Discards regulation vs Mediterranean fisheries sustainability
M. Demestre and F. Maynou (eds)

INTRODUCTIONTop

Coastal and maritime activities have traditionally been important for the national economy and the historical, social, and cultural identity of Portugal (Leitão and Baptista 2017Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83). The country has long relied on fishing as a major means of subsistence and many coastal communities depend almost exclusively on small-scale coastal and estuarine fisheries and related activities. However, over time substantial technological improvements and changes have been made in the fisheries. For example, in the around 1850, steam-powered vessels were introduced to the fishing fleets, resulting in a reduction of total fishers (Alves 1991Alves J.F. 1991. A Pesca e os Pescadores do Litoral Portuense em 1868. In Anon. (ed.) Revista da Faculdade de Letras. Vol. III. Faculdade de Letras, Porto. pp. 151-183). Additionally, fishers began to deploy for the first time an industrial gear, the otter trawl, which immediately created conflicts between the small-scale sector and this newly developing industrial sector (Baldaque da Silva 1891Baldaque da Silva A.A. 1891. Estado Actual das Pescas em Portugal. Impr. Nacional, Lisboa, Portugal. 524 pp., Alves 1991Alves J.F. 1991. A Pesca e os Pescadores do Litoral Portuense em 1868. In Anon. (ed.) Revista da Faculdade de Letras. Vol. III. Faculdade de Letras, Porto. pp. 151-183). According to Hill and Coelho (2001)Hill L., Coelho M.L. 2001. Portuguese fisheries in Portugal for the period 1950-1999. Comparison with ICES data. In: Zeller D., Watson R., Pauly D. (eds), Fisheries impacts on North Atlantic ecosystems: Catch, effort and national/regional data sets. Fisheries Centre Research Reports 9. University of British Columbia, Vancouver, pp. 187-190., there was a decrease in the number of vessels in the Portuguese fishing fleet between 1989 and 1999, but this was compensated by an increase in vessel power. By 1996, 98% of the fishing fleet was motorized—a 2% increase from 1986. Today, in mainland Portugal, a variety of gears/métiers are used in the coastal fisheries, ranging from trawls to static gears such as gill nets and traps. Therefore, a wide variety of unwanted species are captured along with the target species (Gaspar et al. 2003Gaspar M.B., Leitão F., Chicharo L., et al. 2003. A Comparison of direct mortality inflicted on macrofaunal organisms by three types of dredges used in the Portuguese clam fishery. ICES J. Mar. Sci. 60: 733-742., Gonçalves et al. 2007Gonçalves J.M.S., Stergiou K.I., Hernando J.A., et al. 2007. Discards from experimental trammel nets in Southern European small-scale fisheries. Fish. Res. 88: 5-14., Bordalo-Machado et al. 2009Bordalo-Machado P., Fernandes A.C., Figueiredo I., et al. 2009. The black scabbardfish (Aphanopus carbo Lowe, 1839) fisheries from the Portuguese mainland and Madeira Island. Sci. Mar. 73S2: 63-76.). Different types of gear often compete for the same resources (Borges et al. 2001Borges T.C., Erzini K., Bentes L., et al. 2001. By-catch and discarding practices in five Algarve (Southern Portugal) métiers. J. Appl. Ichthyol. 17: 104-114.). However, as different gear types and métiers target different organisms (Watson et al. 2006aWatson R., Revenga C., Kura Y. 2006a. Fishing gears associated with global marine catches. I. Database development. Fish. Res. 79: 97-102., bWatson R., Revenga C., Kura Y. 2006b. Fishing gears associated with global marine catches. II. Trends in trawling and dredging. Fish. Res. 79: 103-111.), unreported catches such as discards also differ from métier to métier.

Discards refer to the part of the catch that is not retained on board during commercial fishing operations but is returned to the sea. Discarding of marine organisms is a widespread feature of commercial fishing operations. Discard patterns are affected initially by catch compositions, which are determined by environmental factors, the fishing gear and fishing tactics used, and ultimately by fishermen themselves when they decide which parts of the catch to retain. This decision is influenced by both market and regulatory conditions, and is constrained by space and time: storage space on board the vessel and sorting time (Catchpole et al. 2014Catchpole T.L., Feekings J.P., Madsen N., et al. 2014. Using inferred drivers of discarding behaviour to evaluate discard mitigation measures. ICES J. Mar. Sci. 71: 1277-1285.). None of the historical accounts published between 1800 and 1950 on Portuguese fisheries address unreported catches, by-catch or discards. The first study for the purse seine was published by Borges et al. (2001)Borges T.C., Erzini K., Bentes L., et al. 2001. By-catch and discarding practices in five Algarve (Southern Portugal) métiers. J. Appl. Ichthyol. 17: 104-114. in 1997. The lack of information on by-catch and discards from this period might suggest that either most of the nearshore, artisanal catches were consumed or used and not discarded, or that discarding may have been low and utterly ignored. Brandão et al. (2000)Brandão M.A., Godinho M.M., Kovács I. 2000. Pescas e pescadores, futuros para o emprego e os recursos. Celta ed., Oeiras, 353 pp. describes how all fish were processed, salted, and dried by Portuguese women, indicating that fish discards may have been minimal between 1800 and 1950.

The way different gears operate suggests that long-term monitoring is required to improve our understanding of the factors affecting discarding and of the implications of the levels of discarding on the marine community structure (Hollingworth 2000Hollingworth C.E. 2000. Ecosystem effects of fishing. Proceedings of an ICES/SCOR Symposium Held in Montpellier, France, 16-19 March 1999. ICES J. Mar. Sci. 57: 465-792., Kaiser and de Groot 2000Kaiser M.J., de Groot S.J. 2000. Effects of Fishing on Non-Target Species and Habitats. Biological, Conservation and Socio-Economic Issues. Oxford: Blackwell Science, 399 pp.). The knowledge of the overall quantity of species caught in coastal marine systems (including unreported catches) is key to understanding the indirect effects of removal of particular taxa from the system. In fact, over the past three decades, renewed interest in a more ecological approach to fisheries (an ecosystem-based management approach) has emerged.

