INTRODUCTION
⌅Mediterranean bottom trawl fisheries are typically multi-species, that is, they are characterized by a large variety of species and the absence of large single-species stocks. Three groups of vessels operate in these fisheries: coastal trawlers, which target a wide variety of species (e.g. red mullet, common squid, curled octopus, European hake and monkfish); trawlers operating on the shelf break and upper slope, with Norway lobster as the main target species; and trawlers operating in submarine canyons, with blue and red shrimp as the target species (Martín et al. 2014Martín P., Muntadas A., de Juan S., et al. 2014. Performance of a northwestern Mediterranean bottom trawl fleet: How the integration of landings and VMS data can contribute to the implementation of ecosystem-based fisheries management. Mar. Policy. 43: 112-121. https://doi.org/10.1016/j.marpol.2013.05.009 ). Mediterranean fisheries are characterized by marked seasonality in landing composition and the presence of very young individuals (Lleonart and Maynou 2003Lleonart J., Maynou F. 2003. Fish stock assessments in the Mediterranean: state of the art. Sci. Mar. 67: 37-49. https://doi.org/10.3989/scimar.2003.67s137 ). Bottom trawl discards, including species with and without commercial value, have been estimated to be around 30% of the total catch (Sánchez et al. 2004Sánchez P., Demestre M., Martín P. 2004. Characterisation of the discards generated by bottom trawling in the northwestern Mediterranean. Fish. Res. 67: 71-80. https://doi.org/10.1016/j.fishres.2003.08.004 ; Tsagarakis et al. 2014Tsagarakis K., Palialexis A., Vassilopoulou V. 2014. Mediterranean fishery discards: review of the existing knowledge. ICES Mar. Sci. 71: 1219-1234. https://doi.org/10.1093/icesjms/fst074 ).
The exploitation of many demersal stocks in the western Mediterranean Sea far exceeds the levels required to achieve the maximum sustainable yield (MSY) (Colloca et al. 2017Colloca F., Scarcella G., Libralato S. 2017. Recent trends and impacts of fisheries exploitation on Mediterranean stocks and ecosystems. Front. Mar. Sci. 4: 244. https://doi.org/10.3389/fmars.2017.00244 ). At present, a multi-year plan for demersal fisheries (2019-2025) is being implemented in the western Mediterranean (EU 2019aEU 2019 a. Multiannual plan for the fisheries exploiting demersal stocks in the western Mediterranean Sea. Official Journal of the European Union. Regulation EU 2019/1022. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32019R1022&from=EN. (Last accessed 3 September 2021).) with the aim of reaching and maintaining the MSY for the target stocks. On the Catalan coast (northern part of the General Fisheries Commission for the Mediterranean (GFCM-FAO) geographical subarea GSA 6), these are Norway lobster (Nephrops norvegicus), blue and red shrimp (Aristeus antennatus), deep-water rose shrimp (Parapenaeus longirostris), European hake (Merluccius merluccius) and red mullet (Mullus barbatus). The plan should also adopt the ecosystem-based approach to fisheries management in order to minimize negative impacts of fishing activities on the marine ecosystem, i.e. it should also take into account bycatch species caught in demersal fisheries and stocks, which play an important role in the ecosystem functioning but insufficient data are available on them.
Blue whiting (Micromesistius poutassou) is an oceanic, semi-pelagic species that is widely distributed in the NE Atlantic and in the Mediterranean Sea over the shelf break and upper slope (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 , Farriols et al. 2019Farriols M.T., Ordines F., Carbonara P., et al. 2019. Spatio-temporal trends in diversity of demersal fish assemblages in the Mediterranean. Sci. Mar. 83: 189-206. https://doi.org/10.3989/scimar.04977.13A ). It plays an important role in the marine food webs, feeding on pelagic crustaceans and Myctophidae fish (Bachiller et al. 2016Bachiller E., Skaret G., Nøttestad L., Slotte A. 2016. Feeding ecology of Northeast Atlantic mackerel, Norwegian spring-spawning herring and blue whiting in the Norwegian Sea. PLoS One 11: e0149238. https://doi.org/10.1371/journal.pone.0149238 , Mir-Arguimbau et al. 2020aMir-Arguimbau J., Navarro J., Balcells M., et al. 2020a. Feeding ecology of blue whiting (Micromesistius poutassou) in the NW Mediterranean: The important role of Myctophidae. Deep Sea Res. Part I. Oceanogr. Res. Pap. 166: 103404. https://doi.org/10.1016/j.dsr.2020.103404 ) and being consumed by a wide range of predators, such as fish, sharks and flying squids (e.g. Cabral and Murtra 2002Cabral H. N., Murta A. G. 2002. The diet of blue whiting, hake, horse mackerel and mackerel off Portugal. J. Appl. Ichthyol 18: 14-23. https://doi.org/10.1046/j.1439-0426.2002.00297.x , Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 ). In the NE Atlantic, large scale migrations of the species linked to reproduction and feeding grounds have been reported (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 , Hátún et al. 2007Hátún H., Arge J., Sandø A.B. 2007. Environmental influence on the spawning distribution and migration pattern of northern blue whiting (Micromesistius poutassou). ICES CM B: 6.), but within the Mediterranean such migrations have not been described (Martín et al. 2016Martín P., Maynou F., Recasens L. Sabatés, A. 2016. Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open‐sea convection processes in the northwestern Mediterranean. Fish. Oceanogr. 25: 229-240. https://doi.org/10.1111/fog.12147 ). In the Atlantic, the spawning period starts in January-February in its southern distribution area, extending northwards with a time lag in relation to the latitudinal temperature gradient (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 ). In the Mediterranean, the species reproduces in winter, from December to March (Serrat et al. 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 , Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ), when temperatures reach the minimum values (12°C−13°C) (Calvo et al. 2011Calvo E., Simó R., Coma R., et al. 2011. Effects of climate change on Mediterranean marine ecosystems: the case of the Catalan Sea. Clim. Res. 50: 1-29. https://doi.org/10.3354/cr01040 ).
