Spawning area of the tropical Skipjack Tuna, Katsuwonus pelamis (Scombridae), in the western Mediterranean Sea

INTRODUCTION

The skipjack tuna Katsuwonus pelamis (Linnaeus, 1758), hereafter referred to as SKJ, is a cosmopolitan, migratory species distributed in tropical and subtropical waters of all oceans, including the Mediterranean Sea. Its geographical limits are 55-60°N and 45-50°S and it is more abundant in the equatorial region throughout the year (Collete and Nausen 1983Collete B.B., Nausen C.E. 1983. FAO Species Catalogue. Vol. 2. Scombrids of the world. An annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date. Rome: FAO. FAO Fish. Synop. 125: 1-137., Matsumoto et al. 1984Matsumoto W.M., Skillman R.A., Dixon A.E. 1984. Synopsis of biological data on skipjack tuna, Katsuwonus pelamis. U.S. Nat. Mar. Fish. Serv. Nat. Oceanic Atmos. Adm. Tech. Rep. NMFS Circ., vol 451. U.S. Department of Commerce.). The SKJ is a commercially important species, being the third species in terms of captures for the eleventh consecutive year, with 2.8 million t in 2020 (FAO 2022FAO. 2022. El estado mundial de la pesca y la acuicultura 2022. Hacia la transformación azul. Roma, FAO. https://doi.org/10.4060/cc0461es ). Its catches in the Mediterranean Sea are common, especially in recent decades. In the western Mediterranean Sea, the first catches were recorded in 1968, and the average landing of the last six years (2015-2020) is 168 t (ICCAT 2021ICCAT. 2021. International Commission for the Conservation of Atlantic Tunas - Access to ICCAT statistical databases. Task I. https://ICCAT.int/en/accesingdb.html.). The main Spanish catches come from the traps located in Ceuta (Strait of Gibraltar) and longline surface fisheries targeting albacore tuna. The SKJ is also caught by Spanish recreational offshore trolling fisheries in the Alboran Sea during the summer months, mainly in August and September, and in the Balearic Sea in June and July, coinciding with the reproductive season for tunas in the area (Saber et al. 2015Saber S., Muñoz P., Ortiz de Urbina J., et al. 2015. Análisis de las tendencias de las capturas de atún listado Katsuwonus pelamis (Linnaeus, 1758) de la pesca deportiva en el Mediterráneo occidental (2006-2014). In: Díaz del Río V., Bárcenas P., Fernández-Salas L.M., et al. (eds), Volumen de Comunicaciones presentadas en el VIII Simposio sobre el Margen Ibérico Atlántico (MIA15). Málaga, Spain, 21-23 September 2015. Ediciones Sia Graf, Málaga, pp. 517-520., 2020Saber S., Macías D., Gómez-Vives M.J., et al. 2020. Standardized catch rates of Skipjack from the Mediterranean Spanish recreational fishery (2006-2018). Collect. Vol. Sci. Pap. ICCAT 76(6): 867-873.).

The SKJ has been classified as a multiple batch spawner fish with asynchronous oocyte development (Goldberg and Au 1986Goldberg S.R., Au D.W. 1986. The spawning of skipjack tuna from the southern Brazil as determinated from histological examination of ovaries. In: Symmons P.E.K., Miyaque P.M., Sahagawa G.T. (eds), Proc. ICCAT Conf. Int. Skipjack Year Program, Int. Comm. Conser. Atl. Tunas, Madrid, Spain, pp. 277-284., Schaefer 2001aSchaefer K.M. 2001a. Assessment of skipjack tuna, Katsuwonus pelamis spawning activity in the eastern Pacific Ocean. Fish. Bull. 99: 343-350., Grande et al. 2012Grande M., Murua H., Zudaine I., Korta M. 2012. Oocyte development and fecundity type of the skipjack Katsuwonus pelamis, in the Western Indian Ocean. J. Sea Res. 73: 117-284. https://doi.org/10.1016/j.seares.2012.06.008 ). Its reproductive potential is considered high because it reaches sexual maturity at around one year of age and spawns opportunistically throughout the year in large areas of the ocean (ICCAT 2019ICCAT. 2019. International Commission for the Conservation of Atlantic Tunas - Report of the Standing Committee on Research and Statistics (SCRS). 459 pp.). Knowledge of the biology of the species is required for stock assessment, so it is important to know the reproductive characteristics and phenology of the species.

