Flatfish survivors have tales to tell: Cold seawater and reduced deployment duration contribute to the survival of European plaice (Pleuronectes platessa) discarded by Belgian beam trawlers

Abstract

The ‘high survival’ exemption to the European Landing Obligation regulation intensified the relevance of quantifying the survival of commonly discarded European plaice (Pleuronectes platessa) within the discard-intensive Belgian beam-trawl fishery, together with the search for ways to maximize discard survival. In this study we worked together with vessel owners and the crews of three Belgian beam trawlers representing the small (≤221 kW) engine power, coastal (n = 1 vessel), ‘Eurocutter’ (1), and large (>221 kW) (1) fleet segments. During 12 commercial trips which included 99 beam-trawl deployments in 2014 and 2015, three scientific observers assessed the survival of 3849 plaice aboard the vessel (by checking for the presence of reflex responses and bleeding injury; on-board survival assessment) and counted how many survived among a sub-sample (n = 489 plaice) monitored for at least 9 days in captivity (post-release survival). These fish were monitored alongside 160 captive controls. Only three control fish died (<2%) during daily monitoring. Survival of plaice discarded from conventional trawls of Belgian beam-trawl vessels representing the three fleet segments was estimated to range between 41–58 %, 11–28 %, 2–4 % (95 % confidence interval; Kaplan-Meier models) for trips of the coastal (≤221 kW), Eurocutter (≤221 kW) and >221 kW vessel, respectively. More challenging operational fishing conditions of >221 kW compared to ≤221 kW vessels such as ∼150 min gear deployment and ∼37 min sorting durations meant that down to only 20 % of undersized plaice were still alive when landed on deck. Such immediate mortality was hardly observed among ≤221 kW vessels. These effects were cumulative to the effect of elevated seawater temperatures of >14 °C during summer, because despite warm waters, immediate mortality among ≤221 kW vessels was low compared to >221 kW vessel-trips. Out of those fish that were still alive after sorting at the point of discarding and monitored for 9 days, on average 23 % (0%–100 %, 95 % CI) survived. Post-release survival was associated with stress and injury sustained during the capture and sorting process (proxied by the reflex impairment and injury index) and seawater temperature. Survival rates can be improved by choosing when to fish, and restricting gear deployments to practicable minimum durations. Without further adaptations to current fishing practices, justifying a high survival exemption for discarded plaice in the Belgian beam-trawl fishery will remain a challenge in compliance with the Landing Obligation.