Seasonal variation in the survival of discarded Nephrops norvegicus in a NW Mediterranean bottom-trawl fishery

Abstract

The landing obligation in the revised European Union Common Fisheries Policy allows for exemptions to obligatory landing of the entire catch for species for which "high survival" of discards can be demonstrated. Nephrops norvegicus is an important target species in many fisheries across Europe in the Mediterranean Sea, NE Atlantic Ocean and North Sea. Historically, Mediterranean fisheries have had a high discard rate of small-sized Nephrops, and it is suspected that this unwanted component of the catch may have a high survival potential that is comparable to those of other EU fisheries, where survival rates of up to 0.56 have been demonstrated. However, to date, no investigations have confirmed a high discard survival rate for Nephrops in the Mediterranean Sea. Furthermore, the environmental, technical and biological characteristics that could affect Nephrops survival have been shown to be substantially different from those in the survival assessments conducted in the NE Atlantic and the North Sea. To address this knowledge gap, this study was conducted to determine the survival of Nephrops discarded from trawls in the Mediterranean Sea. The survival and vitality status of the discarded Nephrops removed from trawl catches were monitored onboard and for 14 days in the laboratory. The results showed seasonality in survival, with the highest survival rate in winter (0.74; CI: 0.7−0.78), lower survival in spring (0.36; CI: 0.31−0.41) and the lowest survival in summer (0.06; CI: 0.04−0.09). Survival was monitored to the asymptote in all cases, and season and vitality status were shown to have statistically significant relationships with survival.

Enhancing onboard post-catch vitality of discard Norway lobster (Nephrops norvegicus) for more sustainable Mediterranean trawl fishery

“…Improvements in onboard vitality in N. norvegicus treated with cool water may be due to a reduction in stress that is accompanied by a recovery in plasma values and muscle metabolites such as lactate found in non-stressful conditions, although the only study carried out in this regard had a longer duration (Barragán-Mendez et al 2020). For the implementation of this new discard-handling method, it should be taken into account that trawlers in the Mediterranean usually carry out several hauls during the fishing day (Demestre et al 2018, García-de-Vinuesa et al 2020. For this reason, after their arrival on board, the catch remains on the deck waiting for the crew members to finish carrying out the manoeuvre of shooting the trawl net to start the next haul, which usually takes at least 20 minutes.…”

“…This refrigeration can be carried out by simple means such as contact with the ice in the container (all trawlers carry ice compulsorily to keep the commercial catch cool for sale) well in advance until the manoeuvres are finished. This practice would improve the post-release survival of trawling of N. norvegicus, especially during the warm months, when their mortality has been shown to be higher both in the Mediterranean and in the Atlantic (Méhault et al 2016, García-de-Vinuesa et al 2020, Fox et al 2020. In this trawl fishery, the amount of catches under the MCRS is very low (ca.…”

“…In recent years, the survival probability for unwanted fractions of Atlantic N. norvegicus catch has been improved through implementation of new rapid discard techniques, such as the "discard chute system" in trawlers, that allow for a shorter time exposure of N. norvegicus to stressors (Mérillet et al 2018). A vitality study carried out in aquariums (captivity methods) (García- de-Vinuesa et al 2020) showed that discard survival of N. norvegicus from the Mediterranean Sea has a large component of seasonal dependence and is directly related to their vitality on board. The survival was high during the winter months (0.74) and low or almost null during the warm months in spring (0.36) and summer (0.06); that is, survival is strongly linked to seasonal changes in air and water temperature.…”

Population Dynamics, Fishery, and Exploitation Status of Norway Lobster (Nephrops norvegicus) in Eastern Mediterranean

“…Due to its importance and high economic value as a commercial species in the Atlantic and North Sea [2,3,9], Nephrops has been the subject of several biological and fishery studies , leading to the collection of long time series data for that region. In regard to the Mediterranean Sea, most of the studies have focused on the Western region and the Adriatic sea [7,8,13,15,[30][31][32][33][34][35][36][37][38], with the majority of them within the framework of the EU research project "MEDITS" [39], contrary to even fewer studies in the Eastern Mediterranean and the Aegean Sea [40][41][42][43][44]. The aim of the present study was to fill that knowledge gap in the Eastern Mediterranean, since no such studies were found for the region.…”

Effects on faunistic composition and population characteristics of decapod crustaceans after the implementation of a fisheries no-take area in the NW Mediterranean

“…However, not all species are affected in the same way. Some may be more vulnerable and others may be more resilient (Dimech et al 2012, García de Vinuesa et al 2020. Additionally, some of them may be key species for the proper functioning of the ecosystem.…”