The reform of the European Union Common Fisheries Policy brought into force the prohibition of discarding catches of regulated species (EC 2013). A Landing Obligation (LO) was included in this reform (Article 15 of EU Reg. 1380/2013) and affects all commercial species subject to catch limits or minimum landing sizes. In Mediterranean demersal fisheries, the LO is implemented in two phases: discards of species that define the fishery are prohibited from 1 January 2017 and the prohibition is extended to all other regulated species from 1 January 2019. The catches of regulated species below the minimum conservation reference size (MCRS) must be hauled and retained on board the fishing vessels, recorded and landed at ports; they may enter the productive economy for uses other than direct human consumption. Forcing producers to land former discards of regulated species is expected to be a significant step towards more selective fisheries (Prellezo et al. 2016). The total volume of discards (including non-regulated species) in EU Mediterranean trawl fisheries is estimated to range typically between 30% and 40% of the total catch (with extremes varying from 20% to 60%) (Tsagarakis et al. 2014). However, the volume of discards of regulated species (Annex III of EU Regulation 1967/20016) is less well researched and it was estimated at less than 10%, for instance, in a bottom trawl fishery of the Alicante area (SE Spain) (García-Rivera et al. 2015). Conversely, a recent study (Sartor et al. 2016) analysing Italian official fisheries data (EU Data Collection Framework, DCF) from 2009 to 2014 on the discards of the species characterizing the otter bottom trawl fisheries show that discards of European hake varied from 5% to 20% of the total catch, depending on the geographical subarea.

Geographical SubArea GSA06 (“Northern Spain”) is one of the 30 areas established by the General Fisheries Commission for the Mediterranean (GFCM) for fisheries stock assessment and management (http://www.gfcm.org). Fisheries management of GSA06 stocks, as for other Mediterranean fish stocks except large pelagics, is based on input measures that control fishing effort by limiting the capacity of the fleet (licence scheme) and the activity of vessels (Lleonart and Maynou 2003). Fishing is not permitted on weekends and an obligatory daily return to base port (maximum 12 working hours per day) is enforced. There are MCRS for most target species, but they are not fully enforced, contributing to the low economic efficiency of fisheries exploitation, as elsewhere in the Mediterranean and the European Atlantic (Colloca et al. 2014, Macher and Boncoeur 2010). Bottom trawlers operate with fishing gear that has a legal minimum mesh size of 40 mm (square mesh). Because output management measures (i.e. total allowable catch or quotas) are not in place for Mediterranean fisheries, again with the exception of large pelagics, fisheries management does not strictly depend on annual stock assessments of the target species, as is the case in European Atlantic fisheries (Lleonart and Maynou 2003).

Demersal fisheries in GSA06 are carried out with a variety of fishing gear, although in terms of production otter bottom trawl is by far the most productive (Lleonart and Maynou 2003). Over two hundred species (or market categories) are routinely landed in fish markets by bottom trawlers, but the bulk of the catches and economic value is made up by only 25 species (80% of value, Maynou 2014). Bottom trawlers in GSA06 practice two main fishing strategies: the mixed demersal fishery on the continental shelf and the deepwater fishery targeting valuable crustaceans.

Regardless of the LO, many studies point to the benefits of improving selectivity in trawl fisheries under biological and ecological criteria (Scott and Sampson 2011, Colloca et al. 2014, Maynou 2014). Selectivity improvements inevitably have short-term negative consequences for the fisheries operators’ profitability, and consequently a low incentive for adoption, but will in the long term help rebuild stocks and produce higher yields, particularly in heavily overexploited resources such as Mediterranean stocks (Colloca et al. 2014). On the other hand, the analysis of the Basque trawl fishery in the Bay of Biscay by Prellezo et al. (2016, 2017) showed that private incentives can arise to improve the selectivity and reduce discards through the adoption of more selective fishing gear by fishers as a logical outcome of the LO, on bioeconomic grounds only. Naturally, any modifications to the current trawl design should be practical and inexpensive to facilitate its wide and fast adoption by fishers. Here we use mathematical simulation modelling to carry out a bioeconomic analysis of the effects of adopting the experimental net described in Sola and Maynou (2018), which has good selection properties for the two target species of the transition phase (2017-2018) of the discards ban.