In the Atlantic, blue whiting distribution in the water column extends between 200 and 600 m depth. At less than 500 m bottom depth the species is found associated with the seabed, while in deeper waters (bottom depth >500 m) the species is located in the water column, at around 400 to 500 m (Johsen and Godø 2007Johsen E., Godø O.R. 2007. Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou). ICES J. Mar. Sci. 64: 1202-1209. https://doi.org/10.1093/icesjms/fsm110 ). Blue whiting performs dial vertical migrations and is generally distributed deeper and more concentrated during daylight (Cohen 1990Cohen D.M., Inada T., Iwamoto T.S. 1990. Gadiform fishes of the world (Order Gadiformes). An annotated and illustrated catalogue of cods, hakes, grenadiers and other gadiform fishes known to date. FAO - Food and Agriculture Organization, Rome, Vol 10, 442 pp., Johsen and Godø 2007Johsen E., Godø O.R. 2007. Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou). ICES J. Mar. Sci. 64: 1202-1209. https://doi.org/10.1093/icesjms/fsm110 ). These characteristics allow the species to be exploited in different ways, by pelagic trawl in the Atlantic and bottom trawl in the Mediterranean. Information on the blue whiting fisheries in the NE Atlantic dates back to 1977 (Hátún et al. 2007Hátún H., Arge J., Sandø A.B. 2007. Environmental influence on the spawning distribution and migration pattern of northern blue whiting (Micromesistius poutassou). ICES CM B: 6.). There, the highest biomass is found along the edge of the continental shelf in areas west of the British Isles and on the Rockall Bank plateau, where blue whiting occurs in large schools at depths ranging between 300 and 600 m (ICES 2020ICES. 2020. Working Group on Widely Distributed Stocks (WGWIDE). ICES Scientific Reports. 2: 82. ICES, Copenhagen, 1019 pp.). Over the period 1981-2020 landings displayed marked variations. Landings remained at around 0.5 million t until the mid-1990s, then steadily increased and exceeded 2 million t in 2003 and 2004. After a sharp decrease and very low landings in 2011, landings started rising again, and current landings (2020) are around 1.5 million t (ICES 2007ICES. 2007. Report of the working group on northern pelagic and blue whiting fisheries. ICES, Vigo, Spain, 229 pp., 2020ICES. 2020. Working Group on Widely Distributed Stocks (WGWIDE). ICES Scientific Reports. 2: 82. ICES, Copenhagen, 1019 pp.).
In the Mediterranean, blue whiting is not a bottom trawl target species but a bycatch of the Norway lobster fishery, and it does not generate high income (Martín et al. 2014Martín P., Muntadas A., de Juan S., et al. 2014. Performance of a northwestern Mediterranean bottom trawl fleet: How the integration of landings and VMS data can contribute to the implementation of ecosystem-based fisheries management. Mar. Policy. 43: 112-121. https://doi.org/10.1016/j.marpol.2013.05.009 ). Most bycatch species are unregulated and are not assessed (Mytilineou et al. 2021Mytilineou C., Herrmann B., Mantopoulou-Palouka D., et al. 2021. Escape, discard, and landing probability in multispecies Mediterranean bottom-trawl fishery. ICES Mar. Sci. https://doi.org/10.1093/icesjms/fsab048 ). No minimum conservation reference size has been defined for blue whiting (EU 2019bEU 2019 b. Conservation of fisheries resources and the protection of marine ecosystems through technical measures. Official Journal of the European Union. Regulation EU 2019/1241. https://eur-lex.europa.eu/legal-content/EL/TXT/PDF/?uri=CELEX:32019R1241&from=ES. (Last accessed 3 September 2021).) although on the Spanish Mediterranean coast a minimum landing size has been established at 15 cm total length (TL) (BOE 2006BOE. 2006. REAL DECRETO 1615/2005. Real Decreto por el que se establecen las tallas mínimas de determinadas especies pesqueras. BOE, 16. https://www.boe.es/boe/dias/2006/01/19/pdfs/A02304-02305.pdf. (Last accessed 3 September 2021).). In 2010 a change of the mesh size was implemented, from 40 mm diamond to 40 mm square mesh size (EC 2006EC 2006. Council Regulation concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean Sea. Official Journal of the European Union. No 196/2006. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32006R1967&from=en ). Landings are far lower than in the Atlantic. Over a thirty-year period, from 1970 to 2000, annual landings fluctuated and remained above 8000 to 10000 t, with the highest values around 14000 to 16000 t. Since the mid-2000s landings have displayed a decreasing trend and are now at the lowest values of the whole time series, at around 2000 t, although a slight increase in landings has been observed in the most recent years (http://www.fao.org/fishery/statistics/global-capture-production/en). In the northwestern basin, blue whiting has been traditionally exploited. Landings records on the Catalan coast (around 580 km) are available since the early 1940s (Bas 1963Bas C. 1963. Fluctuations de la pêche de Merlangus poutassou et quelques considérations sur son contrôle. Proc. Tech. Pap. Gen. Fish. Coun. Mediterr. 7: 417- 420. ). Over two decades, the 1940s and 1950s, landings ranged between 1300 and 3500 t; minimum landings were observed at the beginning of the time series in 1942 (ca. 700 t). In the most recent years, since 2012, landings have remained at very low levels, around 500 t or lower (Generalitat de Catalunya 2021aGeneralitat de Catalunya. 2021a. Evolució Anual De Les Captures 2000-2018. http://agricultura.gencat.cat/ca/ambits/pesca/dar_estadistiques_pesca_subhastada/dar_captures_especies/maire/. (Last accessed 25 September 2021).).
Most of the Mediterranean fisheries are based on the youngest ages (0 and 1) (Lleonart and Maynou 2003Lleonart J., Maynou F. 2003. Fish stock assessments in the Mediterranean: state of the art. Sci. Mar. 67: 37-49. https://doi.org/10.3989/scimar.2003.67s137 ). According to the most recent stock assessments (STECF 2014STECF. 2014. Assessment of Mediterranean Sea Stocks - Part 1 (STECF-14-17). Publ. Off. Eur. Union, Luxembourg, pp. 393 EUR 26955 EN, JRC 93120.), in the western Mediterranean (GSA 6) blue whiting exploitation was based on age classes 1 (dominant in the catch) and age 2, while in the Ligurian and northern Tyrrhenian Seas most of the caught specimens ranged between ages 1 and 3. Discards were reported to amount to 15% of the catch in weight in the Ligurian and less than 5% in the northern Tyrrhenian. The conclusion of the assessment in both areas was that blue whiting was exploited unsustainably.
The present study aimed to characterize the fishery dynamics of blue whiting in the NW Mediterranean, i.e. discards, landings trend and seasonality, exploited sizes and age structure, and recruitment timing. From this fisheries-dependent information, biological traits of the species will be identified, such as spatial distribution by size and age over the year and the recruitment size to the demersal habitat. To this end, we combined landings and effort data over the last two decades with current information on the spatial distribution of fishing effort, catch, size and age structure of blue whiting over the year. The study will ultimately shed light on the status of blue whiting, a bycatch species, providing valuable information that is required in the ecosystem approach to fisheries management.