The new European Union Common Fisheries Policy, which started to be implemented in 2014, sets out a gradual elimination of discards by reducing unwanted catches and ensuring that all catches are landed. Illegal, unreported and unregulated catches (IUU) are one of the most important topics in fisheries from both an economic and an environmental point of view (Alverson and Hughes 1996Alverson D.L., Hughes S.E. 1996. Bycatch: from emotion to effective natural resource management. Rev. Fish Biol. Fish. 6: 443-462., Kelleher 2005Kelleher K. 2005. Discards in the world’s marine fisheries: an update. FAO Fish. Tech. Pap. 470: 1-131., FAO 2010FAO. 2010. Report of the Technical Consultation to Develop International Guidelines on Bycatch Management and Reduction of Discards. FAO Fisheries and Aquaculture Report 957. Rome, 32 pp.). The quantification and composition of the unreported catches and the understanding of the fate and impact of these unreported actions are key issues in fisheries (Zeller et al. 2007Zeller D., Booth S., Davis G., et al. 2007. Re-estimation of small-scale fisheries catches for U.S. flag island areas in the Western Pacific: The last 50 years. Fish. Bull. 105: 266-277., 2011Zeller D., Rossing P., Harper S., et al. 2011. The Baltic Sea: Estimates of total fisheries removals 1950-2007. Fish. Res. 108: 356-363.).

In Portugal it was estimated that an average of 123495 t/y (35.5% of the total catch) was unreported between 1938 and 2009 (Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50). Overall, reconstructed total catches in Portugal amounted to just under 21.6 million t in 1950-2010, which is slightly more than twice the 10592310 t of landings officially reported by Portugal for the same time period. Discards contributed the most to the unreported catches, accounting for 7.6 million (i.e. 35%) t of total catches.

Many fisheries around the world have reached unsustainable levels and therefore deliver poor income to fishers. An effective fisheries management is urgently needed to improve the economic situation of fishing communities. Part of the solution is to reduce discards by finding market-based approaches that will increase the value for all by-catch fish (Leitão and Baptista 2017Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83). The necessity of each country to manage all fisheries within their Exclusive Economic Zones (EEZ), a consequence of the United Nations Convention on the Law of the Sea (UNCLOS), led to attempts to find sustainable indicators for marine fisheries and ecosystems at the national level, including economic effects. However, information about unreported discard ban species that can comprise additional alternative value to the fishery sector is still scarce. Prohibited for the first time in some EU fisheries in 2009, economic-led high-grading is today illegal for all quota species, under amendments to fisheries technical measures enacted by the European Parliament and Council in March 2013 (Regulation (EU) No 227/2013). This means that fish that were discarded before should now have an economic value independently of their final use. Furthermore, sales of this fish will have to be accounted for and included in the country’s economy (Leitão and Baptista 2017Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83). However, so far the amount of information regarding total volumes and species discarded is lacking. Underestimation of catches is especially important in countries where fishing fleets are highly diversified, the enforcement of fishery management is low, data availability is poor, and there is high demand for fish products in local markets (Coll et al. 2014Coll M., Carreras M, Cornax M.J., et al. 2014. Closer to reality: reconstructing total removals in mixed fisheries from Southern Europe. Fish. Res. 154: 179-194.). Estimation of unreported catches for Portuguese fishery was based on a fishery-by-fishery approach by Leitão et al. (2014)Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50. Herein we used information of unreported catches, for each commercial métier (from Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50) and we reconstructed taxonomic profiles of unreported catches, namely discards by commercial fishing sector.

MATERIALS AND METHODSTop

Taxonomic rebuilding of unreported discards

Details on the estimation of the amount of unreported catch per métier and for the recreational/subsistence sector are provided elsewhere (Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50). Briefly, Leitão et al. (2014)Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50 used two data sources from the INE (Portuguese National Statistical office: https://www.ine.pt/xportal/xmain?xpgid=ine_main&xpid=INE) to acquire data: the digital data series starting in 2000, and the data from manuscripts (http://inenetw02.ine.pt:8080/biblioteca/logon.do;jsessionid=6D32727EEDCD9F2223353F2D3D81DB70; last accessed in April 2012) for the years 1934-1999. Many species were described by the fishing sector during the time series (e.g. sardine, European hake, horse mackerel, mackerel and octopus, which together accounted for most of the landed catches). As of the 1970s, data were available by fishing gear and many species were reported by fishing sector (trawl, seine and multi-gear). Since the gear-specific data were less complete, and taxon-specific landings before the 1970s were usually higher than gear-specific data, probable actual catches by gear-type were derived from taxonomic landings. In summary:

– First the amount of landings was estimated for each major métier (seine, trawl and multi-gear, corresponding to small-scale artisanal fishery) to allow estimation of unreported discards, using available information on gear- and sometimes target-specific discard ratios. Overall, the authors used this gear-specific data period to assign catches to major gear types for the earlier period/years when data per gear were not available. Considering that the three segments of the Portuguese fleet kept their relative proportions (Baeta 2009Baeta A.F.R. 2009. Environmental impacts and sustainability of Portuguese fisheries. Ph. D. thesis, Faculdade de Ciências, Universidade de Lisboa, 212 pp.), no significant changes were assumed between 1938 and 1968. This approach is supported by the long time series of landings of sardine, the dominant species in Portuguese landings, which is caught mainly by purse seine. Moreover, the multi-gear sector fishery has been the main component of coastal fisheries (numbers of boats), with few technological changes. The assignment of several periods was carried out by subtracting different reported sectors from total landings (the simplest procedure). In other cases, for instance purse seine, landings estimates were based on sardine data, considering purse seine catchability and selectivity to be constant over time.

– Multi-gear estimates were straightforward, as reported landings for 1979-1982 were only available as ‘total’, ‘trawl’ and ‘purse seine’ categories. Thus, multi-gear reported landings for this period were estimated by subtracting trawl and purse seine landings from total landings.

– Trawl estimates (based on total and seine results) were assigned for 1938-1968. As both trawl components have similar discard rates (see Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50), unreported estimates would not be biased by estimation of unreported landings from combined crustacean and finfish trawl statistics. Therefore, the percentage contribution of each gear to total landings was estimated for years with gear-specific data and used to reconstruct those trawl years where data were missing.

– Multi-gear landings were further disaggregated into more specific métiers. Therefore, the average percentage contribution of a single multi-gear fishery was estimated in relation to the overall multi-gear catches and used for years with no gear-specific data. The following multi-gear target fisheries were identified and differentiated and unreported discards in them were estimated: i) sardine (demersal coastal nearshore purse seine), ii) cephalopods using pots (e.g. octopus) or traps (e.g. octopus and cuttlefish); iii) bivalves; iv) crustaceans (lobster); v) other fishes (scabbardfish and large pelagics); and vi) recreational/subsistence and big-game sport fishing.

