The EU discards ban embedded in Art. 15 of EC 1380/2013 EC 2013, or the Landing Obligation, is aimed at reducing discards in EU fisheries and working towards more selective fishing by incentivizing fishers to apply appropriate technical solutions, among other changes in fishing practices. The policy objective behind the Landing Obligation is the perception that fisheries discards are a structural deficiency of European fisheries (EC 2012) and that resources that can be used productively in the maritime economy are wasted. The discards ban aims at rationalizing the fishing process by means of more selective gears and sustainable practices (Gullestad et al. 2015). In Mediterranean fisheries, the quantity of discards is perceived to be high, but with important variation across fleet segments or fishing gear (Uhlmann et al. 2014). Bottom trawl fisheries are among the fisheries with the highest amount of discards, both total and of regulated species. For example, Tsagarakis et al. (2014) estimate discard rates of 20% to 65% of the total catch in Spanish Mediterranean bottom-trawl fisheries, while discards of undersize individuals of regulated species such as Trachurus spp. or Sparidae vary from 18% to 77% of the catch of each species (Bellido et al. 2017). However, the quantity of discards is generally known with low precision due to the high variability in the quantity of discards, even within a single fishery, because of the varied reasons for discarding (Martinet et al. 2007, Uhlmann et al. 2014).

The Landing Obligation entered into force progressively, with the discards ban in small pelagics fisheries effective as of 1 January 2015 and its application to all regulated marine species by 1 January 2019. In western Mediterranean bottom trawl fisheries, the discards ban for species that define the fisheries, the European hake Merluccius merluccius and the red mullet Mullus barbatus, entered into force on 1 January 2017. With the current legal bottom-trawl mesh sizes of 40 mm (square mesh) or 50 mm (diamond mesh), it is not possible to avoid catching unwanted individuals of these species, and particularly so for M. merluccius, which has an estimated mean retention length of 10 to 16 cm TL in these meshes, compared with a minimum conservation reference size (MCRS) of 20 cm (Bahamon et al. 2006, Guijarro and Massutí 2006, Sala and Luchetti 2011).

Investigating the effect of simple modifications to the current bottom trawl is interesting because these modifications may work towards reducing unwanted bycatch without undue changes to the fishing practices of fishers or expensive modifications. It is important to note that the Landing Obligation may result in additional sorting and handling costs estimated at 0.50 €/kg or higher (Sartor et al. 2016), hence the importance of devising inexpensive solutions that mitigate the effects of the Landing Obligation without imposing excessive costs.

The objective of this work is to investigate the relative catch performance of a simple modification to the extension piece of a bottom trawl with regard to the two species that characterize the fishery, the European hake Merluccius merluccius and the red mullet Mullus barbatus, with additional data on the striped red mullet M. surmuletus, which is also caught in this fishery. Our research focused on the substitution of a standard diamond-mesh extension piece of 53 mm, as used in NW Mediterranean bottom trawls, with a similar piece with netting turned 90° (T90). This modification is based on the improved selection properties that this netting has shown when used in codends in Northern European fisheries (Madsen et al. 2012, Wienbeck et al. 2011), due to increased resistance of the T90 netting to close under pressure compared with the traditional orientation.

The size frequency distributions of the three species under study are reported in Figure 3. The size frequency of hake (Fig. 3, left) was shifted to the right in the experimental trawl, with a modal length of capture around 18 cm TL, while the mode in the control trawl was 15 cm. In the case of red mullet (Fig. 3, center) the size frequency observed lies in a narrow range of 13 to 22 cm (10 size classes), with a single peak at 15 cm for the control trawl and a mode around 16 cm for the experimental trawl. The striped red mullet (Fig. 3, right) showed a bimodal size frequency for the control trawl, with peaks at 13 and 18 cm, while the experimental trawl produced a larger quantity of individuals in the first mode (13-14 cm) and a barely noticeable second mode.

A Kolmogorov-Smirnov test of difference between the size frequencies of the standard and the experimental net showed no significant differences (hake, D=0.231, p=0.493; red mullet, D=0.203, p=0.995; striped red mullet, D=0.176, p=0.954).

The length-dependent catch comparison analysis for the three species produced the models whose parameters are given in Table 2 and plotted in Figure 4 (left panels). In the case of hake and red mullet, the best models selected included polynomial terms of degrees 0, 1, 2, while in the case of the striped red mullet a constant intercept polynomial produced the best results (Table 2). The fit statistics of the models are given in Table 3. The p-values obtained were above 0.05 for all three species, suggesting that the data are well described by the models.