MATERIALS AND METHODS
⌅On the Catalan coast (NW Mediterranean), blue whiting is caught exclusively with bottom trawl during daylight hours as a bycatch from the vessels targeting Norway lobster (Martín et al. 2014Martín P., Muntadas A., de Juan S., et al. 2014. Performance of a northwestern Mediterranean bottom trawl fleet: How the integration of landings and VMS data can contribute to the implementation of ecosystem-based fisheries management. Mar. Policy. 43: 112-121. https://doi.org/10.1016/j.marpol.2013.05.009 , Ordines 2014Ordines F., Farriols M.T., Lleonart J., et al. 2014. Biology and population dynamics of by-catch fish species of the bottom trawl fishery in the western Mediterranean. Mediterr. Mar. Sci. 15: 613-625. https://doi.org/10.12681/mms.812 ). The fishing vessels operate five days a week, with a maximum of 12 hours at sea. Landings are commercialized daily at the auction upon the arrival of the trawlers at the port. At present, the bottom trawl fleet on the Catalan coast consists of 221 vessels, 21 of them based in Roses fishing port (Generalitat de Catalunya 2021bGeneralitat de Catalunya. 2021b. Situació de la flota pesquera (a desembre de 2020). http://agricultura.gencat.cat/ca/ambits/pesca/dar_flota_pesquera/dar_estadistiques/dar_situacio_flota_pesquera. (Last accessed 25 September 2021)., Roses Fishers Association 2021Roses Fishers Association. 2021. Flota pesquera. http://www.confrariapescadorsroses.cat/flotapesquera.html. (Last accessed 23 September 2021).).
Data on blue whiting landings (daily and monthly) and effort (fishing days with reported blue whiting landings) on the coast of Catalonia and in Roses harbour over the period 2000-2020 were taken from the statistics of the Fisheries Service of the Generalitat de Catalunya. In 2012-2014 a blue whiting daily quota was implemented by internal agreement among fishers (i.e. no legal obligation) by establishing a maximum number of boxes depending on the size of the vessel. The fishing days with blue whiting landings were taken as representative of the fishing effort applied to the species because only a part of the trawl fleet captures blue whiting (Martín et al. 2016Martín P., Maynou F., Recasens L. Sabatés, A. 2016. Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open‐sea convection processes in the northwestern Mediterranean. Fish. Oceanogr. 25: 229-240. https://doi.org/10.1111/fog.12147 ). Many fishing days correspond to trawlers that operate in fishing grounds different from those of blue whiting. Daily landings combined with vessel monitoring system (VMS) data were used to characterize the areas where the bottom trawl fleet of Catalonia operates, based on the amount of blue whiting landings extracted and the effort applied in each area. VMS is a satellite-based monitoring system that provides data on the location, course and speed of vessels to the fisheries authorities at regular intervals, at least once every two hours, and is compulsory for vessels larger than 12 m. It is important to note that VMS data do not indicate whether the vessels are fishing, steaming or inactive. To filter fishing activity, following methodologies described in previous studies (Bueno-Pardo et al. 2017Bueno-Pardo J., Ramalho S.P., García-Alegre A., et al. 2017. Deep-sea crustacean trawling fisheries in Portugal: quantification of effort and assessment of landings per unit effort using a Vessel Monitoring System (VMS). Sci. Rep. 7: 1-10. https://doi.org/10.1038/srep40795 , Lee et al. 2010Lee J., South A.B. Jennings S. 2010. Developing reliable, repeatable, and accessible methods to provide high-resolution estimates of fishing-effort distributions from vessel monitoring system (VMS) data. ICES J. Mar. Sci. 67:1260-1271. https://doi.org/10.1093/icesjms/fsq010 , Russo et al. 2014Russo T., D’Andrea L., Parisi A., Cataudella S. 2014. VMSbase: an R-package for VMS and logbook data management and analysis in fisheries ecology. PLoS ONE. 9: e10019. https://doi.org/10.1371/journal.pone.0100195 ), we applied a speed filter between 1.5 and 5 knots that included the speed range for trawling while excluding steaming and inactive moments. Once we had obtained the fishing VMS data we calculated fishing time per day and vessel (fishing trip) and simultaneously calculated blue whiting landings per fishing trip from the auction landings dataset. We homogeneously associated each variable with VMS fishing points corresponding to the same trip. Finally, we aggregated fishing points and their associated variables in a 1×1 km2 grid. Landings per unit effort (LPUE) (kg/h) were calculated by grid cell and represented in maps.
Blue whiting sampling was conducted on the northern Catalan coast, off Cap de Creus and Roses (Fig. 1). The continental shelf width varies from ~2.6 km near the Cap de Creus canyon, to ~30 km in its wider part (Ribó et al. 2018Ribó M., Durán R., Puig P., et al. 2018. Large sediment waves over the Gulf of Roses upper continental slope (NW Mediterranean). Mar. Geol. 399: 84-96. https://doi.org/10.1016/j.margeo.2018.02.006 ). The bottom trawl fishing grounds encompass the rather narrow continental shelf, the upper slope and the submarine canyon. Monthly samples, from March 2017 to October 2018, were collected on board a commercial otter bottom trawler (length overall = 25.05 m, gross tonnage = 100.13, power = 367.7 kW; 40 mm squared mesh size at the cod-end) that usually goes fishing to the blue whiting fishing grounds. The fishing gear was equipped with a SCANMAR system, which allowed us to know the depth where it was operating. Two fishing grounds where the trawler regularly operates were visited, at the shelf break, at around 130-250 m depth, and on the upper slope, at 250−550 m depth (Fig. 1). In total, 60 hauls were conducted (28 on the shelf break and 32 on the slope). A temporal closure was implemented in February. No particular indications were given to the skipper, so sampling reflects normal fishing activity. During the recruitment period, from April to July, the sampling of the catch on board was conducted biweekly to ensure that recruitment timing and duration were detected. When sampling on board was not possible because of weather conditions, blue whiting samples were collected at the auction. These samples corresponded to December 2017, i.e. outside the recruitment period, when discards were generated (Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ).
All individuals were measured on board to the nearest millimetre (TL) and grouped into 1 cm intervals. The sampling on board revealed the monthly length frequency distributions (LFDs) corresponding to the commercial and discarded catch as well as the monthly ratio of discards to commercial catch, i.e. the amount of discards generated in relation to the fraction of the total catch that was commercialized. The discards were made up of the recruits from the spawning of the same year. Prior to any analysis, LFD data were standardized to n/h (numbers/hour). The total monthly LFDs of the commercial catch, expressed in numbers (thousands), were generated by combining the LFDs from the sampling on board and the monthly landings (tonnes) available from the fishing statistics. The ratio of discards to commercial catch was applied to generate the total monthly discarded catch. The length-weight relationship used was taken from Mir-Arguimbau et al. (2020b)Mir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 .
The monthly LFDs in the two fishing grounds, the shelf break and the slope, were obtained from the individuals collected in the sampling on board. The age-length key in Mir-Arguimbau et al. (2020b)Mir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 was applied to the LFDs to convert them to age-frequency distributions. The recruitment timing and area was based on the total monthly LFDs of the commercial and discarded catch and on the monthly LFDs on the shelf break and the slope.
