The cuttlefish trammel net fishery operates in Mallorca from January to July, with a clear spring (March and May) peak in catches (Quetglas et al. 2015). The cuttlefish trammel nets are generally deployed at shallow depths of approximately 10 to 30 m, just at the lower boundary of the seagrass (Posidonia oceanica) meadows (Palmer et al. 2017).

The fishery is locally regulated by the Balearic Government (Regulation of Artisanal Fisheries in Balearic Inland Waters, Decree 17/2003, Balearic Official Gazette 28, 01/03/2003). Trammel nets used in this fishery have a total maximum length restriction of 2000 m per crew member (up to a maximum cumulative length of 5000 m). Because of the size of the local boats and crew, fishers in Mallorca usually fish with nets of approximately 1000 m length. Each trammel net is composed of 20 netting walls, each of which is approximately 50 m long. The mesh has a minimum legal size of 67 mm in the inner panel, although fishers often use nets with a mesh size of 80 mm, which corresponds to the minimum legal size allowed in the Balearic marine protected areas.

A total of 23 fishing trips was surveyed along Mallorcan coasts in the period February to July from 2015 to 2017. Sampling was conducted on two commercial boats, each rigged with two sets of trammel nets (Table 1), fishing at depths ranging from 10 to 32 m (mean±sd: 17.9±4.5 m).

For each fishing trip, the date, name of the boat, location, weather and sea conditions and depth were recorded by an observer on board. Also, for each trammel net, the initial and end coordinates and the number of netting walls were noted. As the net was progressively retrieved on board the boat, most of the items caught, either commercial or discard species, were assigned to a specific netting wall and were immediately identified to the lowest taxonomic level possible (usually to species level). Moreover, they were measured with a ruler, or if that was not possible, they were assigned by eye into ten-centimetre-increment size classes. Finally, the status of the discarded captures was determined before they were returned to the sea, based on their vitality (i.e. the individuals without reflexes or movement were considered dead, and otherwise they were considered alive). These data were used to estimate the immediate survival, detecting the probability of survival at the moment when the gear arrived on board. A subsample of live animals was retained on board and transported for the assessment of medium-term survival (detailed below).

Small animals, such as small invertebrates, and fragments of sessile organisms, such as Posidonia oceanica leaves and stems, were continuously falling from the net onto the deck. This fraction of the catch, which is part of the discard, could not be assigned to a specific netting wall, so was instead assigned at the trammel net level. All these organisms were collected separately for each trammel net and were kept for further analysis. In the laboratory, all organisms were identified to the species level when possible and weighed.

All data collected, both on board and in the laboratory, were organized into a hierarchical (fishing trip, trammel net, netting wall, item) database.

A subsample of animals that were to be discarded but that were alive when they came on board was used to assess medium-term survival. These animals were transferred to an onboard tank with sea water and continuous aeration and were transported to the Marine Research and Aquaculture Laboratory (LIMIA) of the Balearic Government, Spain, at the end of the fishing journey. The selected species were those with high abundance and an immediate survival higher than 20%.

The collected species were maintained separately in tanks according to the size of the individual, with continuous seawater overflow and controlled temperature. The status of the tested specimens was determined using a vitality scale (Benoît et al. 2010): 1, excellent; 2, good; 3, poor; and 4, moribund. The status of the specimens was recorded on their arrival (Day 0) and after seven days in the laboratory. The survival of each specimen was checked daily.

A questionnaire was designed to investigate the fishers’ perception about the new European discards ban. The questionnaire was designed for face-to-face interviews in the native language of the interviewee. The interview process started with the interviewee signing a confidentiality form, followed by the interviewer providing a short verbal description of the objectives of the project and a project brochure (available at http://minouw-project.eu/resources). Several questions, some of them open ended, were asked:

– What is the proportion of discards in the capture (0%-10%, 10%-25%, 25%-50%, 50%-75% or 75%-100%)?
– What are the most frequently discarded species (open question)?
– What percentage of discards corresponds to captures affected by the landing obligation (open question)?
– Will the application of the landing obligation affect the cost of the activity, the sorting time or the needs of personnel (Yes or No)?

Seven professional fishers who usually work in the trammel net cuttlefish fishery in different Mallorcan ports were interviewed. This fraction of interviewed fishers corresponded to 14% of fishers working in the cuttlefish fishery on Mallorca Island in 2016 (official data from the Regional Fisheries Administration (Direcció General de Pesca i Medi Marí, DGPMM). The interviews were conducted from November 2015 to January 2016.