Based on the yearly total amounts of unreported catches, namely discards, per métier (and in several circumstances per species due to available information in the INE (see Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50), the amounts of discards by commercial fisheries per taxa/species are estimated herein. Literature with high taxonomic detail regarding discards (covering the period 1996 to 2007) was used for this purpose (Table 1). Thus, for the commercial fishery (trawl, seine and multi-gear or small-scale artisanal fisheries) the percentage of discards per métier per taxa was compiled and total discards per taxa for each métier were estimated per year. Whenever more than one study was available for the same métier and taxa, the average value was used. The discard rates of unreported catches per sector are presented in Supplementary Material Table S1.

In addition to the INE data, we used the detailed database of the Direcção Geral das Pescas e Aquicultura (DGPA), available for the years 1989 to 2009. The DGPA database comprises information of landing per fishing sector and by species (or groups, e.g. Diplodus spp). Based on the landings of each taxonomic group (and gear) from the DGPA, we estimated the number of species and proportion in catches for the INE data, to group the following categories.

The amount of shrimps, prawns and Nephrops norvegicus since 1969 depends mostly on trawl crustacean fisheries that specifically target these groups. Before 1969 little was known about the crustacean fishery in Portugal. Therefore, for these groups catches before 1969 were not rebuilt.

For Mollusca the same procedure as for crustacea was followed, since the resolution of the data also only increased after 1969 in the INE database. In the DGPA database cephalopods account for four reported taxa: octopus (Octopus vulgaris), squid (Loligo vulgaris), cuttlefish (Sepia officinalis) and shortfin-squid (which can include Illex coindetti, Todarodes sagittatus and Ommastrephes bartrami). The “Other Mollusca” in the INE database, according to the DGPA database, can include Gastropoda (whelks) and other mollusc species (potentially other Octopodidae: Eledone cirrhosa and Eledone moschata).

Until 1969, most landed fish taxa were included as non-specified marine fish (INE category = “Diverse marine fish”), which include both Osteichthyes and Chondrichthyes. Based on INE data available from recent years (between 1991 and 2009), the proportion of “Other fish” was re-estimated (5.83%), with the remaining proportion being used to re-distribute “Other fish” by taxa category whenever data per taxa were missing (using the DGPA database). The average percentage contribution of each taxa to total catch was therefore used to rebuild and redistribute “Other fish” by each taxa. The categories of commercial groups in the INE database include the following:

– Other crustaceans (crabs, such was Maja squinado and Cancer pagurus and other non-specified crustaceans).

– Bivalves, including subtidal coastal clams (Donax spp., clams and razor clam), herein considered to be mainly caught by the artisanal/multispecies dredge fishery/sector.

– Pagellus spp. (Pagellus acarne, Pagellus bogaraveo and Pagellus erythrinus).

– Pleuronectiformes (Turbot, Microchirus spp., Microchirus variegatus, Platichthys flesus, Psetta maxima, Solea spp., Solea lascaris, Solea solea, Lepidorhombus boscii, Lepidorhombus whiffiagonis and Pleuronectes platessa).

– Sparidae (Spondyliosoma cantharus, Sparus aurata, Diplodus spp. and Sarpa salpa).

– Thunnus spp. and other tunas (Thunnus thynnus, Katsuwonus pelamis, Thunnus albacares and Auxis rochei).

– The “Other fish” category includes Pagrus spp., Dentex spp., Merluccius spp., Beryx splendens, Merlangius merlangus, Polyprion americanus, Argyrosomus regius, Dicentrachus spp., Alosa spp., Lophius spp., Gurnards, Mullets, Helicolenus dactylopterus, Serranidae, Zeus faber, Beryx decadactylus, Anguilla anguilla and Brama brama.

RESULTSTop

There are few studies on reconstruction of unreported discards with higher taxonomic resolution, because of the enormous time required for obtaining sound fisheries information, processing the data and developing/applying accurate methodologies. After we estimated IUU in a previous study (see methods in Leitão et al. 2014Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50), we reconstructed unreported discard profiles, because enhancing taxonomic information on discards is imperative for fisheries management within the new CFP and the landing obligation directive. Inevitably, reconstructions of catches are largely based on assumptions derived by analyses of recent data (e.g. discards studies, Table 1). For example, in the present work, the catch composition and discard ratios were mainly based on studies dating from after the mid-1990s but the reconstruction goes back to 1938. Furthermore, the recent studies may cover a small region, raising the question of whether the discards estimates apply to the whole Portuguese mainland. The Portuguese fishery is characterized by nearshore fisheries with the top rank preference in terms of species changing little over time (Almeida et al. 2015Almeida C., Altintzoglou T., Cabral H., et al. 2015. Does seafood knowledge relate to more sustainable consumption? Brit. Food J. 117: 894-914.). In fact, small pelagics (Sardine pilchardus, Trachurus spp. and Scomber spp.) and European hake, for instance, account for the greater proportion of the catches (landed and discards). These groups/species are data rich in the INE long-term database. Therefore, for the main métiers and traditionally consumed species, the rebuilding and taxonomic disaggregation of the discarded species (most of which match the landed species) should be considered more accurate, whereas in the case of species with smaller catches, rebuilding procedures may introduce larger estimation errors. However, the number of unreported species discarded is independent of the percentage in weight of the unreported discards estimated. That is, we can assume that qualitative analyses might be less affected than quantitative estimations in rebuilding methods. The average total number of marketable taxa landed per year is around 296 (Source: DGPA 1989-2009), with 225 taxa being discarded. Of the 225 species discarded, approximately half (109 taxa, 48%) are also landed or reported/discriminated at auction (according to the DGPA database). Therefore, this study showed that 89% of unreported bony fish and shark species are thought to have commercial value.

The compositions of unreported and landed catches do not vary much in terms of the main groups caught (Fig. 1, Table 2). In the Portuguese mainland fisheries the landings comprised mostly fish (84.5%), with cephalopods and crustaceans accounting for 2.9% and 1.3% of the total catches, respectively. For the period 1938 to 2009, the average landings of fish, cephalopods and crustaceans were 207419, 7162 and 3187 t (Fig. 1, Table 2). Fish accounted for 93.5% of the total unreported catches, with an average of 115493 t/y. The contributions of cephalopods and crustaceans to the unreported catches are minor compared with those of fish: 1.9% and 1.4% of total unreported catches, with averages of 2345 and 1754 t/y, respectively (Fig. 1, Table 2). The shark group accounts for 1.3% of total unreported catches in weight (1638 t/y). The recreational/subsistence fishery focuses mainly on fish, but there are no studies or data available on recreational fisheries/harvesting that target small invertebrates such as mussels, goose-barnacles or, more recently, sea urchins.

