Discarding from commercial fisheries is one of the biggest sustainability problems at a global level, because it involves a waste of living resources, causes biodiversity loss and affects food chains, thus possibly contributing to stock decline in species of fisheries interest (Bozzano and Sardá 2002, Bellido et al. 2011, García-de-Vinuesa et al. 2021). Trawling is the type of fishing that produces the highest amounts of discards because of its multi-specificity and low levels of selectivity (Thrush and Dayton 2002, Kaiser et al. 2006, Damalas et al. 2015). A clear example is the Mediterranean trawl fleet targeting Norway lobster (Nephrops norvegicus, Linnaeus 1758), in which the discard fraction often exceeds 30% of the catch (García-de-Vinuesa et al. 2018). Discards of N. norvegicus are mainly juvenile individuals below the mean size at maturity, which is estimated to range from 30 to 36 mm in the Mediterranean (Relini et al. 1998, Tsagarakis et al. 2014, García-de-Vinuesa et al. 2018).

In order to mitigate the problem of discards, through the current Common Fishery Policy the European Union has established a landing obligation (LO) for all animals with a minimum conservation reference size (MCRS). Under the provisions of the LO, species with an MCRS cannot be discarded and must be landed, at the fishermen’s expense and without the possibility of generating economic returns from the sale. The current regulation should encourage member states to adopt measures to enhance discard survival (EU Reg. 1380/2013; (EC) No 850/1998, Reid et al. 2019). N. norvegicus is affected by the LO, because it has an MCRS of 20 mm CL (carapace length) in the Mediterranean Sea (Council Regulation (EC) No. 1967/2006). However, the regulation allows member states to implement an LO exemption for species whose high survival of discarding can be demonstrated under experimental conditions (Art. 15 of EU Reg. 1380 /2013). This exemption could be especially useful to increase the biomass of future stocks of target species whose discarded juveniles show high survival.

In the Atlantic Ocean, high survival (0.51) has been demonstrated for discarded N. norvegicus (Méhault et al. 2016). Survival varies seasonally (Castro et al. 2003, Albalat et al. 2010, Mérillet et al. 2018) and can be affected by sorting and handling practices on board, injuries induced by the trawl (Bergmann et al. 2001, Macbeth et al. 2006), duration of exposure to air, and temperature changes (Davis and Olla 2002, Broadhurst et al. 2006, Benoît et al. 2012). 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. Therefore, it could now be possible to improve the discard survival of N. norvegicus, especially during the warm months, by enhancing vitality on board. The implementation of new techniques or methodologies on board, in the same way as in the Atlantic Ocean but adapted to the Mediterranean environmental conditions, could be especially useful for improving vitality on board of N. norvegicus, thus increasing their survival probabilities when they are discarded.

The goal of this study was to examine technical solutions for improving the sustainability of the N. norvegicus Mediterranean trawl fishery by improving the vitality on board of N. norvegicus juveniles through the implementation of new methods for handling discards.

A total of 1875 CVAs were carried out on 375 N. norvegicus. The environment temperature varied between 31°C and 34°C (average 32.3°C) for the control, between 22°C and 25°C (average 24.2°C) for treatment 1 and between 14°C and 16°C (average 15.3°C) for treatment 2.

The results of this study show that relatively simple modifications to the current discard-handling methods used by trawl fishermen in the Mediterranean can significantly increase the onboard vitality status of N. norvegicus juveniles, which could increase their discard survival, because there is a direct relationship between onboard vitality and discard survival (García-de-Vinuesa et al. 2020). This relatively simple solution would contribute significantly to the efficiency and sustainability of the fishery, because N. norvegicus stocks are currently exploited excessively (Quetglas et al. 2017, García-de-Vinuesa et al. 2018).

Although enhanced vitality on board of N. norvegicus has been directly related to discard survival in the Mediterranean Sea (García-de-Vinuesa et al. 2020), the onboard survival rate obtained in this study could be higher than discard survival, because N. norvegicus can suffer additional mortality in the longer time required by discard survival studies (ICES 2014, 2020). 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. Therefore, quick separation and introduction of juvenile N. norvegicus in cool water seems unfeasible, so the whole catch should be introduced in a container with seawater previously cooled to between 14°C and 16°C. 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. 1%) with the current regulatory trawl net, but juvenile N. norvegicus of legal size in the range 20 to 30 mm CL are routinely and legally discarded for market reasons, sometimes exceeding 40% of the discarded biomass (García-de-Vinuesa et al. 2018). Increasing the post-release survival likelihood of the discarded individuals would mean a better use of resources and economic benefits for fishermen, because surviving N. norvegicus could be added to the stocks of adult animals for this fishery. In addition, if this method were extrapolated to the rest of the Mediterranean trawl fleet, it could increase the chances of survival of other invertebrates belonging to trawl discards that have already been shown to have a high survival probability (Giomi et al. 2008, Tsagarakis et al. 2018, Demestre et al. 2018), by protecting them from thermal shock. The increased survival of the invertebrates discarded could lead to better conservation of the habitat of N. norvegicus (and other Mediterranean habitats affected by trawling), which would have a positive effect on the health of the stocks of commercial species that inhabit them by reducing mortality based on a better functioning of the benthonic and demersal food web (Demestre et al. 2018, García-de-Vinuesa et al. 2020).

The additional implementation of other technical measures, such as the “discard chute system” (Mérillet et al. 2018), would considerably reduce the discard time once the individuals have been dumped from the container with water, exposing the animals to stress for the shortest possible time and thus increasing their chances of survival (Davis and Olla 2002, Broadhurst et al. 2006, Benoît et al. 2012). In addition, in the Mediterranean it has been shown that short-duration hauls also have a positive effect on invertebrates’ survival probabilities when they are discarded from trawling (Demestre et al. 2018). Therefore, carrying out three or four hauls of short duration rather than one or two of long duration could also improve survival of discarding of N. norvegicus and other animals.

The implementation of technical and methodological improvements such as the introduction of the catch in cold water during the warm months prior to discarding improves the onboard vitality of N. norvegicus juveniles. This result can improve the sustainability and efficiency of the trawl fishery, with positive consequences for the economy of fishermen in the medium to long term and progress in the conservation of management-based habitats and ecosystems. The implementation of this methodological enhancement should be encouraged by the European Union, including through direct financial aid to fishermen (Veiga-Malta et al. 2019). However, as this new method can be implemented relatively easily, it can be transferred to fishermen sooner, because it can increase stocks of commercial species and the fishermen’s economy in the short to medium term, while at the same time helping to recover and conserve Mediterranean marine habitats.