After compiling the data from all the fishing trips of all the boats of the cuttlefish trammel net fleet, the seasonal variation in landing composition (i.e., biomass of the different commercial categories) was found to be larger than the between-boat variation: the convex polygon including all the fishing trips of each of the two sampled boats (dashed polygons in Fig. 1) included the average scores of all the boats in the fleet (i.e. averaged landing composition of any given boat; points in Fig. 1). Accordingly, in spite of the small number of surveyed boats (4% of the boats that used cuttlefish trammel nets at least once during the study period), the data should be considered representative of the métier activity.

A total of 1224 individuals captured and retained for sale were surveyed during this study, corresponding to a total of 46 different species. The most frequently commercialized species was the target species, Sepia officinalis, with an abundance of 0.35 items and 173.35 g per netting wall (Fig. 2). The second most common commercial capture was Scorpaena porcus (0.15 items per netting wall), followed by the gastropods Hexaplex trunculus, Stramonita haemastoma and Bolinus brandaris (Fig. 2A). However, due to their small size, these gastropods had little importance in the weight comparison. The high-value species Scorpaena scrofa, Zeus faber and Dentex dentex were relatively abundant, with a weight of 66.90, 8.28 and 7.55 g per netting wall, respectively (Fig. 2B).

The monthly temporal trends of commercial captures of the target species were compared with the trends of the rest of the commercialized species pooled together (i.e. commercialized by-catch; Fig. 3). The captures of the target species peaked in March with an abundance of 14.79 kg per trip but decreased sharply as the fishing season progressed. Conversely, the capture of the commercialized by-catch increased as the fishing season progressed, with a maximum weight of captures observed in July (18.02 kg per trip). Similarly, the gross revenues, estimated as the weight captured multiplied by the average price, followed the same pattern. The gross revenues of the target species were higher in March (€127.17 per trip) but then decreased with the decline of captures. However, the gross revenues of the commercialized by-catch were higher at the end of the fishing season, reaching values in July that were even higher than those of the target species in March (€286.08 per trip). After combining all the data (i.e. target and by-catch species), the mean revenues throughout the season were €174.82 per trip (95% confidence interval: €123.48-311.24).

A total of 825 discarded captures (i.e. items captured and discarded) was surveyed during this study, distributed across a total of 95 different species. The most discarded species was Holothuria sp. (Fig. 4A), with an abundance of 0.34 items per netting wall. This group includes the species Holothuria tubulosa and Holothuria forskåli, which are abundant in the coastal zone of the Mediterranean but are difficult to differentiate due to a lack of external diagnostic features. The rest of the discards were low-value species such as Leucoraja naevus and Raja clavata, small-sized species and damaged items that otherwise might have a higher price. The group OTHER, which encompasses all the discarded species with fewer than eight specimens, cannot be neglected (0.15 items per netting wall), indicating that a large number of different species are captured in very low numbers. The analysis of the weight of discards captured (Fig. 4B) was markedly different from the analysis of the number of items. Although Holothuria sp. was still the most abundant species (53.6 g per netting wall), in this case some species that were captured at a lower frequency but with a high weight showed similar values (e.g. Torpedo marmorata, Dasyatis pastinaca or Raja clavata). Furthermore, fragments of sessile organisms, such as Posidonia oceanica and calcareous seaweeds (mainly composed of Lithothamnion spp.), represented a non-negligible fraction of the discards (41.6 and 21.2 g per netting wall, respectively).

Up to 47.7% of the catch in terms of weight was discarded; however, this percentage was reduced to 42.0% when the fragments of the aforementioned sessile organisms were not considered. Similarly, the discarded percentage in terms of number of items was 40.3%.

In the cuttlefish trammel net fishery in Mallorca, only the species for which there exists a European minimum legal size (Annex III to Regulation EC No 1967/2006) will be affected by the landing obligation of the new European discards ban. Just 67 of the 825 discarded items (i.e. 8.1%) were captured under their minimum legal size (Fig. 5). These captures corresponded to 1.5% of the total discarded weight.

To determine immediate survival, 729 non-commercialized individuals of 62 species were examined when the net came on board, and 456 were found alive. The immediate survival of the 18 most frequent discarded species (n>8) is shown in Figure 6. Invertebrate species, such as Holothuria sp., Palinurus elephas, Maja crispata and Dardanus arrosor, had a high survival rate, close to 100%. However, fish species showed lower immediate survival, ranging from 62% for Scorpaena porcus to 5.4% for Raja clavata. Conversely, Synodus saurus, Labrus viridis, Dactylopterus volitans and Auxis rochei showed 100% mortality when the net came on board.

The immediate survival of the 67 discarded items affected by the landing obligation was analysed separately. However, some of them were captured in a very low number, so only the most frequent species (n>8) were considered for the immediate survival estimation. The probability of immediate survival of Palinurus elephas was 100%, while that of Solea sp. was 62% (95% Bayesian credibility interval between 0.42 and 0.79).