Figure 4 (top, left) shows that the experimental trawl has higher catches of hake larger than 16 cm (approximately) than the standard trawl, with a strong reduction in catches of individuals in the smaller size classes. In the case of red mullet (Fig. 4, middle, left), a slightly higher, but not statistically significant, rate of catches was detected for individuals in the range 16-20 cm, with significantly reduced catches below 15 cm in the experimental trawl (confidence intervals below the baseline of 0.5). For striped red mullet (Fig. 4, bottom, left) the relative catches of 13 to 15 cm fish were higher in the experimental net, but while the experimental net had lower catch rates for lengths greater than 15 cm, the difference was not statistically significant from the standard trawl (confidence intervals within the baseline of 0.5).

The quantitative differences in the catch efficiencies are shown in the catch ratio curves (Fig. 4, right panel). In the case of hake (Fig. 4, top, right) the experimental net caught fewer individuals smaller than 12 cm and more individuals larger than 20 cm. In the case of red mullet, the catch ratio of the experimental trawl was significantly lower than the standard for sizes below 15 cm and higher than 23, with significantly higher catch rates between 17 and 20 cm. The experimental trawl did not produce significant differences in catch efficiency for striped red mullet. Table 4 shows the catch ratios, in percentage, between the two fishing gears (baseline of no effect=100%) for the typical fish sizes collected in the experiments (10 to 25 cm TL). Although the confidence intervals are wide, the experimental net had a lower catch efficiency of individuals of 15 cm TL and below for both hake and red mullet. On the other hand, the average catch efficiency of the experimental net for hake was 150%, i.e. overall it caught 50% more quantity of hake regardless of size. In the case of red mullet, a loss of 34% could be expected, and for striped red mullet a loss of 13% (Table 4).

Computing the empirical catch ratio (Eq. 4) for sizes below an MCRS of 20 cm TL for hake and 11 cm TL for both red mullets resulted in a value of 52% for hake, i.e. the experimental trawl caught about half the quantity of undersize hake that the standard trawl caught. In the case of red mullets, no undersize specimens were caught by the standard or the experimental trawl.

In addition to the study target species, the catches of accompanying species of commercial interest was also modified in the experimental trawl, as shown in Table 5. In general, both catches and total discards were lower in the modified net, although the proportion of total discards was similar: 47.6% and 48.8% respectively. All fish species except M. merluccius and Pagellus erythrinus showed lower catch rates with the modified net. The overall catch of invertebrates (commercial cephalopods and the echinoderm Stichopus regalis) were lower in the modified net, although some species of cephalopods (Alloteuthis media, Octopus vulgaris and Sepia officinalis) showed higher catches. In economic terms, the modified net produced a 17% decrease in the marketed catch and an 18% decrease in the commercial value.

Our results show that introducing the T90 mesh in the trawl extension piece reduces the likelihood of capturing small individuals for two important commercial species of Mediterranean fisheries, the European hake (Merluccius merluccius) and the red mullet Mullus barbatus. This result is relevant in terms of fisheries management because these species fall under the Landing Obligation in the transition period 2017-2019. In the case of M. merluccius, the adoption of this modification to the trawl extension helps bring L50 well below the MCRS with current trawl regulatory mesh size, more in line with the 20 cm legal size, while at the same time increasing catch rates. For the two red mullet species Mullus spp., regulatory mesh size already catches individuals larger than the MCRS of 11 and the modification would result in a non-negligible catch reduction.

The introduction of modifications to fishing gear that improve fisheries selectivity will be successful only if these modifications are practical (easy to use and inexpensive), can be acceptable to industry and managers, have low environmental impact and are easily enforceable (Catchpole et al. 2008). Simple modifications to the trawl net such as the one proposed here meet these requirements and are particularly interesting for hake, whose exploitation is excessive and relies to a large extent on undersize individuals. Other authors have shown the selective properties of T90 netting either in the codend or the trawl body. T90 netting has interesting properties regarding species and size selection when used in codends. For instance, Deval et al. (2016) tested codends with T90 meshes in four commercial shrimps of eastern Mediterranean trawl fisheries and showed that the percentage of escapes for all four species increases, as well as obtaining an increase in the L50. In Northern European fisheries, T90 codends have been subject to comprehensive studies that show a clear improvement in selectivity (Moderhak et al. 1999) and in the quality of the fish marketed (Hansen 2004). Despite the number of studies carried out with T90 codends, we are not aware of any other studies testing selection at the level of trawl extension.

The proposed modification does, however, significantly reduce the total commercial catch of the vessel, by 17% in volume and 18% in value. Most of the species affected by this reduction are bony fishes, in particular small anglerfish (Lophius sp.) and poor cod (Trisopterus minutus). It is expected that the economic loss can be partially offset by decreased sorting time and costs and decreased costs related to compliance with the Landings Obligation, but certainly short-term losses of income are a barrier to the adoption of more selective technologies. Nevertheless, this short-term reduction of commercial catch can be justified by the need to reduce fishing mortality in the Mediterranean demersal fisheries for overexploited target stocks. It should lead to a recovery of stocks and provide higher yields in the medium to long term Merino et al. (2015).