In the Atlantic Ocean SKJ spawn in two main areas: one in the western Atlantic Ocean, where it spawns over two subtropical areas off the coast of South America, and its migration is influenced by season (Andrade and Santos 2004Andrade H.A., Santos J.A.T. 2004. Seasonal trends in the recruitment of skipjack tuna (Katsuwonus pelamis) to the fishing ground in the southwest Atlantic. Fish. Res. 66: 185-194. https://doi.org/10.1016/S0165-7836(03)00199-1 , Benevenuti et al. 2019Benevenuti J., Monteiro-Neto C., Rodrigues M., et al. 2019. Size structure, reproduction, and growth of skipjack tuna (Katsuwonus pelamis) caught by the pole-and-line fleet in the southwest Atlantic. Fish. Res. 212: 136-145. https://doi.org/10.1016/j.fishres.2018.12.011 ); and one in the eastern Atlantic Ocean, where it spawns year-round in the equatorial area, from the Gulf of Guinea to 20-30ºW. In the tropical area, spawning occurs only in the warm season (Andrade and Santos 2004Andrade H.A., Santos J.A.T. 2004. Seasonal trends in the recruitment of skipjack tuna (Katsuwonus pelamis) to the fishing ground in the southwest Atlantic. Fish. Res. 66: 185-194. https://doi.org/10.1016/S0165-7836(03)00199-1 ). In the Indian Ocean two spawning seasons have been identified, one coinciding with the northeast monsoon and one with the southwest monsoon (Stequert and Ramcharrum 1996Stequert B., Ramcharrun B. 1996. La reproduction du listao (Katsuwonus pelamis) dans le bassin ouest de l’ocean Indien. Aquat. Liv. Res. 9: 235-247. https://doi.org/10.1051/alr:1996027 , Grande et al. 2014Grande M., Murua H., Zudaine I., et al. 2014. Reproductive timing and reproductive capacity of the Skipjack Tuna (Katsuwonus pelamis) in the western Indian Ocean. Fish. Res. 156: 14-22. https://doi.org/10.1016/j.fishres.2014.04.011 ). Though the Balearic Sea is an important spawning area for several tuna species (Alemany et al. 2010Alemany F., Quintanilla L., Velez-Belchi P., et al. 2010. Characterization of the spawning habitat of Atlantic bluefin tuna and related species in the Balearic Sea (Western Mediterranean). Progr. Oceanogr. 86: 21-38. https://doi.org/10.1016/j.pocean.2010.04.014 ), there is no information about the reproduction of SKJ in the area, except for a study in which a few spawning females were recorded (Saber et al. 2012Saber S., Gómez-Vives M.J., García-Barcelona S., et al. 2012. Recreational catch rates and biology of Skipjack tuna from the western Mediterranean Sea. In: Morris E.P., Mañanes R., Fernández M.C., Gómez J. (eds), Libro de resúmenes del III Simposio Internacional de Ciencias del Mar (ISMS12), Cádiz, ISBN: 978-84-695-1394-1, p. 112.). Temperature is a very important factor determining the distribution of both adult top predators and larvae (Worm et al. 2005Worm B, Sandow M., Oschlies A., et al. 2005. Global patterns of predator diversity in the open oceans. Science 309: 1365-1369. https://doi.org/10.1126/science.1113399 , Reglero et al. 2014Reglero P., Tittesor D.P., Álvarez-Berasategui D., et al. 2014. Worldwide distributions of tuna larvae: revising hypotheses on environmental requirements for spawning habitats. Mar. Ecol. Prog. Ser. 501: 207-224. https://doi.org/10.3354/meps10666 ). An increase in water temperatures before and during spawning is needed for the adult tunas to release eggs (Margulies et al. 2007Margulies D., Suter J.M., Hunt S.L., et al. 2007. Spawning and early development of captive yellowfin tuna (Thunnus albacares). Fish. Bull. 105: 249-265.). The SKJ, like other tunas, breeds at a sea surface temperature (SST) exceeding 24°C, and its spawning season varies according to locality (Cayre and Farrugio 1986Cayre P., Farrugio H. 1986. Biologie de la reproduction du listao (Katsuwonus pelamis) del ocean Atlantique. In: Symmons P.E.K., Miyaque P.M., Sahagawa G.T. (eds), Proc. ICCAT Conf. Int. Skipjack Year Program, Int. Comm. Conser. Atl. Tunas, Madrid, Spain, pp. 252-272., Schaefer 2001bSchaefer K.M. 2001b. Reproductive biology of tunas. In: Block B.A., Stevens E.D. (eds), Tuna physiology, ecology and evolution. Academic Press, San Diego, CA, pp. 225-272. https://doi.org/10.1016/S1546-5098(01)19007-2 ). Geographical differences in the reproductive traits of the SKJ probably occur because the spawning depends on oceanographic conditions (Ashida 2020Ashida H. 2020. Spatial and temporal differences in the reproductive traits of skipjack tuna Katsuwonus pelamis between the subtropical and temperate western Pacific Ocean. Fish. Res. 221: 105352. https://doi.org/10.1016/j.fishres.2019.105352 ), and especially on SST (Margulies et al. 2007Margulies D., Suter J.M., Hunt S.L., et al. 2007. Spawning and early development of captive yellowfin tuna (Thunnus albacares). Fish. Bull. 105: 249-265.).