Between 1938 and 2009, sardine (S. pilchardus, 44.8%), horse mackerel (T. trachurus, 14.8%), hake (M. merluccius, 5.7%), chub mackerel (Scomber colias, 5.6%) and octopus (Octopus vulgaris, 2.3%) together accounted for an average of 73.2% of the landings (163826 t/year) and 36.8% of unreported catches (450411 t/year) (Table 3, Fig. 2). The unreported species included mainly S. colias (17.6%), Boops boops (9%), Trachurus picturatus (8.6%), M. merluccius (8.3%), S. pilchardus (7%), Liza aurata (7%), Micromesistius poutassou (6.9%) and T. trachurus (3.6%), all marketable species that together accounted for 68.2% of the total average annual unreported catches of approximately 84222 t (Table 3, Fig. 2; see also Supplementary Material Table S2). Species such as the two-banded sea bream (Diplodus vulgaris) and the Senegal sea bream (Diplodus bellottii) are often discarded in purse seine fisheries when they are small-sized but above the minimum legal size, as taking them to auction is considered not worthwhile (Gonçalves et al. 2008Gonçalves J.M.S., Bentes L., Monteiro P., et al. 2008. Reducing discards in a demersal purse-seine fishery. Aquat. Living Resour. 21: 135-144.).

The composition of landings varies considerably according to a number of factors, including the nature of the fishery, the type of fishing gear used, gear selectivity, tow duration, the target species and their price value, the depth of capture and the time of year (Oliver 1993Oliver P. 1993. Analysis of fluctuations observed in the trawl fleet landings of the Balearic Islands. Sci. Mar. 57: 219-227., García-Rodriguez and Esteban 1999García-Rodriguez M., Esteban A. 1999. On the biology and fishery of Aristeus anntenatus (Risso, 1816) (Decapoda, Dendobranchiata) in the Ibiza Channel (Balearic Islands, Spain). Sci. Mar. 63: 27-37., Rochet et al. 2002Rochet M.-J., Péronnet I., Trenkel V.M. 2002. An analysis of discards from the French trawler fleet in the Celtic Sea. ICES J. Mar. Sci. 59: 538-552.). As expected, all the above factors also affect the discard species composition of each métier. In mainland Portuguese fisheries, the number of unreported/discarded species was higher in the multi-gear sector (N=184), followed by trawl (N=79) and seine (N=9) (Table 4). The large number of taxa in the multi-gear category is due to the differences between gears, fishing grounds and target species. Within the multi-gear métiers, the number of species discarded from trammel nets (N=120 taxa) was far greater than that of other static gears such as scabbardfish longlines (N=29), gill net and longline (N=22), artisanal dredges (N=18), trap (N=16) and demersal seine (N= 5) (see Supplementary Material Table S2). This is due to the greater diversity of trammel net catches compared with other static gear (Martins et al. 1992Martins R., Santos M.N., Monteiro C.C., et al. 1992. Contribuição para o estudo da selectividade das redes de emalhar de um pano fundeadas na costa Portuguesa no biénio 1990-1991. INIP. Relatórios técnicos e científicos/Instituto Nacional de Investigação das Pescas. 62: 1-26., Erzini et al. 2003Erzini K., Gonçalves J.M.S., Bentes L., et al. 2003. Quantifying the roles of competing static gears: comparative selectivity of longlines and monofilament gill nets in a multi-species fishery of the Algarve (southern Portugal). Sci. Mar. 67: 341-352.) and can be accounted for by the species and size selectivity of trammel nets (Erzini et al. 2006Erzini K., Gonçalves J.M.S., Bentes L., et al. 2006. Size selectivity of trammel nets in southern European small-scale fisheries. Fish. Res. 79: 183-201., Stergiou et al. 2006Stergiou K.I., Moutopoulos D.K., Soriguer M.C., et al. 2006. Trammel net catch species composition, catch rates and métiers in southern European waters: a multivariate approach. Fish. Res. 79: 170-182.). In the Algarve (south coast of Portugal), one exhaustive study showed that more than 900 species can be caught and discarded by the commercial fishery (trawls, purse seine and trammel nets): 69% are always discarded, 27% are frequently discarded and only 4% are occasionally discarded (Borges 2007Borges T.C. 2007. Biodiversidade nas pescas do Algarve (Sul de Portugal) / Biodiversity in the fisheries of Algarve (South Portugal). Universidade do Algarve, Faro, 699 pp.). The number of taxa recorded above is far greater than those reported herein that were based on specific scientific literature. This finding might be related to the fact that scientific surveys are usually restricted to short time periods and are also limited in terms of the geographic area surveyed (scientific surveys onboard commercial boats allow exhaustive faunistic records to be obtained).

In multi-gear fisheries the unreported catches consisted mainly of L. aurata, S. colias, S. pilchardus, Trachinus draco and B. boops (Fig. 2, Table 3). Together, the latter species account for 76.4% of the multi-gear discards, with an average of 17935 t/y. The unreported multi-gear catches of S. colias, S. pilchardus and B. boops were mostly due to demersal seine and trammel net discards, while those of L. aurata were mostly due to demersal purse seine discards (see Supplementary Material Table S2).

As in the multi-gear category, B. boops, S. colias and S. pilchardus were the species most discarded by purse seiners (Table 3). Together with Belone belone, these species accounted for 79% of the unreported purse seine discards, with an average of approximately 19027 t/y. In purse seiners that use electronic equipment to detect the schools around which the seine net is set, the lack of success in determining the species and/or size composition of the fish in the school before setting the net is a major factor leading to high volume discards. In fact, the target species (sardine or horse mackerel) may also be captured and discarded when mixed with by-catch species, making the sorting of large catches uneconomical, and when the sizes caught are not suitable for the market or for canning (Borges et al. 2001Borges T.C., Erzini K., Bentes L., et al. 2001. By-catch and discarding practices in five Algarve (Southern Portugal) métiers. J. Appl. Ichthyol. 17: 104-114.).