The most important species in the two fishing strategies practised in GSA06 by bottom trawlers, the continental shelf mixed demersal fishery and the deepwater crustacean fishery, are subject to regular stock assessments by the European Commission Scientific, Technical and Economic Committee for Fisheries (STECF) and the GFCM. Specifically, in recent years, the target species of the mixed demersal fishery (hake, red mullet, black anglerfish and blue whiting), as well as the three deepwater crustaceans (deepwater rose shrimp, Norway lobster and red shrimp) have been assessed by means of catch-at-age models from data submitted by national authorities to the STECF in the Data Collection Framework (STECF 2013, 2014, 2015a, 2015b, 2017). The biological parameters used in these assessments and their results (number at age, fishing mortality and biological reference points) were used to parameterize the bioeconomic model applied in this analysis (see below).

The economic parameters for the model were obtained from data sources such as the Annual Economic Report (STECF 2016) published by the STECF from official fisheries data (DCF) and national generic data available from https://stecf.jrc.ec.europa.eu/, complemented with our own observations in the MINOUW project (the H2020 Research and Innovation Action “Science, Technology and Society Initiative to Minimize Unwatched Catches in European Fisheries” (2015-2019), contract nº 634495). The economic parameters used were the average of the values for bottom trawlers in the 18-24 m and 24-40 m length classes because they are the most representative segments in terms of demersal fish production and comprise 75% of the bottom trawl fleet in GSA06.

The size frequency of bottom trawl catches in GSA06 for hake and red mullet are shown in Figure 1, with the modal distribution of age classes as established by the MIX model (Table 5). Only three age classes for hake and two for red mullet can be statistically detected from these data. For our purposes, this is sufficient because the first classes are the ones that produce unwanted catches under MCRS. For age class 0, the retention ogive of the modified net configuration was estimated at practically 0 (Table 5), while retention for ages 2 were practically 1.0 for both species.

The projections of the biological indicators for each species, under the assumption of stochastic mean recruitment, are shown in Figures 2 and 3. In the control scenario, with no modification in the trawl net, the average fishing mortality of the last three years is assumed to continue over the simulation horizon. Given the relatively high stock status estimated for most species in recent years (as shown here by the SSB indicator), catches are expected to be around the mean values observed in the years of the historical data series or higher. The introduction of a modification to the trawl net simulated from 2017 onwards providing better escape for juvenile hake and red mullet would involve a reduction in the fishing mortality of age class 0 in both species, and is projected to result in a rapid increase in spawning stock biomass (Fig. 2, right panel) and short-term catch losses for both species. However, the catch losses are not expected to last more than one year, given the rapid rebuilding of biomass. In economic terms, a comparison of Figures 4 and 5 shows a corresponding short-term reduction in the income from landings (and an associated reduction in other indicators, such as trade costs, labour costs and average crew wage), but income from landings was projected to reach higher values due to the increase in landings from hake and red mullet after 2018 (Fig. 5).

The amount of discards projected for 2015 and 2016, about 900 t for hake and red mullet combined, would decrease significantly, to about 100 t yr^–1, from 2017 onwards after the introduction of the trawl modification (Fig. 6). The value of former discards, in case of non-commercial use, would produce losses of ca. €0.5 million before the adoption of the new net. However, these losses would decrease rapidly by 90% with the adoption of the new net from 2017 onwards (Fig. 7). Likewise, if former discards were used commercially, the revenues from this source would amount to €0.2 million before the adoption of the new net and ca. €0.02 million after 2017 (Fig. 7). In both cases, the amount of income or loss from unwanted catches of hake and red mullet is estimated at less than €1 million or less than 0.5% of the income generated by the bottom trawl fishery (Figs 4 and 5, top right panel).