RESULTS
⌅Fishing grounds
⌅The spatial distribution of blue whiting landings resulting from the activity of the bottom trawl fleet with reported blue whiting landings is shown on the production map (Fig. 1). The map clearly identified the blue whiting fishing grounds located over the shelf break and the slope, mainly at 200 to 600 m depth. The biomass extraction was highest in the northern areas, up to 25 kg/h, especially in the Gulf of Lion, suggesting that the species would be more abundant to the north. The most productive areas overlapped with the implemented Fisheries Restricted Area (FRA) in the Gulf of Lion (FAO-CGFM 2009FAO-CGFM 2009. Recommendation GFCM/33/2009/1 On the establishment of a Fisheries Restricted Area in the Gulf of Lions to protect spawning aggregations and deep sea sensitive habitats. http://www.fao.org/3/ax884e/ax884e.pdf. (Last accessed 3 September 2021)) (Fig. 1). It is important to note that the maps refer to the commercialized catch, that is, discards were not included in these maps, and that only the Catalan fleet is represented by this VMS set, so the interaction of fleets from France is not accounted for.
Landings and LPUE trend
⌅Over the period 2000-2020, blue whiting landings from the Catalan coast and landings and LPUE (landings per fishing day and vessel) from the fishing port of Roses displayed the same trend (Fig. 2). Landings from Roses represented around 20% to 25% of those from the Catalan coast. Overall, landings showed a decreasing trend with fluctuating values, and in the most recent years landings remained at the lowest values of the time series. Blue whiting LPUE from Roses displayed the same trend as landings. Fishing effort, expressed as the number of fishing days with reported blue whiting landings, displayed a steadily decreasing trend to reach a 60% decrease over the whole period (3598 days in 2000 and 1403 in 2020). Despite the progressive decrease in the number of fishing days in the most recent years, blue whiting landings increased slightly in 2019. The LPUE increase observed in 2019 was more marked than that of landings (Fig. 2).
During the first part of the series (2000-2010) with high annual values, the landings in winter and spring large accounted for the annual total, while the landings in summer and autumn were low in relation to those in the first half of the year. However, in years of good recruitment, e.g. 2006, summer-autumn landings were also high because they were dominated by recruits of the year. In the second part of the series, when annual landings were low and interannual variation was low, this seasonal pattern was much less evident (Fig. 3).
Length frequency distributions
⌅The sizes of the blue whiting captured ranged between 5 and 39 cm TL (Fig. 4). Blue whiting discards corresponded to very small sized individuals (from 5 to 15 cm TL). The size of the smaller discarded individuals (from 5 cm TL) corresponded to the size when blue whiting shifted from the pelagic to the demersal habitat, so discards were taken as an indicator of recruitment to the fishery.
The LFDs showed the dynamics of the species in the area over the year, that is, the time of the year when the incorporation of recruits occurs and large individuals are present in the bottom trawl catch (Fig. 4). The incorporation of recruits that would be discarded, between 5-12 cm TL with the mode at 7-9 cm TL, started in April and was highest in May; later on, the recruits’ mode shifted towards larger sizes, and discards decreased in the following months. The time of the year with highest discards was spring, especially May, although some discards were still observed until October (Fig. 4). The larger individuals (>26 cm TL) were more abundant in the coldest period (from January to March); their presence decreased in the following months, and no large individuals were detected in late summer and autumn (from August to December). The period of highest presence of large individuals corresponded to the reproduction period of the species in the area (December-March) (Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ).
In the two years of sampling, discards represented a very large fraction of the blue whiting catch, 31% and 23% in 2017 and 2018, respectively, when expressed in weight. These values are very high, but when discards are expressed in number, the percentage of the discarded catch amounted to more than 80% and 75% of the total individuals fished in 2017 and 2018, respectively (Table 1). It is worth mentioning that in May 2017 and 2018 almost the whole catch was discarded (95% and 98%, respectively). In 2017 recruitment took place earlier than in 2018, and consequently discards in April were also very high (94%), while in April 2018 they were quite low (6%) (Table 1).
| Weight (t) | Number (thous.) | ||||||
|---|---|---|---|---|---|---|---|
| Month | Commercial | Discards | % discards | Commercial | Discards | % discards | |
| 2017 | 1 | 6.6 | 0.0 | 0.0 | 131.4 | 0.0 | 0.0 |
| 2 | |||||||
| 3 | 10.6 | 0.0 | 0.0 | 94.3 | 0.4 | 0.4 | |
| 4 | 8.4 | 4.0 | 32.3 | 90.7 | 1428.8 | 94.0 | |
| 5 | 12.4 | 12.1 | 49.4 | 135.8 | 2604.4 | 95.0 | |
| 6 | 7.8 | 2.8 | 26.5 | 80.4 | 153.4 | 65.6 | |
| 7 | 4.7 | 4.9 | 50.9 | 136.9 | 407.1 | 74.8 | |
| 8 | 2.6 | 1.9 | 42.3 | 83.6 | 124.2 | 59.8 | |
| 9 | 2.5 | 1.4 | 35.0 | 90.9 | 82.3 | 47.5 | |
| 10 | 2.1 | 1.2 | 35.9 | 87.2 | 60.9 | 41.1 | |
| 11 | 1.6 | 0.0 | 0.0 | 35.6 | 0.0 | 0.0 | |
| 12 | 0.9 | 0.0 | 0.0 | 19.4 | 0.0 | 0.0 | |
| Total | 53.8 | 28.2 | 34.4 | 986.2 | 4861.5 | 83.1 | |
| 2018 | 1 | 13.0 | 0.0 | 0.0 | 186.6 | 0.0 | 0.0 |
| 2 | |||||||
| 3 | 11.6 | 0.0 | 0.1 | 155.4 | 7.7 | 4.7 | |
| 4 | 6.3 | 0.0 | 0.2 | 80.3 | 5.3 | 6.2 | |
| 5 | 5.8 | 14.8 | 71.9 | 35.6 | 2900.4 | 98.8 | |
| 6 | 9.4 | 0.7 | 7.3 | 103.2 | 32.2 | 23.8 | |
| 7 | 12.3 | 0.4 | 3.3 | 350.2 | 21.0 | 5.7 | |
| 8 | 4.1 | 0.8 | 16.6 | 79.1 | 40.9 | 34.1 | |
| 9 | 4.0 | 3.6 | 47.5 | 135.6 | 187.0 | 58.0 | |
| 10 | 4.0 | 0.0 | 0.0 | 96.9 | 0 | 0.0 | |
| 11 | 5.6 | 0.0 | 0.0 | ||||
| 12 | 4.8 | 0.0 | 0.0 | ||||
| Total | 80.8 | 20.4 | 20.2 | 1222.8 | 3194.6 | 72.3 |
Spatial distribution by size and age
⌅Blue whiting displayed spatial segregation by size. The smaller individuals (recruits) remained on the shelf break (130-250 m depth), while the large ones (adults) were more abundant on the upper slope (250-550 m). The largest individuals displayed movements over the year. They approached the coast during the spawning period (March 2017 and January 2018) and remained on the upper slope until spring, gradually disappearing from the catches in the following months (Fig. 5).