Global warming will directly affect the phenology and spatial distribution of marine organisms and have indirect effects on the productivity and structure of marine ecosystems with important consequences for fisheries. Future changes in the marine environment are expected to affect the physiological rates (reproduction, growth), geographical distribution and migration of SKJ with consequences for their life history. Vargas-Yañez et al. (2010Vargas-Yañez M., García M.C., Moya F., et al. 2010. Cambio Climático en el Mediterráneo español. Segunda edición. Instituto Español de Oceanografía. 176 pp., 2019Vargas-Yañez M., García M.C., Moya F., et al. 2019. Estado de los ecosistemas marinos en el Mediterráneo español en un contexto de cambio climático. Instituto Español de Oceanografía. 284 pp.) analysed trends in different temperature time series in Spanish Mediterranean waters and found warming in intermediate, deep and surface waters from the middle of the last century. We hypothesized that the western Mediterranean warming in the last decades could lead to the expansion of skipjack tuna spawning areas from tropical areas to the Mediterranean Sea. To study the reproductive status of SKJ in the western Mediterranean, we used the gonadosomatic index, microscopic analysis of ovaries and estimates of the daily spawning fraction.

MATERIALS AND METHODS

Study areas and data collection

 ⌅

Specimens of SKJ were obtained from sport fishery during fishing tournaments targeting large pelagic species (mainly albacore Thunnus alalunga, skipjack, dolphinfish Coryphaena hippurus and Mediterranean spearfish Tetrapturus belone) from June to September 2014-2019. Sampling areas located in the western Mediterranean Sea were divided into two areas based on the oceanographic features: (i) the Balearic Sea (Area I) and (ii) the Alboran Sea (Area II) (Fig. 1). The fishing grounds extended 50 to 60 nautical miles around six base ports. Fishes were caught during daylight hours using rod and reel gear by trolling, which consists of fishing lines with hooks and artificial lures, targeting fish in surface waters between 0 and 5 m approximately.

Fig. 1.  The western Mediterranean Sea showing the fishing grounds of recreational fishery (circles around the base ports) during the fishing tournaments. Base ports. Alboran Sea (Area 2): 1, Fuengirola; 2, Benalmadena. Balearic Sea (Area 1): 3, Moraira; 4, Denia; 5 S’Estanyol; 6, Cala D’Or.

A total of 454 SKJ were randomly collected for sampling collection. Immediately after landing, the round weight (RW, kg), the straight fork length (to the nearest 0.1 cm) and the gonad weight (GW, to the nearest g) were measured. Sex was determined by visual inspection and the sex ratio was calculated as the ratio of females to males. Assuming no systematic differences in ovarian homogeneity throughout the whole ovary, including between the two ovarian lobes (Otsu and Uchida 1959Otsu T., Uchida R. 1959. Study for age determination by hard parts of albacore from central Pacific and Hawaiian waters. U.S. Fish. Bull. 59: 353-363., Stequert and Ramcharrun 1995Stequert B., Ramcharrun B. 1995. The fecundity of skipjack tuna (Katsuwonus pelamis) from the western Indian Ocean. Aquat. Liv. Res. 8: 79-89. https://doi.org/10.1051/alr:1995006 ), a full cross section (2-3 cm wide) from the central part of one of the lobes was fixed in Bouin’s fluid for 4 h and then preserved in 70% ethanol for later analysis.

