Edward Schram

We measured discards survival probabilities of thornback (Raja clavata) and spotted skate (Raja montagui) in tickler chain beam trawling (5 trips, n = 183 for thornback skate, n = 137 for spotted skate), pulse beam trawling (9 trips, n = 94 for thornback skate) and flyshoot fishieres (4 trips, n = 137 for thornback skate, n = 24 for spotted skate). Survival probabilities were measured by captive observation for 15 to 25 days post catch. All fishery operations were conducted in the southern North Sea (ICES division 27.4.c) and in the Eastern English Channel (ICES division 27.7.d) according to the regular commercial practices of the fishing vessels. Trips were spread out over the seasons to account for the effect of variable environmental and fishing conditions on discards survival. Operational and environmental conditions during sea trips were recorded. For beam trawling survival probabilities (95% CI) were 50% (43–57) for thornback and 44% (37–54) for spotted skate. For pulse trawling, survival probability of thornback skate was 54% (40–65). For flyshooting survival probabilities were higher: 80% (73–87) for thornback and 75% (60–94) for spotted skate. Survival probabilities were significantly affected by gear, catch processing time, water temperature, wave height and the interaction between water temperature and wave height. We found no evidence for effects of species and length on survival probability. Vitality class provides a useful qualitative prediction of survival probability within gear. These findings are relevant for the fisheries management of skates stocks in the North Sea and English Channel.

Between 2009 and 2021 almost the complete beam trawl fleet of the Netherlands switched from conventional beam trawls (BT) to pulse trawls (PT) using electrical stimulation to catch sole, Solea solea. Electric fishing, being banned in the EU in 1988, was made possible in 2006 under a derogation. Over the years stakeholders expressed concern about ecosystem effects. Here we review the research conducted. PT improved the selectivity of the fishery and reduced the ecological side effects. PT caught more sole per hour fishing but less discards and benthos than BT. The transition to PT reduced the surface area swept (lower towing speed), sediment depth of disturbance and associated benthic impacts, as well as fuel consumption. Laboratory experiments with 9 fish and 17 benthic invertebrate species showed that exposure to a commercial bipolar pulse stimulus did not result in harmful effects except in cod. Autopsy of cod sampled from PT revealed that 40% had an internal injury. Injury rates in other roundfish species was low (< 2%) and absent in flatfish. Electrical-induced impacts on biogeochemistry were not observed. The transition increased competition with other fishers which fed the resentment against PT. Governance arrangements under which the number of temporary licenses expanded, undermined legitimacy of the gear, resulting in a ban in 2021. Although questions about the ecological impact of electrical stimulation remain, adverse effects are considered negligible in comparison with the benefits.

Undersized European plaice dominate the discarded fraction of the catch of the beam trawl fisheries for sole in the Southern North Sea. Effects of environmental conditions at sea and of the use of a water-filled hopper on the survival of undersized European plaice discarded by pulse trawl fisheries were explored. During trips with commercial pulse-trawlers catches were discharged in either water-filled hoppers or conventional dry hoppers. For both hoppers, undersized plaice were sampled from the sorting belt. After assessment of vitality status, sampled fish were housed in dedicated survival monitoring tanks on board. Upon return in the harbour fish were transferred to the laboratory to monitor their survival for up to 18 days post-catch. Conditions at sea, such as wave height and water temperature, as prevailing during these trips were recorded or obtained from public data sources. The overall estimate for the survival probability for plaice discarded by pulse trawl fisheries is 12% (95% CI: 8% - 18%). Both water temperature and vitality status had strong effects on survival probabilities of discarded plaice. Increasing water temperature increased mortality. The vitality of the fish could be moderately increased by using a water-filled hopper to collect the fish on deck, but we found no significant direct effect of hopper type on plaice discard survival. It seems that to increase discards survival, fish need to be landed on deck in much better condition by a reduction of the impact of capture and hauling processes on fish condition.