Our results show that the volume of unwanted catches under the MCRS for hake and red mullet (under the remit of the LO from the beginning of 2017) is relatively low in terms of overall bottom trawl fisheries production. Their obligatory landing and sale for non-human consumption would produce small additional revenue to trawler operators, of the order of €0.2 million assuming a relatively high price of 0.20 € kg^–1 for this product paid by fishmeal producers, for example. Adding the production of former discards from other demersal species that will produce unwanted catches from 2019, with the full implementation of the LO, is unlikely to double the additional revenue from former discards to trawl operators. In our analysis we have ignored the additional handling costs of former discards brought to land, but from personal interviews with producers (Maynou et al. 2018), these are not expected to be disproportionate. Our model does not include any investments by fishers that would be necessary to comply with the LO, but these are expected to be low. If former discards cannot be utilized industrially due to the lack of market, and producers are obliged to destroy the product at an estimated 0.50 € kg^–1 (Sartor et al. 2016), a minimum loss of income of €0.5 million can be expected. This figure does not include additional costs of disposal for 2019 and later or handling costs. Considering the magnitude of the income of bottom trawl fisheries in GSA06 (ca. €200 million), the introduction of the LO without technical adaption of the fishing gear is not expected to produce significant losses of revenues to operators. Modifying trawl nets to allow the escape of the undersize fraction of regulated species would yield important losses in profits in the short term (Table 6, Scenarios 1-3 for 2018), but the expected recovery of the stocks would quickly offset this loss. It is important to note that even if the amount of catches below the MCRS of hake is relatively low in terms of weight (typically 5%-10% of catch), the amount of catches in number of individuals is very high (30% or more: Fig. 1). In our model, the simulated increased escape of individuals below 20 cm total length with the adoption of the trawl modification described in Sola and Maynou (2018) significantly decreases fishing mortality (from Fbar_1-3=1.71 in 2015 to Fbar_1-3=1.22 in 2017) and helps explain the rapid rebuilding of the stock under constant recruitment conditions.

All in all, the adoption of more selective fishing gear is expected to help rebuild stocks and increase the profitability of bottom trawl fisheries, regardless of the commercial fate of unwanted catches under the remit of the LO. Our scenarios provide only minimum estimates because the adoption of a more selective trawl design, such as the one described in Sola and Maynou (2018), will certainly increase escape and probably decrease fishing mortality of juvenile fish of other species in this mixed bottom-trawl fishery. The scenarios analysed work with the F given by the change in fishing gear, disregarding other factors resulting from effort control or optimization of the means of production (labour, capacity).

Colloca et al. (2014) recommended changes in the selectivity patterns to decrease fishing mortality and delay the age at first capture as a more practical and efficient way to recover Mediterranean fish stocks than mere effort-reduction measures. Additionally, in light of the current Common Fisheries Policy, in which a ban of discards on regulated species can provide additional incentives for improving fisheries selectivity (Prellezo et al. 2016, 2017), our results show that it is possible to reduce fishing mortality and contribute to stock rebuilding by relatively simple, inexpensive modifications to trawl nets that enhance size selectivity.

The adoption of simple but efficient technological changes to bottom trawls can be a positive step towards reducing catches of the unwanted fraction of regulated species. In the short term, the introduction of the new fishing gear would have negative economic consequences, which are unlikely to last more than one or two years given the rapid turnover rates of Mediterranean fish stocks. The real impact in the medium to long term is harder to forecast for two main reasons: First, the strong stock rebuilding predicted by simple biological models under constant recruitment assumptions may be far from the reality of dynamic ecosystems with complex relationships among the components (e.g. intra-specific density dependence and inter-species interactions). Heath et al. (2014), based on a complex ecosystem model, showed how a discards ban in the North sea, without accompanying fisheries management measures, would even have adverse consequences on seabirds and other taxa that depend on discards for breeding success. Second, the extent to which the discards ban will be truly enforced in Mediterranean fisheries is uncertain. At current market prices, undersize fish have a much higher commercial value for human consumption than their value for purposes other than human consumption, such as reduction to fishmeal. As short-term economic incentives are lacking, the implementation of the LO will rely to a great extent on persuading the industry that stock rebuilding will produce future higher yields, or on strongly disincentivizing non-compliance.