The gonadosomatic index (GSI) was calculated as the ratio of GW to fish gonad-free weight (RW-GW) times 10². GW and RW were both measured in grams for males and females to examine monthly changes. The equation of Gibson and Ezzi (1980)Gibson R.N, Ezzi I.A. 1980. The biology of the scaldfish, Arnoglossus laterna (Walbaum) on the west coast of Scotland. J. Fish Biol. 17: 565-575. https://doi.org/10.1111/j.1095-8649.1980.tb02788.x was used:

$GSI=GW/(RW-GW)\times 100$  

An exploratory analysis showed that the relationship between GSI, length and month was non-linear, so generalized additive models (GAM) were used to model the relationships between months, specimen length and monthly means of GSI:

$GSI=\alpha +s\left(FL,month\right)+\epsilon$  

All statistical analyses and graphs were performed using R software (R Core Team 2017R Core Team. 2017. R: A language and environmental for analysis computing. R Foundation for Statistical Computing, Vienna, Austria.). For the GAM, the mgcv (Wood 2017Wood S.M. 2017. Generalized Additive Models: An Introduction with R (2nd ed.) Chapman and Hall/CRC. https://doi.org/10.1201/9781315370279 ) and ggplot2 packages were particularly useful (Wickhman 2016Wickhman H. 2016. ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag. New York.).

RESULTS

Between 2014 and 2019 we attended a total of 21 sport fishing championships. A total of 224 SKJ were sampled in the Balearic Sea (Area I) and 230 in the Alboran Sea (Area II). For more detail of the number of individuals sampled per year, month or fishing port, see Table 2.

The straight fork length of 233 females ranged between 41.8 and 76 cm (mean±sd=61.5±6.2 cm) and the weight between 9.6 and 1.54 kg (5.4±1.7 kg). The sizes of 221 males ranged between 46.5 and 81 cm (61.2±6.3 cm) and the weight between 2.1 and 12.1 kg (5.4±1.8 kg). The sex ratio was 1:0.98 (F:M). Females were more abundant than males in the study sample. However, using a binomial test we concluded that there were no significant differences in the population analysed (p=0.6057, p>0.05). The SKJ belonging to Area I were slightly larger than those in Area II in both size (65.2±5.1 cm versus 57.7±5.0 cm) and weight (6.5±1.5 kg vs 4.3±1.3 kg).

We used total weight to calculate the GSI. The Balearic Sea samples had the highest mean GSI values (4.46±1.04 vs 1.04±0.66). In general, females always showed slightly higher values than males (2.81±1.8 vs 2.64±2.04). The highest GSI value for females was 7.82 (56.5 cm, 4.3 kg) and that for males was 7.44 (67 cm, 6.8 kg), both found in spawning individuals sampled in July in Balearic waters.

The mean monthly GSI was calculated for both males and females (Fig. 2). In June, a slight increase was observed until the maximum was reached at the beginning of July. After that date, the mean GSI decreased rapidly until it reached the minimum values in September. The monthly evolution showed a similar trend in both sexes, suggesting that the spawning season for SKJ in western Mediterranean occurs between June and August. Mean GSI showed an increasing trend during the maturation progress. The immature females showed the lowest GSI values, followed by developing and postspawning ones. The spawning capable females followed by spawning females showed the highest values. The GAM analysis (Fig. 3) revealed that larger-sized individuals showed the highest GSI values in June and July and the lowest in August and September.

The monthly frequencies of gonad stages by area are shown in Figure 4. Regarding the ovaries analysed, 76 females belonged to fishing championships in the Alboran Sea and 116 to ones in the Balearic Sea. The majority (n=113) of the samples from the Balearic Sea were spawning females and the rest were spawning capable. We consistently found spawning females in the Balearic Sea throughout the study period. The ovaries collected in the Alboran Sea showed greater variability in all possible gonad stages: two immature individuals, two specimens in the developing stage, two in the spawning capable stage, 15 in the spawning stage, 24 in the regressing stage and 31 in the regenerating stage. In 2018 all spawning capable and spawning females were captured in the Benalmadena sport fishing championship. This unusual episode was recorded for the first time.

Figure 5 shows the mean daily SST in 2018 for northern zone of the Alboran Sea (5°W to 2°W and 36.4°N to 37°N) and the monthly mean values for the period 1981-2021. A few days before the sport championship, an SST peak occurred there (25.4°C), with higher temperatures than expected for these dates (23.7±0.94°C).

Figure 6 shows microphotographs of ovaries in spawning and postspawning phases, confirming the reproduction of the species in the western Mediterranean. The spawning female fraction calculated considering the proportion of spawning females (n=116) to all active females (n=119, that is, the spawning capable and spawning stages) was 0.97, and the mean spawning frequency was 1.03 days. Therefore, SKJ from the Balearic Sea are capable of spawning every day.

