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

Schram, Edward; Molenaar, Pieke; Koning, Susan de

Summary

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. Until 2019 this had never been investigated in situ. In 2019 a pilot study developed a method for in situ assessments and collected the first data on direct mortality among fish and benthic invertebrates. More data using the same methodology were collected in a second experiment in 2020. The data collected in 2019 and 2020 were merged into one dataset. We here report on the data collection in experiment 1 and 2 and the results of the analysis of the combined data set. A pulse trawler equipped with double rigs made pulse trawl tracks specific for the current experiments. 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. Experiment 1 (2019) 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. In experiment 1 there was no difference in direct mortalities among biota sampled from the two pulse trawl treatments, therefore in Experiment 2 only the complete pulse trawl treatment was employed. The condition of three fish species and four species of invertebrates was assessed. Fish species included plaice (Pleuronectes platessa), dab (Limanda limanda) and solenette (Buglossidium luteum). Invertebrate species included flying crab (Liocarcinus holsatus), hermit crabs (Paguroidea spp.) brittle stars (Ophiuroidea spp.) and brown shrimp (Crangon crangon). Underwater video observations confirmed deployment of the sampling trawl inside the pulse trawl tracks, although part of the swept area was outside the pulse trawl tracks and data were corrected for this. Direct mortality was low and ranged from 0-10% among treatments for the fish and 0-16% for the invertebrates. Our study did not find any evidence of direct mortality nor deteriorated condition among plaice, dab, solenette, flying crab and brittle stars as a result of a passing pulse trawler. Throughout the study period (2019-2020), we have conducted workshops and interviews with Dutch small-scale fishers and representatives. The workshops and interviews were aimed at understanding the general perception of pulse fisheries and the specific concerns of small-scale fishers with regard to pulse fisheries. Our results show that after the pulse was banned and the first field work in 2019 had been completed, worries of small-scale fishers shifted from mass mortality among benthic organisms towards alleged misuse of pulse gear resulting in (local) overfishing and displacement of other types of fisheries. The interviews and workshops gave valuable input into study design, resulting in a change of location for experiment 2, as small-scale fishers attach great importance to the location of the fieldwork.