The age composition of catches ranged between 0 and 8 years, and ages 0-1 were the most abundant ones. Age 0 individuals were clearly dominant in the catches. A different distribution by age was observed in the two fishing grounds during the year. Ages 0 and 1 were predominantly found on the shelf break, while ages 2 and over were more abundant on the upper slope. Age 0 individuals peaked in April and May, notably in May, on the shelf break and their presence on the slope was very low. The older individuals (from 3 yr) appeared in the catch from January to July, but then gradually decreased and showed practically no presence from August (Table 2).
| Age (year) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
|---|---|---|---|---|---|---|---|---|---|---|
| Month | Zone | |||||||||
| 2016-Oct | S b | |||||||||
| 2017-Mar | S b | 67.6 | 111.7 | 25.4 | 7.0 | 3.7 | 4.8 | 0.2 | 0.1 | |
| 2017-Apr | S b | 2732.4 | 41.7 | 0.8 | 0.0 | |||||
| 2017-May | S b | 5594.4 | 187.8 | 6.5 | 0.1 | |||||
| 2017-Jun | S b | 526.7 | 121.6 | 6.6 | 0.4 | |||||
| 2017-Jul | S b | 3467.3 | 122.4 | 6.4 | 0.7 | 0.0 | ||||
| 2017-Aug | S b | 879.4 | 57.5 | 0.1 | ||||||
| 2017-Sep | S b | 767.2 | 43.8 | |||||||
| 2017-Oct | S b | 583.6 | 24.2 | |||||||
| 2017-Nov | S b | 55.9 | 23.7 | 1.3 | 0.2 | |||||
| 2016-Oct | U s | 101.8 | 36.6 | 0.9 | 0.2 | |||||
| 2017-Mar | U s | 14.5 | 56.5 | 50.5 | 21.3 | 18.3 | 11.3 | 5.8 | 1.2 | 1.5 |
| 2017-Apr | U s | 27.7 | 48.2 | 15.7 | 8.2 | 7.3 | 4.6 | 1.6 | 0.6 | 0.3 |
| 2017-May | U s | 18.9 | 50.5 | 18.3 | 8.3 | 9.6 | 6.1 | 2.2 | 0.2 | 0.5 |
| 2017-Jun | U s | 805.4 | 31.9 | 17.4 | 5.3 | 3.1 | 1.7 | 0.3 | 0.1 | |
| 2017-Jul | U s | 62.4 | 34.8 | 14.4 | 3.5 | 1.4 | 0.7 | 0.1 | 0.1 | |
| 2017-Aug | U s | 0.1 | 2.5 | 1.5 | 0.5 | 0.2 | 0.1 | 0.0 | 0.0 | |
| 2017-Sep | U s | 6.3 | 6.7 | 1.5 | 0.3 | 0.1 | 0.0 | 0.0 | 0.0 | |
| 2017-Oct | U s | 12.0 | 4.2 | 0.0 | ||||||
| 2017-Nov | U s | 6.9 | 6.8 | 1.0 | 0.2 | 0.1 | 0.0 | |||
| 2018-Jan | S b | 39.4 | 17.0 | 0.1 | 0.0 | |||||
| 2017-Mar | S b | 201.7 | 200.1 | 12.0 | 4.8 | 3.9 | 2.0 | 0.8 | 0.1 | |
| 2018-Apr | S b | 89.4 | 177.8 | 7.3 | 0.2 | |||||
| 2018-May | S b | 4923.0 | 2.9 | 0.7 | 0.1 | 0.0 | 0.0 | |||
| 2018-Jun | S b | 146.5 | 72.4 | 12.1 | 1.2 | 0.0 | ||||
| 2018-Jul | S b | 3080.0 | 621.0 | |||||||
| 2018-Aug | S b | 113.9 | 12.3 | |||||||
| 2018-Sep | S b | 949.2 | 26.6 | |||||||
| 2018-Oct | S b | 892.4 | 374.2 | |||||||
| 2018-Jan | S b | 20.6 | 44.8 | 12.9 | 4.8 | 2.5 | 1.5 | 1.3 | 0.1 | 1.6 |
| 2017-Mar | U s | 2.0 | 33.6 | 25.8 | 6.6 | 3.6 | 2.1 | 0.6 | 0.1 | 0.1 |
| 2018-Apr | U s | 0.9 | 3.8 | 3.5 | 4.2 | 2.7 | 1.7 | 0.3 | 0.3 | |
| 2018-May | U s | 1.2 | 15.5 | 14.4 | 9.2 | 8.6 | 6.0 | 2.2 | 0.3 | 0.1 |
| 2018-Jun | U s | 29.4 | 35.9 | 19.0 | 4.5 | 2.3 | 1.4 | 0.4 | 0.1 | 0.0 |
| 2018-Jul | U s | 0.9 | 35.5 | 31.1 | 10.7 | 6.5 | 3.4 | 1.4 | 0.3 | 0.1 |
| 2018-Aug | U s | 1.1 | 25.0 | 13.9 | 2.9 | 0.9 | 0.6 | 0.1 | 0.0 | 0.1 |
| 2018-Sep | U s | 28.1 | 31.6 | 7.4 | 1.8 | 0.7 | 0.4 | 0.0 | 0.0 | |
| 2018-Oct | U s |
DISCUSSION
⌅Blue whiting is considered a cold-water species in the Mediterranean Sea, the southern edge of the species distribution (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 ). In the area where the Catalan bottom trawl fleet operates, the species is mainly fished in the northern sector of the coast and in the Gulf of Lion (Fig. 1), suggesting that the species is more abundant there than southwards. The higher abundance of cold-water species such as blue whiting in the northern areas of the Mediterranean is a well-documented phenomenon linked to the marked latitudinal gradient of temperatures (Bianchi and Morri 2000Bianchi C.N., Morri C., 2000. Marine biodiversity of the Mediterranean Sea: situation, problems and prospects for future research. Mar. Pollut. Bull. 40: 367-376. https://doi.org/10.1016/S0025-326X(00)00027-8 , Lloret et al. 2015Lloret J, Sabatés A., Muñoz M., et al. 2015. How a multidisciplinary approach involving ethnoecology, biology and fisheries can help explain the spatio-temporal changes in marine fish abundance resulting from climate change. Glob. Ecol. Biogeogr. 24: 448-461. https://doi.org/10.1111/geb.12276 ). The production map allowed us to identify the blue whiting fishing grounds in the NW Mediterranean and evidenced that the species is mainly found between 200 and 600 m depth, as reported in the Atlantic (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 , Johsen and Godø 2007Johsen E., Godø O.R. 2007. Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou). ICES J. Mar. Sci. 64: 1202-1209. https://doi.org/10.1093/icesjms/fsm110 ). However, it was not possible to determine the distribution of the species at greater depths because at bottom depths greater than 500 m the species is not associated with the seabed, and is therefore not efficiently sampled by bottom trawling.