In Portugal, the “trawling” category includes two different fleet components: deepwater trawlers that target crustaceans, and fish trawlers that operate mainly on the continental shelf (CEC 1993Commission of the European Communities (CEC). 1993. Report of EC Group of experts on Review of Biological Information and Technical Measures Applicable to the Gulf of Cadiz. Commission of the European Communities, pp. 1369.). Fishing trip duration is one of the most important factors influencing the proportion of the fish by-catch that is commercialized, and the quantity of by-catch landed is inversely related to trip duration (Clucas 1997Clucas I. 1997. A study of the options for utilization of bycatch and discards from marine capture fisheries. FAO Fisheries Circular 928, FAO, Rome., Costa et al. 2008Costa M.E., Erzini K., Borges T.C. 2008. Bycatch of crustacean and fish bottom trawl fisheries from southern Portugal (Algarve). Sci. Mar. 72: 801-814.). The main species unreported due to trawl discards differed from both the multi-gear (small-scale/artisanal) and purse seine fleets. For the trawls, T. picturatus (16%) and M. merluccius (13%) were the most discarded species, accounting for 29% of the unreported catches and approximately 19514 t/y. Together with S. colias (13%) and M. poutassou (13%) these species comprised more than half (55%) the unreported trawl catches. The occurrence of high concentrations of small, non-commercial species such as Capros aper and Macroramphosus scolopax accounts for the occasional high volume discards witnessed onboard trawlers (Borges et al. 2001Borges T.C., Erzini K., Bentes L., et al. 2001. By-catch and discarding practices in five Algarve (Southern Portugal) métiers. J. Appl. Ichthyol. 17: 104-114.). However, in this analysis different trawl studies were used and C. aper was the sixth most important species in terms of trawl discards.

Despite some overlap in the species that contribute most to unreported discards of different metiers, some significant differences were found. In fact, the discards of sharks were always higher than landings in all the time series, which is not surprising in view of discard rates for most species (see studies on trawl, Table 1). In fact, the catch ratio of IUU sharks/total sharks showed that 71.6% of the sharks are discarded without being reported (Table 2). The discards of sharks have increased in the last few decades although landings of sharks have not. This finding may also be related to discards of deepwater sharks, which were formerly used to produce liver oil, including during the Second World War. Compared with other sectors (see also Supplementary Material Table S2), trawlers (mainly crustacean trawls) discard considerable quantities of mainly deepwater sharks such as Scyliorhinus cannicula, Galeus melastomus, Etmopterus pusillus and Hexanchus griseus, which may have poor resilience to high levels of fishing mortality because of their life history characteristics (Stevens et al. 2000Stevens J.D., Bonfil R., Dulvy N.K., et al. 2000. The effects of fishing on sharks, rays, and chimaeras (Chondrichthyans) and the implications for marine ecosystems. ICES J. Mar. Sci. 57: 476-494.). In such deepwater communities with long-lived, slow growing, low-fecundity species, fishing activity with associated discard-related mortality may be expected to severely impact some populations of non-commercial species and in the long-term result in community changes (Kaiser and de Groot 2000Kaiser M.J., de Groot S.J. 2000. Effects of Fishing on Non-Target Species and Habitats. Biological, Conservation and Socio-Economic Issues. Oxford: Blackwell Science, 399 pp.).

DISCUSSIONTop

The results showed that independently of the métier, common marketable species account for most of the unreported discarded catches. In fact, the species that most contribute to overall unreported catches are among the most frequently landed and are those that contribute most to overall landings. This point is important, because these species are considered choke species under the new Common Fisheries Policy landing obligation. So what are the implications of the landing obligation in relation to this finding and what is the value of this study? The identification of discarded species is a key factor for launching the debate regarding their use, particularly because most of them have quotas/total allowed catches (TACs). Until recently, the EU prohibited discards of fish with established quotas which could be legally landed (high-grading). However, it was legal to discard non-commercial fish and other organisms. As discussed by Leitão and Baptista (2017)Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83, it is difficult to know with certainty whether there will be any costs for fishermen if they land more fish than their quota for one or more species. In short, fish caught in excess of individual quotas can be marketed normally and “by-catch quotas” can be set as part of the fishing opportunities established by the EU council each year.

The difficulty of managing Portuguese fisheries can be largely attributed to their multi-gear nature, insufficient research (funding and lack of support for monitoring and analysis of non-target fisheries) and unreported catches, which affect stock assessment and management. Fisheries data collection, advice and management have traditionally been based on single-species approaches. However, ignoring interactions between métiers and species could lead to an undesirable situation in which fishing for one species may lead to discarding of another whose quota has already been exceeded. Moreover, the by-catch and discarding of non-target species may have negative consequences for non-commercial as well as commercial species due to influences on species interactions and consequent cascading effects throughout the trophic web (Harris and Poiner 1990Harris A.N., Poiner I.R. 1990. By-catch of prawn fishery of Torres Strait, Composition and partitioning of the discards into components that float or sink. Austral. J. Mar. Freshw. Res. 41: 37-52., Hill and Wassenberg 1990Hill B.J., Wassenberg T.J. 1990. Fate of discards from prawn trawlers in Torres Strait. Austral. J. Mar. Freshw. Res. 41: 53-64., Yamamura 1997Yamamura O. 1997. Scavenging on discarded saury by demersal fishes off Sendai Bay, northern Japan. J. Fish Biol. 50: 919-925.).

Borges (2007)Borges T.C. 2007. Biodiversidade nas pescas do Algarve (Sul de Portugal) / Biodiversity in the fisheries of Algarve (South Portugal). Universidade do Algarve, Faro, 699 pp. state that the main reasons for discarding are economic restrictions (e.g. low or no commercial value of the species with no immediate market) and technical restrictions (fishing gear selectivity). Moreover, Bellido et al. (2011)Bellido J.M., Santos M.B., Pennino M.G., et al. 2011. Fishery discards and bycatch: solutions for an ecosystem approach to fisheries management? Hydrobiologia 670: 317-333. reported that discarding may have a number of adverse ecological impacts on marine ecosystems, causing changes in the overall structure of trophic webs and habitats, which could in turn pose risks for the sustainability of current fisheries. Discarding is less frequently associated with legal/administrative restrictions such as quotas, minimum landing size and TACs. However, given the overfished state of many of the world’s most important stocks (Pauly et al. 2002Pauly D., Christensen V., Guénette S., et al. 2002. Towards sustainability in fisheries management. Nature 418: 689-695., Leitão 2015Leitão F. 2015. Landing profiles of Portuguese fisheries: assessing the state of stocks. Fish. Manag. Ecol. 22: 152-163.), there has been great interest in documenting and finding solutions to the economic, political, and ecological implications of by-catch and discarding (Costa et al. 2008Costa M.E., Erzini K., Borges T.C. 2008. Bycatch of crustacean and fish bottom trawl fisheries from southern Portugal (Algarve). Sci. Mar. 72: 801-814.). Research on by-catch utilization is rapidly moving to the field of food and nutrition research, creating value-added fish products from by-catch or discarded fish: extracting gelatin from Alaska pollock (Theragra chalcogramma; Zhou and Regestein 2005Zhou P., Regenstein J.M. 2005. Effects of Alkaline and Acid Pretreatments on Alaska Pollock Skin Gelatin Extraction. J. Food Sci. 70: 392-396.) and shark cartilage (Isurus oxyrinchus, Cho et al. 2004Cho S.M., Kwak K.S., Park D.C., et al. 2004. Processing optimisation and functional properties of gelatin from shark (Isurus oxyrinchus) cartilage. Food Hydrocoll. 18: 573-579.), and even using these value-added products as alternatives to the use of mammalian gelatin (Karim and Bhat 2009Karim A.A., Bhat R. 2009. Fish gelatin: properties, challenges, and prospects as an alternative to mammalian gelatins. Food Hydrocoll. 23: 563-576.).

