Direct mortality among demersal fish and benthic organisms in the wake of pulse trawling

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Abstract

Pulse trawl fisheries involve the use of electrical pulses to immobilize (cramp) target species and make them available for capture. A major concern related to pulse trawl fisheries is the passing pulse trawl causing direct, mass mortalities among benthic organisms, resulting in a ‘graveyard’ in the wake of a pulse trawler. This has never been investigated in situ and the current pilot study therefore aimed to develop a method for in situ assessments and to perform a first assessment. A pulse trawler equipped with double rigs made pulse trawl tracks specific for the current experiment. Within 15 to 30 minutes after passage of the pulse trawler, one of its trawl tracks was sampled with a shrimp trawler by a 10 minute tow with a small mesh shrimp beam trawl while the other, similar shrimp trawl was deployed outside the pulse trawl track to obtain control samples. The experiment consisted of two pulse trawl track treatments: a complete pulse trawl and a pulse trawl with its netting and ground rope removed to minimize its mechanical impact. This allowed for isolating electrical from mechanical impacts. In total two paired samples of treatments and controls were obtained per pulse trawl track treatment. The condition of three fish species and three species of invertebrates was assessed. Fish species included plaice (Pleuronectus platessa), dab (Limanda limanda) and solenette (Buglossidium luteum). Invertebrate species included flying crab (Liocarcinus holsatus), hermit crabs (Paguroidea spp.) and brittle stars (Ophiuroidea spp.). Direct survival ranged from 91-100% among treatments for the fish and 88-100% for the invertebrates. No significant differences in direct survival were detected between the two treatments and their respective controls for any of the species. Underwater video observations confirmed deployment of the sampling trawl inside the pulse trawl tracks, although part of swept area was outside the pulse trawl tracks. However, also when correcting the observed direct mortality for this, no differences between treatments and controls were detected. We conclude that with the right equipment, skilled skippers and a calm sea, it is possible to collect biota samples from a track recently trawled by a pulse trawler. However, it is very difficult to sample exclusively from a pulse trawl track; it is inevitable that part of the area swept by a sampling tow lies outside the pulse trawl track aimed for. Despite these limitations, any direct mass mortality caused by a passing pulse trawler would have been recorded in the current study, not only for the systematically observed species but also for other species in the samples. Our study thus did not find any evidence of direct mortality among plaice, dab, solenette, flying crab and brittle stars.
Original languageEnglish
Place of PublicationIJmuiden
PublisherWageningen Marine Research
Number of pages42
DOIs
Publication statusPublished - 2019

Publication series

NameWageningen Marine Research report
No.C097/19

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