To determine medium-term survival, a total of 14 specimens of 4 different species (Holothuria sp., Dardanus arrosor, Torpedo marmorata and Scyliorhinus canicula) were transported to the laboratory in order to estimate the delayed survival (Table 2). All the specimens survived after 7 days in captivity. The initial vitality state (Day 0) ranged between 1.5 and 2.86. However, the final vitality state (Day 7) was 1 in all cases, which corresponds to an excellent condition.

The fishers’ perception of the discarded captures and the fishers’ opinion of the expected outcomes of the new landing obligation measures were recorded by means of seven face-to-face interviews. Four of the interviewed fishers believed that the discard ratio for the cuttlefish trammel net fishery was between 0% and 10%. The other three fishers believed that it was between 10% and 25%. According to fishers’ perceptions, the most frequently discarded species were Holothuria sp., Dardanus sp. (i.e. Dardanus arrosor and Dardanus calidus), Torpedo marmorata, Scorpaena porcus, Dasyatis pastinaca, Dactylopterus volitans and specimens of the Asteroidea family.

According to the fishers’ perceptions, the percentage of discards affected by the landing obligation was very small (mean±sd: 4.3±6.7%). Finally, most (85.7%) of the interviewed fishers reported that the application of the landing obligation will not increase the costs of the activity, the sorting time or the needs of onboard personnel.

The most abundant commercial capture was the nominal target species, Sepia officinalis, with 0.35 items per netting wall, but a long list of untargeted species was also marketed (up to 45 different species), suggesting a low selectivity of this gear and supporting the importance of the by-catch in this fishery and probably in most of the Mediterranean small-scale fisheries (Palmer et al. 2017). The revenues corresponding to such an apparently untargeted fraction became higher than the revenues of the target species as the fishing season progressed, thus supporting its great importance for total revenues (Batista et al. 2009). Accordingly, fishers may be progressively changing their fishing tactics using essentially the same net and fishing grounds to maximize profits. These changes are largely driven by external constraints such as cuttlefish catchability, catchability of by-catch and market demand. It appears that while fishers and managers continue to label the métier as cuttlefish trammel net, the fishers’ intentions are to maximize not the weight of cuttlefish catches but rather the total revenues. Most fishers are at or near the economic sustainability level (Maynou et al. 2013), so proper assessment and management of the fisheries should take into account that fishing tactics may be continuously adapted from mainly targeting cuttlefish to mainly targeting some of the by-catch species. In some studies, trammel nets deployed around the lower limit of P. oceanica are split into two different métiers based on a given threshold of catch similarity (Quetglas et al. 2016). However, a pragmatic alternative, which may better reflect fishers’ intentions, would be to consider them a single métier (cuttlefish/fish) because even fishers themselves are not able to accurately discriminate between them (Palmer et al. 2017).

During the fishing season, the captures of the target species peaked in March and April and decreased afterwards. The captures of commercialized by-catch showed the opposite trend, and captures increased from March and April onwards. The most plausible explanation for such a pattern is that the catchability or abundance of cuttlefish and by-catch change during the fishing season. The cuttlefish peak may be related to an increased catchability associated with the seasonal spawning migrations (Boletzky 1983, Belcari et al. 2002). Therefore, cuttlefish are available to the cuttlefish trammel net fishery from the end of winter (Boletzky 1983), while from autumn to winter the cuttlefish retreat to deeper waters, where they are mostly exploited by trawlers (Belcari et al. 2002, Keller et al. 2014). Conversely, the commercialized by-catch is mainly composed of demersal species that, in most cases, exhibit a home range behaviour. Therefore, the increase in catchability may not be related to migration but to increased activity, which may be ultimately driven by a water temperature increase. The positive relationship between catchability with passive gear and water temperature has been proposed elsewhere (Villegas-Ríos et al. 2014). In fact, the increased activity during the warmer seasons is related to intensified feeding to recover from the previous spawning effort in winter or to store energy for the next reproductive season, representing an increase in the probability of encounter between fish and fishers (Villegas-Ríos et al. 2014).

In this study, the discard ratio in weight (47%) was higher than those reported for similar trammel nets: about 10% in the Patraikos Gulf (Tzanatos et al. 2007) and 22% on the Portuguese coast (Batista et al. 2009). Conversely, the discard ratio in number of items reported here (40%) was smaller than in other studies. For example, discard ratios of 53% and 49% were respectively reported by Batista et al. (2009) and Gonçalves et al. (2007). In this study, the most frequently discarded item by far was Holothuria sp., which represented 15.2% of the discarded captures in weight and 41.8% in number of items. However, some large specimens of elasmobranchs such as Torpedo marmorata, Dasyatis pastinaca and Raja clavata had great importance for the total discarded weight, although they were captured in low numbers. Some of these elasmobranch species were also frequently discarded captures in the cuttlefish trammel net fishery on the coast of Sicily (Tiralongo et al. 2018).