There are still few summaries of estimates of unreported discards, especially with taxonomic detail. However, a complete review of IUU catches was made by Pauly and Zeller (2016)Pauly D., Zeller D. 2016. Global Atlas of the marine fisheries. A critical appraisal of catches and ecosystems impacts. Island Press. USA. 497 pp.. Overall, world results show that the taxonomic composition of unreported catches of the main target species vary considerably among areas, which is an expected result as fish assemblages, and target species, differ among regions. However, small pelagics and some demersal species are some of the most frequently reported species in the Mediterranean and Southern Europe (Coll et al. 2014Coll M., Carreras M, Cornax M.J., et al. 2014. Closer to reality: reconstructing total removals in mixed fisheries from Southern Europe. Fish. Res. 154: 179-194., Pauly and Zeller 2016Pauly D., Zeller D. 2016. Global Atlas of the marine fisheries. A critical appraisal of catches and ecosystems impacts. Island Press. USA. 497 pp.). Coll et al. (2014)Coll M., Carreras M, Cornax M.J., et al. 2014. Closer to reality: reconstructing total removals in mixed fisheries from Southern Europe. Fish. Res. 154: 179-194. showed that in Southern Europe unreported catches were due to (i) illegal catches of commercial species (undersized or with quotas, such as bluefin tuna), (ii) illegal fishing techniques (such as the Spanish driftnet fishery after the 1992 ban), and (iii) portions of misreported catches of protected species or species at risk (such as pelagic sharks). Illegal catches in the study area were mainly identified as juvenile commercial species such as juveniles of demersal species as hake or small pelagic fish such as sardines and anchovies. These results are similar to those found herein for unreported discards of species/groups. In Italy the main taxa discarded were clams (Bivalvia; 12.0%), sharks (Selachimorpha; 8.9%), jacks (Trachurus spp. 6.7%) and rays (Rajidae; 5.6%) (Piroddi et al. 2015Piroddi C., Gristina M., Zylich K, et al. 2015. Reconstruction of Italy’s marine fisheries removals and fishing capacity, 1950-2010. Fish. Res. 172: 137-147.). In fact, the worldwide unreported proportion is most often due to the discarded component (Pauly and Zeller 2016Pauly D., Zeller D. 2016. Global Atlas of the marine fisheries. A critical appraisal of catches and ecosystems impacts. Island Press. USA. 497 pp.). It remains, however, to be determined whether worldwide total unreported discard species also match the commercial species most frequently landed and with the highest contribution to total catch, as in Portugal.

From an economic perspective, there are possibilities for making better use of some discarded species, thereby possibly reducing the pressure on target species (Leitão and Baptista 2017Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83.). Most of the discarded species in Portuguese fisheries have been shown to have economic potential. The critical fact is that sustainable management of fishing resources must take place in the ecosystem context, with a good understanding of all the possible effects of fishing activities (Borges 2007Borges T.C. 2007. Biodiversidade nas pescas do Algarve (Sul de Portugal) / Biodiversity in the fisheries of Algarve (South Portugal). Universidade do Algarve, Faro, 699 pp.). Any effect on one stock, population or species may produce a change in another, resulting in readjustment in both populations (Hongskul 1979Hongskul V. 1979. Report on the studies of multispecies systems in fisheries. In: Stock Assessment in Tropical Small-Scale Fisheries, Proceedings of an international workshop, September 19–21, U.R.I., pp. 173-182., Saila 1983Saila S.B. 1983. Importance and Assessment of Discards in Commercial Fisheries. FAO Circular no. 765, Rome, Italy, 62 pp., Kennelly 1995Kennelly S.J. 1995. The issue of bycatch in Australia’s demersal trawl fisheries. Rev. Fish Biol. Fish. 5: 213-234.). Moreover, discard estimates are necessary, not only to evaluate the impact of fishing on non-commercial species but also on ecosystems as a whole (Alverson et al. 1994Alverson D.L., Freeberg M.H., Murawski S.A., et al. 1994. A global assessment of fisheries bycatch and discards. FAO Fish. Tech. Pap. 339., Hall 1999Hall S.J. 1999. The Effects of Fishing on Marine Ecosystems and Communities. Fish Biology and Aquatic Resources Series 1, Blackwell Science, Oxford, 274 pp.), since they are not usually taken into account in stock assessments (Borges et al. 2005Borges L., Rogan E., Officer R. 2005. Discarding by the demersal fishery in the waters around Ireland. Fish. Res. 76: 1-13.). Knowledge of unreported catches may change the way we assess the marine ecosystem, including the poorly understood trophic effects of fisheries in the marine environment, thereby improving our understanding of fishing trend variability and catch predictions.

ACKNOWLEDGEMENTSTop

Vânia Baptista and Francisco Leitão hold scholarships from Fundação para a Ciência e Tecnologia (references: SFRH/BD/104209/2014 and SFRH/BPD/108949/2015). This work received national funds through the Foundation for Science and Technology (FCT) through project UID/Multi/04326/2013. Karim Erzini was supported by funding from the European Commission’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 634495 for the project Science, Technology, and Society Initiative to minimize Unwanted Catches in European Fisheries (MINOUW).

REFERENCESTop

Almeida C., Altintzoglou T., Cabral H., et al. 2015. Does seafood knowledge relate to more sustainable consumption? Brit. Food J. 117: 894-914.
https://doi.org/10.1108/BFJ-04-2014-0156In Anon. (ed.) Revista da Faculdade de Letras, Porto 3: 151-183

Alverson D.L., Hughes S.E. 1996. Bycatch: from emotion to effective natural resource management. Rev. Fish Biol. Fish. 6: 443-462.
https://doi.org/10.1007/BF00164325

Alverson D.L., Freeberg M.H., Murawski S.A., et al. 1994. A global assessment of fisheries bycatch and discards. FAO Fish. Tech. Pap. 339.