DISCUSSION

Though the SKJ is of great economic importance and several studies have been carried out on it worldwide, there is a lack of knowledge about its biology and migrations in the Mediterranean Sea.

An upward trend in the CPUE of SKJ was observed in the Balearic Sea, where the specimens caught were slightly larger than in the Alboran Sea. Catches in the Alboran Sea also showed an upward trend in CPUE. However, this trend is more irregular than in the Balearic Sea (Saber et al. 2012Saber S., Gómez-Vives M.J., García-Barcelona S., et al. 2012. Recreational catch rates and biology of Skipjack tuna from the western Mediterranean Sea. In: Morris E.P., Mañanes R., Fernández M.C., Gómez J. (eds), Libro de resúmenes del III Simposio Internacional de Ciencias del Mar (ISMS12), Cádiz, ISBN: 978-84-695-1394-1, p. 112., 2015Saber S., Muñoz P., Ortiz de Urbina J., et al. 2015. Análisis de las tendencias de las capturas de atún listado Katsuwonus pelamis (Linnaeus, 1758) de la pesca deportiva en el Mediterráneo occidental (2006-2014). In: Díaz del Río V., Bárcenas P., Fernández-Salas L.M., et al. (eds), Volumen de Comunicaciones presentadas en el VIII Simposio sobre el Margen Ibérico Atlántico (MIA15). Málaga, Spain, 21-23 September 2015. Ediciones Sia Graf, Málaga, pp. 517-520., 2020Saber S., Macías D., Gómez-Vives M.J., et al. 2020. Standardized catch rates of Skipjack from the Mediterranean Spanish recreational fishery (2006-2018). Collect. Vol. Sci. Pap. ICCAT 76(6): 867-873.). Our results on CPUE trends agree with those reported by Saber et al. (2015)Saber S., Muñoz P., Ortiz de Urbina J., et al. 2015. Análisis de las tendencias de las capturas de atún listado Katsuwonus pelamis (Linnaeus, 1758) de la pesca deportiva en el Mediterráneo occidental (2006-2014). In: Díaz del Río V., Bárcenas P., Fernández-Salas L.M., et al. (eds), Volumen de Comunicaciones presentadas en el VIII Simposio sobre el Margen Ibérico Atlántico (MIA15). Málaga, Spain, 21-23 September 2015. Ediciones Sia Graf, Málaga, pp. 517-520., which indicate an increase in the abundance, weight and average size of individuals caught in the area during the study period.

The sex ratio we found was close to 1:1. However, in Atlantic waters Cayré and Farrugio (1986)Cayre P., Farrugio H. 1986. Biologie de la reproduction du listao (Katsuwonus pelamis) del ocean Atlantique. In: Symmons P.E.K., Miyaque P.M., Sahagawa G.T. (eds), Proc. ICCAT Conf. Int. Skipjack Year Program, Int. Comm. Conser. Atl. Tunas, Madrid, Spain, pp. 252-272. identified a global sex ratio in favour of males. In the Indian Ocean, Stequert and Ramcharrum (1996)Stequert B., Ramcharrun B. 1996. La reproduction du listao (Katsuwonus pelamis) dans le bassin ouest de l’ocean Indien. Aquat. Liv. Res. 9: 235-247. https://doi.org/10.1051/alr:1996027 and Timohina and Romanov (1996)Timohina O.I., Romanov E.V. 1996. Characteristics of ovogenesis and some data on maturation and spawning of skipjack tuna, Katsuwonus pelamis (Linnaeus, 1758), from the western part of the equatorial zone of the Indian Ocean. Indian Ocean Tuna Commission 24. observed the same for larger fishes.

