Pulse Trawling Research Articles

Pulse trawl fishing: characteristics of the electrical stimulation and the effect on behaviour and injuries of Atlantic cod (Gadus morhua)

Abstract In the North Sea flatfish fishery, electric pulse trawls have been introduced to replace the conventional mechanical method. Pulse trawls reduce the fuel consumption, reduce adverse impact on the ecosystem but cause injuries in gadoids. We describe the design and electrical properties of pulse trawls currently in use and study the behavioural response and injuries in cod exposed to electrical pulses under controlled conditions. Pulse trawls operate at an average power of 0.7 kW m−1 beam length and a duty cycle of ∼2%. The electric field is heterogeneous with highest field strength occurring close to the conductors. Cod were exposed to three different pulse types for a range of field strengths, frequencies, and duty cycles. Two size classes were tested representing cod that escape through the meshes (11–17 cm) and market-sized cod that are retained in the net (34–56 cm). Cod exposed to a field strength of ≥37 V m−1 responded by moderate-to-strong muscular contractions. Some of the large cod (n = 260) developed haemorrhages and fractures in the spine, and haemal and neural arches in the tail part of the body. The probability of injuries increased with field strength and decreased when frequency was increased from 100 to 180 Hz. None of the small cod (n = 132) were injured and all survived. The field strength at the lateral boundaries of the trawl was too low to inflict injuries in cod.

Short-term effect of pulsed direct current on various species of adult fish and its implication in pulse trawling for brown shrimp in the North Sea

Electric pulses in fishing gear are increasingly used in the North Sea and are considered a promising alternative to ameliorate the sustainability of demersal trawl fisheries. The electrotrawl for brown shrimp employing low frequency pulsed direct current (PDC) selectively induces a startle response in shrimp engendering decreased environmental impact and reduced by-catch. Prior to commercially introducing this fishing technique, data on its impact on marine organisms are crucial. The aim of this study was to evaluate the short-term effects of this pulse used for electrotrawling for brown shrimp on five marine fish species inhabiting shrimp fishery areas. For this purpose, 25 European plaice (Pleuronectes platessa), 30 Dover sole (Solea solea), 20 Atlantic cod (Gadus morhua), 19 bull-rout (Myoxocephalus scorpius) and 20 armed bullhead (Agonus cataphractus) were exposed to the shrimp pulse for 5s. Before, during and till 20min following exposure, the behaviour of the fish was monitored. Twenty-four hours post-exposure, all fish were sacrificed, inspected and samples for histological analysis were taken from the gills, dorsal muscle and internal organs. To investigate possible spinal injuries radiographs were taken. Behavioural responses were variable and species dependant. Roundfish species, cod in particular, were displaying more active and agitated fast swimming activity during exposure. The majority of flatfish showed only minor reactions and remained close to the bottom throughout the observation period. However, 15% of the exposed sole actively swam upwards during exposure. Mild multifocal petechial haemorrhages and suffusion, encountered mainly in plaice and sole, were not significantly different between exposed and control groups. Upon histological examination, in two exposed plaice, a focal small haemorrhage between muscle fibres was found, which was not encountered in control animals. In addition, the number of melanomacrophage centres in the spleen of exposed cod was significantly higher than in the non-exposed animals. In conclusion, under the circumstances as adopted in this study, the electrical field seemed to have only limited immediate impact on the exposed animals.

Fishing gear transitions: lessons from the Dutch flatfish pulse trawl

Abstract This paper focuses on the transition in the Dutch cutter fleet-targeting flatfish, from the conventional beam trawl to the pulse trawl fishing gear. In doing so, we explore the process of gear transition, presenting the challenges that fishers and policy-makers face. The pulse trawl technique represents a particularly controversial gear transition as it makes use of electricity, which has been banned by the European Union since 1988. However, it is seen by those developing it in the Netherlands as an important alternative fishing gear to the conventional beam trawl technique, which is becoming increasingly inefficient with rising fuel prices and well-documented impact on benthic habitats. By using a multi-level perspective on socio-technical transitions as the analytical framework, we explore the development of the pulse trawl and the interaction between different levels. We also discuss the influence of technology-push on its transition into practice and regulation. This paper demonstrates the importance of social dimensions in the adoption of new fishing gears and in doing so contributes to our knowledge on how technological transitions in fisheries can be managed.

Mixed fisheries management: Is the ban on discarding likely to promote more selective and fuel efficient fishing in the Dutch flatfish fishery?

We model the potential effects of a discard ban on the annual fishing strategy of individual fishers in a mixed fishery under individual quota management. The North Sea beam trawl fishery, which catches large amounts of undersized plaice, is used as a model system. Under a discard ban, fishing is restricted to the fishing grounds and weeks where the maximum revenue can be realised with other species while catching the quota of the restricted species with a reduced bycatch of undersized fish. Model results suggest that, if properly enforced, a discard ban provides an incentive to implement more selective fishing gears that catch fewer small fish and are more fuel efficient (pulse trawl). If a discard ban is not properly enforced, restrictive quota do not necessarily result in the intended decrease in discarding as the fishery continues to fish while discarding the over-quota catch and least valuable size classes caught.

Measuring and assessing the physical impact of beam trawling

Abstract Beam trawling causes physical disruption of the seabed through contact of the gear components with the sediment and the resuspension of sediment into the water column in the turbulent wake of the gear. To be able to measure and quantify these impacts is important so that gears of reduced impact can be developed. Here we assess the physical impact of both a conventional 4 m tickler-chain beam trawl and a “Delmeco” electric pulse beam trawl. We measure the changes in seabed bathymetry following the passage of these gears using a Kongsberg EM2040 multi-beam echosounder and use a LISST 100X particle size analyser to measure the concentration and particle size distribution of the sediment mobilized into the water column. We also estimate the penetration of the gears into the seabed using numerical models for the mechanical interaction between gears and seabed. Our results indicate that the seabed bathymetry changes between ∼1 and 2 cm and that it is further increased by higher trawling frequencies. Furthermore, our results suggest that the alteration following the passage of the conventional trawl is greater than that following the pulse trawl passage. There was no difference in the quantity of sediment mobilized in the wake of these two gears; however, the numerical model introduced in this study predicted that the tickler-chain trawl penetrates the seabed more deeply than the pulse gear. Hence, greater alteration to the seabed bathymetry by the tickler-chain beam trawling is likely to be a result of its greater penetration. The complimentary insights of the different techniques highlight the advantage of investigating multiple effects such as sediment penetration and resuspension simultaneously and using both field trials and numerical modelling approaches.

Catch comparison of flatfish pulse trawls and a tickler chain beam trawl

Pulse trawling is used to a growing extent in the Dutch flatfish beam trawl fleet, and deemed as a promising alternative to tickler chain beam trawling. A comparative fishing experiment was carried out with one vessel using conventional beam trawls, and the other two using flatfish pulse trawls supplied by two different companies. Pulse trawl landings were lower both expressed in kgh−1 (67% based on auction data) or baskets per hectare (81%).The pulse trawls had fewer fish discards (57%, p<0.0001), including 62% undersized plaice (Pleuronectes platessa L.) (p<0.0001), and 80% discarded weight of benthic invertebrates (p=0.0198) per hectare. The pulse fishing technique resulted in a lower fuel consumption (37–49%), and consequently in spite of lower landings net revenues were higher. A downside of using pulse trawls is the possible spinal damage of marketable cod (Gadus morhua L.), but because total cod landings by beam trawls are low (4–5%), the implication will likely be limited.