Baeta A.F.R. 2009. Environmental impacts and sustainability of Portuguese fisheries. Ph. D. thesis, Faculdade de Ciências, Universidade de Lisboa, 212 pp.

Baldaque da Silva A.A. 1891. Estado Actual das Pescas em Portugal. Impr. Nacional, Lisboa, Portugal. 524 pp.

Batista M.I., Teixeira C.M., Cabral H.N. 2009. Catches of target species and bycatches of an artisanal fishery: The case study of a trammel net fishery in the Portuguese coast. Fish. Res. 100: 167-177.
https://doi.org/10.1016/j.fishres.2009.07.007

Bellido J.M., Santos M.B., Pennino M.G., et al. 2011. Fishery discards and bycatch: solutions for an ecosystem approach to fisheries management? Hydrobiologia 670: 317-333.
https://doi.org/10.1007/s10750-011-0721-5

Bordalo-Machado P., Fernandes A.C., Figueiredo I., et al. 2009. The black scabbardfish (Aphanopus carbo Lowe, 1839) fisheries from the Portuguese mainland and Madeira Island. Sci. Mar. 73S2: 63-76.

Borges T.C. 2007. Biodiversidade nas pescas do Algarve (Sul de Portugal) / Biodiversity in the fisheries of Algarve (South Portugal). Universidade do Algarve, Faro, 699 pp.

Borges T.C., Erzini K., Bentes L., et al. 2001. By-catch and discarding practices in five Algarve (Southern Portugal) métiers. J. Appl. Ichthyol. 17: 104-114.
https://doi.org/10.1111/j.1439-0426.2001.00283.x

Borges L., Rogan E., Officer R. 2005. Discarding by the demersal fishery in the waters around Ireland. Fish. Res. 76: 1-13.
https://doi.org/10.1016/j.fishres.2005.05.011

Brandão M.A., Godinho M.M., Kovács I. 2000. Pescas e pescadores, futuros para o emprego e os recursos. Celta ed., Oeiras, 353 pp.

Catchpole T.L., Feekings J.P., Madsen N., et al. 2014. Using inferred drivers of discarding behaviour to evaluate discard mitigation measures. ICES J. Mar. Sci. 71: 1277-1285.
https://doi.org/10.1093/icesjms/fst170

Cho S.M., Kwak K.S., Park D.C., et al. 2004. Processing optimisation and functional properties of gelatin from shark (Isurus oxyrinchus) cartilage. Food Hydrocoll. 18: 573-579.
https://doi.org/10.1016/j.foodhyd.2003.10.001

Clucas I. 1997. A study of the options for utilization of bycatch and discards from marine capture fisheries. FAO Fisheries Circular 928, FAO, Rome.

Coll M., Carreras M, Cornax M.J., et al. 2014. Closer to reality: reconstructing total removals in mixed fisheries from Southern Europe. Fish. Res. 154: 179-194.
https://doi.org/10.1016/j.fishres.2014.01.013

Commission of the European Communities (CEC). 1993. Report of EC Group of experts on Review of Biological Information and Technical Measures Applicable to the Gulf of Cadiz. Commission of the European Communities, pp. 1369.

Costa M.E., Erzini K., Borges T.C. 2008. Bycatch of crustacean and fish bottom trawl fisheries from southern Portugal (Algarve). Sci. Mar. 72: 801-814.
https://doi.org/10.3989/scimar.2008.72n4801

Erzini K., Gonçalves J.M.S., Bentes L., et al. 2003. Quantifying the roles of competing static gears: comparative selectivity of longlines and monofilament gill nets in a multi-species fishery of the Algarve (southern Portugal). Sci. Mar. 67: 341-352.
https://doi.org/10.3989/scimar.2003.67n3341

Erzini K., Gonçalves J.M.S., Bentes L., et al. 2006. Size selectivity of trammel nets in southern European small-scale fisheries. Fish. Res. 79: 183-201.
https://doi.org/10.1016/j.fishres.2006.03.004

FAO. 2010. Report of the Technical Consultation to Develop International Guidelines on Bycatch Management and Reduction of Discards. FAO Fisheries and Aquaculture Report 957. Rome, 32 pp.

García-Rodriguez M., Esteban A. 1999. On the biology and fishery of Aristeus anntenatus (Risso, 1816) (Decapoda, Dendobranchiata) in the Ibiza Channel (Balearic Islands, Spain). Sci. Mar. 63: 27-37.
https://doi.org/10.3989/scimar.1999.63n127

Gaspar M.B., Leitão F., Chicharo L., et al. 2003. A Comparison of direct mortality inflicted on macrofaunal organisms by three types of dredges used in the Portuguese clam fishery. ICES J. Mar. Sci. 60: 733-742.
https://doi.org/10.1016/S1054-3139(03)00023-7

Gonçalves J.M.S., Stergiou K.I., Hernando J.A., et al. 2007. Discards from experimental trammel nets in Southern European small-scale fisheries. Fish. Res. 88: 5-14.
https://doi.org/10.1016/j.fishres.2007.06.017

Gonçalves J.M.S., Bentes L., Monteiro P., et al. 2008. Reducing discards in a demersal purse-seine fishery. Aquat. Living Resour. 21: 135-144.
https://doi.org/10.1051/alr:2008023

Hall S.J. 1999. The Effects of Fishing on Marine Ecosystems and Communities. Fish Biology and Aquatic Resources Series 1, Blackwell Science, Oxford, 274 pp.

Harris A.N., Poiner I.R. 1990. By-catch of prawn fishery of Torres Strait, Composition and partitioning of the discards into components that float or sink. Austral. J. Mar. Freshw. Res. 41: 37-52.
https://doi.org/10.1071/MF9900037

Hill L., Coelho M.L. 2001. Portuguese fisheries in Portugal for the period 1950-1999. Comparison with ICES data. In: Zeller D., Watson R., Pauly D. (eds), Fisheries impacts on North Atlantic ecosystems: Catch, effort and national/regional data sets. Fisheries Centre Research Reports 9. University of British Columbia, Vancouver, pp. 187-190.

Hill B.J., Wassenberg T.J. 1990. Fate of discards from prawn trawlers in Torres Strait. Austral. J. Mar. Freshw. Res. 41: 53-64.
https://doi.org/10.1071/MF9900053

Hollingworth C.E. (ed.). 2000. Ecosystem effects of fishing. Proceedings of an ICES/SCOR Symposium Held in Montpellier, France, 16-19 March 1999. ICES J. Mar. Sci. 57: 465-792.
https://doi.org/10.1006/jmsc.2000.0745

Hongskul V. 1979. Report on the studies of multispecies systems in fisheries. In: Roedel P.M., Saila S.B. (eds), Stock Assessment in Tropical Small-Scale Fisheries. Proceedings of an international workshop. September 19-21, Univ. Rhode Island, pp. 173-182.

