two control treatments, but not to mechanically stimulated shrimps.
Little is known on the effects of electrical stimulation on the development of eggs and larvae. One experiment exposing 8 early life stages of cod (embryos, larvae, early juveniles) to a very strong shrimp pulse stimulus, (a strength which only occurs very close to a commercial electrode), did not find differences in morphometrics between exposed and control animals, but observed a reduced developmental rate in one embryonic stage and an increased mortality in 2 larval stages following exposure. No Report of the Working Group on Electric Trawling (WGELECTRA) adverse effects were noted following exposure of two embryonic, two larval and one juvenile stage(s) in sole. Both experiments only studied possible short-term effects of the pulse and included a limited set of parameters to evaluate the sub-lethal effects. The effects of the sole pulse on reproduction have not been studied yet.
In contrast to the mechanical disturbance of the traditional beam trawl, preliminary results of recent studies on the effect of pulse stimulation on the biogeochemical functioning of the benthic ecosystem have not provided evidence that the electrical pulses used in the fishery for sole result in changes in sediment oxygen consumption, oxygen micro-profiles or surface chlorophyll levels. Effects on benthic ecological functioning has not yet been investigated.
Summarising the available evidence shows that the replacement of the tickler chain beam trawl with pulse trawl with electrodes to exploit sole results in a reduction of the environmental impacts: catch rate of fish discards (-16% to -24%), catch rate of benthos (-62% in large vessels and +6% in small vessels), trawling footprint (-18%), mechanical impact on seafloor and benthos (–50%) and CO2 emissions (-46%). There is insufficient evidence to fully understand the impact of electrical pulse on marine organisms and the benthic ecosystems across the North Sea. The possible adverse effects of electrical pulses on marine organisms and the benthic ecosystem are still being investigated. The available evidence so far suggests that the spinal fractures induced by the cramp response to the sole pulse are observed in two roundfish species, but not in flatfish which comprise more than 80% of the catch. Various gaps in knowledge on the effects of electrical stimulation on marine organisms and ecosystem functioning still exist. The on-going research on the effects of electrical stimulation on marine organisms and ecosystem functioning will improve the scientific basis to assess the ecological effects on the scale of the North Sea.
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