Immediate return to the sea of the species showing high post-release survival is recommended, but proper assessment of survival is very difficult. Specifically, keeping animals in captivity is confounding the effects on survival of captivity itself with the effects of fishing (Pollock and Pine 2007). Nevertheless, the results on immediate and medium-term survival reported here may justify at least precautionary advice. For example, immediate survival for Holothuria sp. was 98% and medium-term survival was 100%. Therefore, the immediate return to the sea is recommended for this species because nearly 100% of captured specimens will survive and will be not affected by the fishing practice. Torpedo marmorata and Scyliorhinus canicula showed lower immediate survival (around 20%) but excellent medium-term survival, so returning them to the sea is also recommended. Releasing practices will be banned under the EU landing obligation for some species according to the new European discards regulation; however, none of these species (i.e. Holothuria sp., Torpedo marmorata and Scyliorhinus canicula) are affected by the new enforcement (Landing obligation, Article 15 of Regulation EU 1380/2013).

Live shoots, stems and rhizomes of the protected species Posidonia oceanica are a non-negligible part of the discards, indicating that the fishing gear interacted with this habitat-forming species. Therefore, the relative importance of effects of the trammel nets on seagrass meadows in relation to other potential impacts requires immediate attention.

The fishers’ perception of discards agreed well with the results reported here in terms of species composition. In contrast, their perceived quantity of discards was lower than the actual amount reported here. As in other cases, it seems that the accuracy of past experiences is biased by memory and subjectivity (Bradburn et al. 1987, Hassan 2005, Damalas et al. 2015). Therefore, studies based on interviews should not attempt to obtain quantitative estimates, though they may be extremely useful for knowing and understanding the general perceptions of the fishers. More interestingly, the landing obligation is not viewed as a threat, but this point is discussed in depth in the next section.

The new orientation of the Common Fisheries Policy (EU Regulation 1380/2013) calls for moving towards a gradual elimination of discards on a case-by-case basis. The landing obligation has been included in this new plan with the aim of encouraging fishers to use more selective fishing gear and practices (Salomon 2009, Veiga et al. 2016). However, only those species subject to catch limits or minimum landing sizes will be affected by the landing obligation in the Mediterranean. In this study, only 67 items (1.5% of discarded weight) would be affected by the landing obligation. This result agrees with the opinion of the interviewed fishers, who reported a low percentage of discards affected by the landing obligation, and most of them (86%) believe that the application of the landing obligation will not have any negative effects on their fishing activities. Note, however, that this fishers’ perception may change depending on the fishery. For example, fishers from a small-scale Galician gillnet fishery, which targets some species managed by total allowable catches, perceived that the landing obligation will negatively impact on their household economies (Villasante et al. 2016).

According to Article 15 of EU Regulation 1380/2013, some exemptions from the landing obligation are allowed:

A) Catches of prohibited species, which must be returned to the sea. In this study, no species prohibited in the Balearic Islands (Decree 139/2011 and Order AAA/1771/2015) were captured.
B) Catches of those species for which there is scientific evidence of high post-release survival rates. The two most abundant species affected by the landing obligation (Solea sp. and Palinurus elephas) could have a high probability of surviving. Notably, undersized Palinurus elephas, which showed 100% immediate survival in this study, showed an almost complete recovery of their vitality status a few days after fishing (Catanese et al. 2018).
C) Specific de minimis values of up to 5% of total catches of all species subject to the landing obligation under certain conditions. However, this exemption seems to have been developed for target species of industrial or large-scale fisheries.

Contrasting with the limited outcomes of exceptions, it is noteworthy that in the case of the cuttlefish fishery and other small-scale fisheries in Mallorca, a large amount of the catch is sold as mixed-fish commercial categories which may be biasing the landings statistics (Gil et al. 2018). This mixing strategy is fully in accordance with EU policy, which is intended to reduce discards by marketing lower-valued specimens (Gil et al. 2018).

In summary, here we demonstrate that assessing and managing small-scale fisheries based on the target species only provides a potentially biased view. In the case of Mallorca Island’s cuttlefish trammel net fishery, by-catch has even greater economic importance than the target species, the discarded fraction cannot be ignored, the survival probability of such a discarded fraction must be properly assessed and fisher’s perceptions should be accounted for. However, more effort should be invested in the future not only to estimate the relative impact of small-scale fishing at the ecosystem level, but also in other potential drivers affecting the long-term ecosystem dynamics, which should be based on long-term, comprehensive monitoring.