In the last decade, blue whiting landings on the Catalan coast have displayed a decreasing trend and have remained at very low levels in the most recent years, lower than the minimum values reported by Martín et al. (2016)Martín P., Maynou F., Recasens L. Sabatés, A. 2016. Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open‐sea convection processes in the northwestern Mediterranean. Fish. Oceanogr. 25: 229-240. https://doi.org/10.1111/fog.12147 over the period 1961-2011. These authors also reported cyclic fluctuations in the landings linked to deep convection processes that occur in the Gulf of Lion in winter (MEDOC group 1970MEDOC Group. 1970. Observation of formation of deep water in the Mediterranean Sea, 1969. Nature, 227: 1037-1040. https://doi.org/10.1038/2271037a0 ). These processes enhance planktonic production and subsequent recruitment of the species, resulting in a strong year class the following year (Martín et al. 2016Martín P., Maynou F., Recasens L. Sabatés, A. 2016. Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open‐sea convection processes in the northwestern Mediterranean. Fish. Oceanogr. 25: 229-240. https://doi.org/10.1111/fog.12147 ). The low landings observed in the last few years might be related to the absence since 2013 of intense deep convention processes in winter (Salat et al. 2019Salat J., Pascual J., Flexas M., et al. 2019. Forty-five years of oceanographic and meteorological observations at a coastal station in the NW Mediterranean: a ground truth for satellite observations. Ocean Dyn. 69: 1067-1084. https://doi.org/10.1007/s10236-019-01285-z , Margirier et al. 2020Margirier F., Testor P., Heslop E., et al. 2020. Abrupt warming and salinification of intermediate waters interplays with decline of deep convection in the Northwestern Mediterranean Sea. Sci. Rep. 10: 20923. https://doi.org/10.1038/s41598-020-77859-5 ), which enhance landings peaks. Major episodes (deep convection down to 1800 m depth) were detected again in winter 2018 (Margirier et al. 2020Margirier F., Testor P., Heslop E., et al. 2020. Abrupt warming and salinification of intermediate waters interplays with decline of deep convection in the Northwestern Mediterranean Sea. Sci. Rep. 10: 20923. https://doi.org/10.1038/s41598-020-77859-5 ), resulting in a slight increase in landings in 2019 (more marked in the LPUE). However, this increase was lower than those previously observed after deep convection episodes (Martín et al. 2016Martín P., Maynou F., Recasens L. Sabatés, A. 2016. Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open‐sea convection processes in the northwestern Mediterranean. Fish. Oceanogr. 25: 229-240. https://doi.org/10.1111/fog.12147 ), pointing to a weak response of the stock to the favourable environmental conditions. Indeed, in 2020, with fishing effort (fishing days) similar to that in 2019, LPUE was much lower. The effects of the exploitation on this weak blue whiting response in 2019 to the deep convection of the previous year cannot be disregarded, particularly when landings display a decreasing trend and the bulk of the catch is made up of immature individuals (size of first maturity 18-19 cm TL; Serrat et al. 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 , Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ).
Blue whiting LFDs showed marked changes over the year and between areas. The larger individuals appeared in the winter months over the upper slope, and the massive incorporation of recruits took place in spring, basically in May, over the shelf break. The presence of large individuals corresponds to spawners (age ≥2) that approach the upper slope for reproduction in winter (Table 2; Fig. 5, March 2017 and January 2018), joining the resident population that is mainly composed of age 0 and 1 individuals. The spawning over the upper slope would allow the blue whiting eggs and larvae to be retained by the shelf-slope density front that is present all along the continental slope (Font et al., 1988Font J., Salat J. Tintoré J. 1988. Permanent features of the circulation of the Catalan Sea. In: Minas H.J., Nival P. (eds), Pelagic Mediterranean Oceanography. Oceanol. Acta n. sp., 9: 51-57.), avoiding their dispersion towards the open sea (A. Sabatés, pers. comm.), as observed in other fish species in the area (Sabatés et al. 2018Sabatés A., Salat J., Tilves U., et al. 2018. Pathways for Pelagia noctiluca jellyfish intrusions onto the Catalan shelf and their interactions with early life fish stages. J. Mar. Syst. 187: 52-61. https://doi.org/10.1016/j.jmarsys.2018.06.013 ). Thus, the fluctuations in the presence of large individuals over the year would suggest population movements linked to reproduction, as already described for the blue whiting in the Atlantic (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 , Hátún et al. 2007Hátún H., Arge J., Sandø A.B. 2007. Environmental influence on the spawning distribution and migration pattern of northern blue whiting (Micromesistius poutassou). ICES CM B: 6.). These displacements over the year and recruitment intensity and period would explain the seasonal pattern of landings. The seasonal pattern changed in the period 2000-2020. Marked peaks were detected in winter-spring in years of high landings, but when annual landings remained at very low levels, as of 2008, this seasonal pattern became less pronounced (Fig. 3). The disappearance of the marked winter-spring landings peak must reflect a low recruitment the previous year i.e. a low abundance of age 1, as well as a low abundance of spawners.
The spawning stock must remain on the upper slope in spring, the recovering season (Serrat et al. 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 , Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ), gradually disappearing from the landings in the following months (Fig. 5; Table 2). This means that the spawning and feeding grounds in the Mediterranean overlap, unlike in the Atlantic, where migrations linked to feeding and reproduction have been described (Bailey 1982Bailey R.S. 1982. The population biology of blue whiting in the North Atlantic. Adv. Mar. Biol. 19: 257-355. https://doi.org/10.1016/S0065-2881(08)60089-9 , Hátún et al. 2007Hátún H., Arge J., Sandø A.B. 2007. Environmental influence on the spawning distribution and migration pattern of northern blue whiting (Micromesistius poutassou). ICES CM B: 6.). The absence of these large individuals in the catch some months after spawning would suggest that blue whiting move to areas not accessible to bottom trawl. In agreement with our results, in the Aegean Sea, where the spawning season spans from December to March, the presence of large individuals was observed in December and March, yet their presence was much lower in June (Papaconstantinou and Petrakis 1989Papaconstantinou C., Petrakis G. 1989. Some data on the population dynamics of blue whiting (Micromesistius poutassou) in the Northern Euviokos Gulf. In: Savini M., Caddy J.F. (eds), Report of the second technical consultation on stock assessment in the eastern Mediterranean, FAO Fish. Rep. No 412: 83-89.). While we cannot describe the direction of the population migratory movements, we could expect inshore-offshore movement of the species at a certain depth, i.e. without a close association with the seabed when the depth exceeds 500 m (Johsen and Godø 2007Johsen E., Godø O.R. 2007. Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou). ICES J. Mar. Sci. 64: 1202-1209. https://doi.org/10.1093/icesjms/fsm110 ). Indeed, inshore-offshore displacements of spawners have been hypothesized for the Portuguese coast (Gonçalves et al. 2017Gonçalves P., de Melo A.Á., Murta A.G., Cabral H.N. 2017. Blue whiting (Micromesistius poutassou) sex ratio, size distribution and condition patterns off Portugal. Aquat. Living Resour. 30: 24. https://doi.org/10.1051/alr/2017019 ). Also, considering that blue whiting is closely associated with the shelf-slope region, and a strong connectivity along this axis has been reported in the Atlantic (Post et al. 2019Post S., Fock H.O., Jansen T. 2019. Blue whiting distribution and migration in Greenland waters. Fish. Res. 212: 123-135. https://doi.org/10.1016/j.fishres.2018.12.007 ), we cannot disregard northward movements in summer to the Gulf of Lion where, based on the production map, the species is more abundant than southwards (Fig. 1). In this regard, the presence of large individuals has been reported in autumn in the Gulf of Lion (Massuti et al. 2008Massuti E., Ordinas F., González N et al. 2008. Informe del seguimiento científico de La acción piloto rai/Ap-26/2007: pesca experimental con arte de arrastre De Fondo En El Golfo De León (Mediterráneo noroccidental). Instituto Español Oceanografia, 112 pp.). However, we can only hypothesize about these possibilities, since our information comes from the commercial bottom trawl, for which blue whiting is not a fishing target, and no specific surveys are available for the species in the NW Mediterranean.