Kaiser M.J., de Groot S.J. 2000. Effects of Fishing on Non-Target Species and Habitats. Biological, Conservation and Socio-Economic Issues. Oxford: Blackwell Science, 399 pp.

Karim A.A., Bhat R. 2009. Fish gelatin: properties, challenges, and prospects as an alternative to mammalian gelatins. Food Hydrocoll. 23: 563-576.
https://doi.org/10.1016/j.foodhyd.2008.07.002

Kelleher K. 2005. Discards in the world’s marine fisheries: an update. FAO Fish. Tech. Pap. 470: 1-131.

Kennelly S.J. 1995. The issue of bycatch in Australia’s demersal trawl fisheries. Rev. Fish Biol. Fish. 5: 213-234.
https://doi.org/10.1007/BF00179757

Leitão F. 2015. Landing profiles of Portuguese fisheries: assessing the state of stocks. Fish. Manag. Ecol. 22: 152-163.
https://doi.org/10.1111/fme.12112

Leitão F., Baptista V. 2017. The discard ban policy, economic trends and opportunities for the Portuguese fisheries sector. Mar. Pol. 75: 75-83
https://doi.org/10.1016/j.marpol.2016.10.012

Leitão F., Gaspar M., Santos M., et al. 2009. A comparison of bycatch and discard mortality in three types of dredge used in the Portuguese Spisula solida (solid surf clam) fishery. Aquat. Liv. Res. 22: 1-10.
https://doi.org/10.1051/alr/2009001

Leitão F., Baptista V., Erzini K., et al. 2014. Reconstructed catches and trends for mainland Portugal fisheries between 1938 and 2009: implications for sustainability, domestic fish supply and imports. Fish. Res. 155: 35-50
https://doi.org/10.1016/j.fishres.2014.02.012

Martins R., Santos M.N., Monteiro C.C., et al. 1992. Contribuição para o estudo da selectividade das redes de emalhar de um pano fundeadas na costa Portuguesa no biénio 1990-1991. INIP. Relatórios técnicos e científicos/Instituto Nacional de Investigação das Pescas. 62: 1-26.

Oliver P. 1993. Analysis of fluctuations observed in the trawl fleet landings of the Balearic Islands. Sci. Mar. 57: 219-227.

Pauly D., Zeller D. 2016. Global Atlas of the marine fisheries. A critical appraisal of catches and ecosystems impacts. Island Press. USA. 497 pp.

Pauly D., Christensen V., Guénette S., et al. 2002. Towards sustainability in fisheries management. Nature 418: 689-695.
https://doi.org/10.1038/nature01017

Piroddi C., Gristina M., Zylich K, et al. 2015. Reconstruction of Italy’s marine fisheries removals and fishing capacity, 1950-2010. Fish. Res. 172: 137-147.
https://doi.org/10.1016/j.fishres.2015.06.028

Rochet M.-J., Péronnet I., Trenkel V.M. 2002. An analysis of discards from the French trawler fleet in the Celtic Sea. ICES J. Mar. Sci. 59: 538-552.
https://doi.org/10.1006/jmsc.2002.1182

Saila S.B. 1983. Importance and Assessment of Discards in Commercial Fisheries. FAO Circular no. 765, Rome, Italy, 62 pp.

Saldanha H.J. 2001. Contribuição para o estudo da pesca artesanal do polvo Octopus vulgaris Cuvier, 1797) com covos, no sotavento Algarvio: caracterização das capturas alvo, capturas acessórias e rejeições ao mar. Relatório de estágio do curso de Licenciatura em Biologia Marinha e Pescas. Universidade do Algarve. Faro. 50 pp.

Santos M.N., Gaspar M.B., Monteiro C.C., et al. 2002. Gill net and long-line catch comparisons in a hake fishery: the case of southern Portugal. Sci. Mar. 66: 433-441.
https://doi.org/10.3989/scimar.2002.66n4433

Stergiou K.I., Moutopoulos D.K., Soriguer M.C., et al. 2006. Trammel net catch species composition, catch rates and métiers in southern European waters: a multivariate approach. Fish. Res. 79: 170-182.
https://doi.org/10.1016/j.fishres.2006.03.003

Stevens J.D., Bonfil R., Dulvy N.K., et al. 2000. The effects of fishing on sharks, rays, and chimaeras (Chondrichthyans) and the implications for marine ecosystems. ICES J. Mar. Sci. 57: 476-494.
https://doi.org/10.1006/jmsc.2000.0724

Watson R., Revenga C., Kura Y. 2006a. Fishing gears associated with global marine catches. I. Database development. Fish. Res. 79: 97-102.
https://doi.org/10.1016/j.fishres.2006.01.010

Watson R., Revenga C., Kura Y. 2006b. Fishing gears associated with global marine catches. II. Trends in trawling and dredging. Fish. Res. 79: 103-111.
https://doi.org/10.1016/j.fishres.2006.01.013

Yamamura O. 1997. Scavenging on discarded saury by demersal fishes off Sendai Bay, northern Japan. J. Fish Biol. 50: 919-925.
https://doi.org/10.1111/j.1095-8649.1997.tb01618.x

Zeller D., Booth S., Davis G., et al. 2007. Re-estimation of small-scale fisheries catches for U.S. flag island areas in the Western Pacific: The last 50 years. Fish. Bull. 105: 266-277.

Zeller D., Rossing P., Harper S., et al. 2011. The Baltic Sea: Estimates of total fisheries removals 1950-2007. Fish. Res. 108: 356-363.
https://doi.org/10.1016/j.fishres.2010.10.024

Zhou P., Regenstein J.M. 2005. Effects of Alkaline and Acid Pretreatments on Alaska Pollock Skin Gelatin Extraction. J. Food Sci. 70: 392-396.
https://doi.org/10.1111/j.1365-2621.2005.tb11435.x

SUPPLEMENTARY MATERIAL

The following supplementary material is available through the online version of this article and at the following link:
http://scimar.icm.csic.es/scimar/supplm/sm04723esm.pdf

Table S1. – Taxonomic list with the average discard rate per métier per taxa (based on studies of Table 1) used for the rebuilding of unreported catches.

Table S2. – Taxonomic list with the average absolute and relative weight contribution of each taxa to total unreported catch, per métier, for the period between 1938 and 2009.