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The European Commission landing obligation, including species-specific “high survival” exemptions, has established a need for accurate discard survival estimates. This study presents the first discard survival estimates on-board Dutch commercial pulse trawlers. During seven, six, and one fishing trip(s), respectively, undersized plaice (Pleuronectes platessa), sole (Solea solea), and dab (Limanda limanda) were collected, assessed for vitality and subsequently monitored up to 21 days. Uncorrected for any potential impacts from predation, tagging, research-related handling, or holding conditions overall survival for plaice (n = 349), sole (n = 226), and dab (n = 187) was assessed as 15% [95% CI: 11–19%], 29% [95% CI: 24–35%], and 16% [95% CI: 10–26%] respectively. Survival was mainly effected by water temperature and factors linked to the fishing vessel. Fish length was not found to affect survival. Catch processing time and haul duration affected plaice survival but not sole. Vitality index, which averages reflex impairment and external damage scores, correlated with survival and may be developed as a proxy for discard survival. Compared to tickler-chain beam trawlers, pulse trawlers showed relatively higher discard survival under fishing conditions pertinent to these studies.

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Ecosystem effects of bottom trawl fisheries are of major concern. Although it is prohibited to catch fish using electricity in European Union waters, a number of beam trawlers obtained a derogation and switched to pulse trawling to explore the potential to reduce impacts. Here we analyse whether using electrical rather than mechanical stimulation results in an overall reduction in physical disturbance of the seafloor in the beam-trawl fishery for sole Solea solea. We extend and apply a recently developed assessment framework to the Dutch beam-trawl fleet and show that the switch to pulse trawling substantially reduced benthic impacts when exploiting the total allowable catch of sole in the North Sea. Using Vessel Monitoring by Satellite and logbook data from 2009 to 2017, we estimate that the trawling footprint decreased by 23%, the precautionary impact indicator of the benthic community decreased by 39%, the impact on median longevity of the benthic community decreased by 20%, the impact on benthic biomass decreased by 61%, and the amount of sediment mobilised decreased by 39%. The decrease in impact is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic physical disturbance of the beam-trawl fishery targeting plaice Pleuronectes platessa in the central North Sea increased with the recovery of the plaice stock. Our study illustrates the utility of a standardized methodological framework to assess the differences in time trends and physical disturbance between gears.

Bottom trawls impact the seafloor and benthic ecosystem. To estimate the trawling impact, information is required about the dimensions of the gear that determine their footprint, sediment penetration depth and hydrodynamic drag that determines the amount of sediment mobilised in the wake of the trawl. Here we present the dimension of gear components of the different beam trawls used in the North Sea flatfish fishery including the traditional tickler chain beam trawl, chain-mat trawl and pulse trawls. The hydrodynamic drag of the gears is estimated by summing the drag of different gear components using empirical equations that describe the hydrodynamic drag of different shaped objects, including cylinders, blocks and fish nets. Netting contributes most to the hydrodynamic drag, followed by the ground rope, tickler chains and chain mat. The hydrodynamic drag of bottom components, which determines sediment mobilisation, is estimated at 6.2 and 6.3 kN.m-1 for a traditional tickler chain and chain-mat trawl, and 3.8 kN.m-1 for a pulse trawler. Drag of pulse trawls depends on their rigging and ranges between 2.8-3.2 kN.m-1 for the rectangular ground rope types and 4.0-4.1 kN.m-1 for the sole rope type. The amount of sediment mobilised in a seabed habitat with 20% silt content is 24 kg.m-2 for a large tickler chain and chain-mat trawler and between 12-16 kg.m-2 for a large pulse trawler.