Our results show that the highest blue whiting yields were obtained in the Gulf of Lion, some of them in the FRA established in 2009 to protect spawning aggregations and sensitive deep-sea habitats (FAO-GFCM 2009FAO-CGFM 2009. Recommendation GFCM/33/2009/1 On the establishment of a Fisheries Restricted Area in the Gulf of Lions to protect spawning aggregations and deep sea sensitive habitats. http://www.fao.org/3/ax884e/ax884e.pdf. (Last accessed 3 September 2021)). Since then, fishing effort in the FRA has been frozen. A bottom trawl survey conducted in autumn 2007 in an area that was later protected showed higher abundance than that observed in the commercial catch of large individuals of the different target species, one of them, blue whiting (Massuti et al. 2008Massuti E., Ordinas F., González N et al. 2008. Informe del seguimiento científico de La acción piloto rai/Ap-26/2007: pesca experimental con arte de arrastre De Fondo En El Golfo De León (Mediterráneo noroccidental). Instituto Español Oceanografia, 112 pp.). The abundance of large individuals points to a healthy status of the population. As stated in Commission Decision 2017/848 (EU 2017EU 2017. Laying down criteria and methodological standards on good environmental status of marine waters and specifications and standardised methods for monitoring and assessment. Comission Decision (EU) 2017/848. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32017D0848&from=EN ), a high proportion of old/large individuals in the populations of commercially-exploited species is indicative of a healthy population. In the Gulf of Lion, based on trawl surveys, an east-to-west gradient was reported regarding average sizes and abundances of target species. The gradient was smaller in the western part, where the French and Spanish fleets operate, than in the eastern part, where fishing effort is lower (UNEP-MAP-RAC/SPA 2013UNEP-MAP-RAC/SPA. 2013. Description of the ecology of the Gulf of Lions shelf and slope area and identification of the areas that may deserve to be protected. By Gili J.M, Domínguez-Carriό C., Ed. RAC/SPA, Tunis. 64 pp.). These findings, combined with the established FRA, suggest that the Gulf of Lion might act as a refuge for the species (Farrugio 2012Farrugio H. 2012. A refugium for the spawners of exploited Mediterranean marine species: the canyons of the continental slope of the Gulf of Lion. In: Würtz M. (ed), Mediterranean Submarine Canyons: Ecology and Governance. Gland, Málaga, IUCN, pp.45-49.).
In 2010 a change in the mesh size was introduced, from 40 mm diamond to 40 mm square mesh size, which in principle would improve protection of the smallest individuals in comparison with the first part of the study period (2000-2010). However, the results of the present study show that blue whiting recruits were highly fished and discarded during the second period. Ancillary information on blue whiting LFDs of the total catch is available from research projects conducted in the study area in the 1980s (Martín 1989Martín P. 1989. Dinámica de la pesquería de arrastre en Cataluña. PhD thesis. University of Barcelona, 364 pp., Lleonart 1990Lleonart J. 1990. La Pesquería Cataluña y Valencia: descripción global y planteamiento de bases para su seguimiento. Comisión de las Comunidades Europeas Dirección General XIV (Pesca). Informe final de Proyecto, Referencia nº 1989/3. http://digital.csic.es/bitstream/10261/147881/1/Pesqueria_Catalu%C3%B1a_Valencia_Informe_Final_1990_vol_1.pdf ). Despite the changes in the gear design and mesh size (smaller at the time) it is worth noting that the smaller reported exploited sizes started at 9 cm TL, and abundance of large individuals was higher than at present. Data from the 1960s indicate that the exploited sizes ranged between 12 and 32 cm TL (Bas 1963Bas C. 1963. Fluctuations de la pêche de Merlangus poutassou et quelques considérations sur son contrôle. Proc. Tech. Pap. Gen. Fish. Coun. Mediterr. 7: 417- 420. ), the larger sizes being absent from the catch because of limited access to the deeper fishing grounds. Blue whiting has been exploited for several decades with immature individuals (size at first maturity around 18 cm TL; Serrat et al. 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 , Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ) as a major component of the catch. In the present study, the age composition of the catches ranged between 0 and 8 years, ages 0-1 being the most abundant ones. Fishery-independent data from 2009-2013 reported that the blue whiting population consisted mainly of only four age classes, from 0 to 3, the abundance of ages 2 and 3 being very low (STECF 2014STECF. 2014. Assessment of Mediterranean Sea Stocks - Part 1 (STECF-14-17). Publ. Off. Eur. Union, Luxembourg, pp. 393 EUR 26955 EN, JRC 93120.). These age compositions indicate that the species has been heavily exploited for years, and the maintenance of the population is almost exclusively dependent on recruitment. Furthermore, since the reproductive potential of blue whiting increases with size (Serrat et al 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 ), the scarcity of large fish must have also affected the reproductive potential of the population.
Recruitment to the demersal habitat takes place over the shelf break in April and May, when the individuals attain 5 cm TL, and the recruits remain there until the next spawning season (Fig. 4). This size is smaller than the recruitment size previously reported for the western Mediterranean, 11 cm TL in June (García et al. 1987García L., Lucena J., Sánchez F.J. Hernández J. 1987. Biologia De La Bacaladilla («Micromesistius poutassou» Risso 1826) Del Mediterraneo Occidental. Boletín del Instituto de Estudios Almerienses, Ciencias 7: 169-198.). Tursi et al. (1992)Tursi A., Sion L., Maiorano L., Panza M. 1992. Le nurseries di Micromesistius poutassou Risso, 1826) nel mar Jonio. Bioloia Marina 1: 377-378. identified blue whiting nursery areas in the Ionian Sea located at 100-300 m depth. Recruits, 8-9 cm individuals in April-May, and 11-12 cm individuals in June, represented between 30% and 96% of the sampled population. These observations on blue whiting attaining 11 cm TL in June suggest a rather regular pattern of the species in the Mediterranean in relation to the recruitment timing, location and size.
In the present study, the major component of the blue whiting discards corresponded to individuals between 5 and 12 cm TL, coming from the recent spawning (Table 2 and Fig 3). Discards were observed over a period of several months, from April to October, associated with the seasonality of the recruitment, but very high discards were concentrated mainly in the month of May (more than 95% of the catch in number, Table 1). These discard values are much higher than those previously reported in the Mediterranean to date by Tsagarakis et al. (2017)Tsagarakis K., Carbonell A., Brčić J., et al. 2017. Old info for a new fisheries policy: Discard ratios and lengths at discarding in EU Mediterranean bottom trawl Fisheries. Front. Mar. Sci. 4: 99. https://doi.org/10.3389/fmars.2017.00099 . In their review on Mediterranean discards, these authors stressed that the bottom trawl discard ratios of non-target commercial bycatch species such as blue whiting were higher than those of the target species. Blue whiting discards rates of less than 5% of the catch have been reported in the Ligurian Sea, discards consisting of individuals ranging between 10 and 19 cm TL (STECF 2014STECF. 2014. Assessment of Mediterranean Sea Stocks - Part 1 (STECF-14-17). Publ. Off. Eur. Union, Luxembourg, pp. 393 EUR 26955 EN, JRC 93120., Mannini and Lanteri 2015Mannini A., Lanteri L. 2015. Micromesistius poutassou. In: Sartor P., Mannini A., et al. (eds), Sintesi delle conoscenze di biologia, ecologia e pesca delle specie ittiche dei mari italiani. Biologia Marina Mediterranea, 22: 304-310 (Suppl. 1).). A similar size range for blue whiting discards, 13 to 19 cm TL, has been reported for the deep-sea decapod crustacean fisheries in Mallorca (Moranta et al. 2000Moranta J., Massutí E., Morales-Nin B. 2000. Fish catch composition of the deep-sea decapod crustacean fisheries in the Balearic Islands (western Mediterranean). Fish. Res. 45: 253-264. https://doi.org/10.1016/S0165-7836(99)00119-8 ). Blue whiting discards in the Atlantic, where the species is fished almost entirely with pelagic trawl (99% of the catch) are reported to be very low (0.25% of the catch) (ICES 2019ICES. 2019. Blue whiting (Micromesistius poutassou) in subareas 1-9, 12, and 14 (Northeast Atlantic and adjacent waters). In: Report of the ICES Advisory Committee, 2019.). Altogether, the reported discard values make blue whiting one of the species with the highest discards in bottom trawl fisheries in the Mediterranean. This finding constitutes an important input for the implementation of the ecosystem approach to fisheries management because it highlights that the impact of fishing activity on the ecosystem might be underestimated when the exploitation status of bycatch species is not taken into account.
The results of the present study suggest that blue whiting is intensively exploited in the NW Mediterranean. This is evidenced by the decline in landings over the last decade, the catch dominated by immature individuals and the very low presence of old, large-sized blue whiting. Considering that larger females have a higher reproductive success than smaller females (Hixon et al. 2014Hixon M.A., Johnson D.W., Sogard S.M. 2014. BOFFFFs: on the importance of conserving old-growth age structure in fishery populations. ICES J. Mar. Sci. 71: 2171-2185. https://doi.org/10.1093/icesjms/fst200 , Serrat et al. 2019Serrat A., Lloret J., Frigola‐Tepe X., Muñoz M. 2019. Trade‐offs between life‐history traits in a coldwater fish in the Mediterranean Sea: the case of blue whiting Micromesistius poutassou. J. Fish Biol. 95: 428-443. https://doi.org/10.1111/jfb.13993 ), the scarcity of large individuals must have also affected the reproductive potential of the population. Furthermore, it is precisely during the reproductive period that the large and more fecund individuals are caught, which must have further worsened the status of the stock. Indeed, the most recent stock assessment of blue whiting in the western Mediterranean, performed some years ago (STECF 2014STECF. 2014. Assessment of Mediterranean Sea Stocks - Part 1 (STECF-14-17). Publ. Off. Eur. Union, Luxembourg, pp. 393 EUR 26955 EN, JRC 93120.), reported that the stock was highly over-exploited. To improve the status of fish stocks, the fishing pressure on immature individuals must decrease (Vasilakopoulos et al. 2011Vasilakopoulos P., O’Neill F.G., Marshall C.T. 2011. Misspent youth: does catching immature fish affect fisheries sustainability? ICES Mar. Sci. 68: 1525-1534. https://doi.org/10.1093/icesjms/fsr075 ) which, according to our results, is not the case for NW Mediterranean blue whiting. The implementation of closures in spring, when the highest discards rates have been observed, should be considered as a management measure for the recovery of the population. Though the above indicators point to over-exploitation, there part of the population may not be accessible to bottom trawling, because the larger individuals disappear from the catch for several months.
In addition to fishing, the unfavourable environmental conditions, such as the absence of deep convection processes for a period of eight consecutive years (Margirier et al. 2020Margirier F., Testor P., Heslop E., et al. 2020. Abrupt warming and salinification of intermediate waters interplays with decline of deep convection in the Northwestern Mediterranean Sea. Sci. Rep. 10: 20923. https://doi.org/10.1038/s41598-020-77859-5 ), must have negatively impacted the species. In fact, the response of blue whiting to the deep convection in 2018 was rather weak, suggesting that low levels of abundance might have hindered their reaction to favourable environmental drivers. We must also consider that for a species with affinity to cold waters, with a strong dependence on the cold winter temperatures for reproduction in the Mediterranean (Mir-Arguimbau et al. 2020bMir-Arguimbau J., Balcells M., Raventós N., et al. 2020b. Growth, reproduction and their interplay in blue whiting (Micromesistius poutassou, Risso, 1827) from the NW Mediterranean. Fish. Res. 227: 105540. https://doi.org/10.1016/j.fishres.2020.105540 ), the sea water temperature increase recorded (Salat et al. 2019Salat J., Pascual J., Flexas M., et al. 2019. Forty-five years of oceanographic and meteorological observations at a coastal station in the NW Mediterranean: a ground truth for satellite observations. Ocean Dyn. 69: 1067-1084. https://doi.org/10.1007/s10236-019-01285-z ) must have been be an additional stress (Lloret et al. 2015Lloret J, Sabatés A., Muñoz M., et al. 2015. How a multidisciplinary approach involving ethnoecology, biology and fisheries can help explain the spatio-temporal changes in marine fish abundance resulting from climate change. Glob. Ecol. Biogeogr. 24: 448-461. https://doi.org/10.1111/geb.12276 ). All these environmental constraints, added to over-exploitation and their synergistic effects, must have affected the resilience of the species, compromising its success in the Mediterranean.