The estimated length at which 50% of the female population of the species reached maturity (L₅₀) is below the sizes sampled in our study. The minimum size at which SKJ reached maturity in the western Mediterranean was at least 53.5 cm straight fork length. This finding agrees with studies in the Indian Ocean, where the L₅₀ reaches 41 to 43 cm in females, corresponding to 1.5 years (Stequert and Ramcharrun 1996Stequert B., Ramcharrun B. 1996. La reproduction du listao (Katsuwonus pelamis) dans le bassin ouest de l’ocean Indien. Aquat. Liv. Res. 9: 235-247. https://doi.org/10.1051/alr:1996027 , Hartaty et al. 2020Hartaty H., Setyadji B., Fahmi Z. 2020. Reproductive biology of Skipjack Tuna (Katsuwonus pelamis) in Indonesian Exclusive Economic Zone. IOTC-2020-WPTT22(AS)-8.). In the southwest Atlantic L₅₀ was estimated at 45.6 cm for both sexes, 43.2 cm for females and 46.2 cm for males (Benevenuti et al. 2019Benevenuti J., Monteiro-Neto C., Rodrigues M., et al. 2019. Size structure, reproduction, and growth of skipjack tuna (Katsuwonus pelamis) caught by the pole-and-line fleet in the southwest Atlantic. Fish. Res. 212: 136-145. https://doi.org/10.1016/j.fishres.2018.12.011 ). Grande et al. (2014)Grande M., Murua H., Zudaine I., et al. 2014. Reproductive timing and reproductive capacity of the Skipjack Tuna (Katsuwonus pelamis) in the western Indian Ocean. Fish. Res. 156: 14-22. https://doi.org/10.1016/j.fishres.2014.04.011 used cortical alveolar oocytes as an indicator of maturation in the western Indian Ocean and the L₅₀ of the female population that reached maturity was 39.9 cm. When these authors used advanced vitellogenic oocytes as an indicator, the L₅₀ was similar to that in other areas and estimated as 43.5 cm. In our study, the lack of small-sized and immature specimens did not allow us to calculate the L₅₀ in the western Mediterranean. We will have to resolve this lack of information in further studies.

The monthly variation of the mean GSI values for males and females and the results obtained from the GAM indicated that the western Mediterranean is a spawning area for SKJ. GAM results indicated that larger specimens reached their maximum GSI earlier than smaller specimens, suggesting that larger specimens start their spawning period earlier than smaller ones. Our results agree with those of other studies conducted on other tuna species in the western Mediterranean Sea (e.g. Macías et al. 2005Macías D., Gómez-Vives M.J., García S., Ortiz de Urbina J.M. 2005. Reproductive characteristics of Atlantic Bonito (Sarda sarda) from the south-western Spanish Mediterranean. Col. Vol Sci. Pap. ICCAT 58: 470-483., Saber et al. 2018Saber S., Ortiz de Urbina J., Lino P.G., et al. 2018. Biological aspects of little tunny Euthynnus alletteratus from Spanish and Portuguese waters. Collect. Vol. Sci. Pap. ICCAT. 75: 95-110.). These results were confirmed by histological analysis. The majority of the females sampled in Area I in June and July were spawning females. SKJ spawned in the Mediterranean in the summer months when the environmental conditions were suitable. Several tuna larval surveys carried out by Alemany et al. (2010)Alemany F., Quintanilla L., Velez-Belchi P., et al. 2010. Characterization of the spawning habitat of Atlantic bluefin tuna and related species in the Balearic Sea (Western Mediterranean). Progr. Oceanogr. 86: 21-38. https://doi.org/10.1016/j.pocean.2010.04.014 and Reglero et al. (2012)Reglero P., Ciannelli L., Álvarez-Berasategui D., et al. 2012. Geographically and environmentally driven spawning distributions of tuna species in the western Mediterranean Sea. Mar. Ecol. Prog. Ser. 463: 273-284. https://doi.org/10.3354/meps09800 have demonstrated that the Balearic Sea is an important spawning ground for tuna species. Moreover, Alemany et al. (2010)Alemany F., Quintanilla L., Velez-Belchi P., et al. 2010. Characterization of the spawning habitat of Atlantic bluefin tuna and related species in the Balearic Sea (Western Mediterranean). Progr. Oceanogr. 86: 21-38. https://doi.org/10.1016/j.pocean.2010.04.014 and Reglero et al. (2014)Reglero P., Tittesor D.P., Álvarez-Berasategui D., et al. 2014. Worldwide distributions of tuna larvae: revising hypotheses on environmental requirements for spawning habitats. Mar. Ecol. Prog. Ser. 501: 207-224. https://doi.org/10.3354/meps10666 reported larvae of SKJ around the Balearic Islands, confirming this area as a spawning ground for the species in the western Mediterranean Sea.

Schaefer and Fuller (2019)Schaefer K.M., Fuller D.W. 2019. Spatiotemporal variability in the reproductive dynamics of skipjack tuna (Katsuwonus pelamis) in the eastern Pacific Ocean. Fish. Res. 209: 1-13. https://doi.org/10.1016/j.fishres.2018.09.002 found that SKJ spawning in the eastern Pacific Ocean occurred from 24°C to 30°C. The optimal temperatures for tuna spawning are only reached in the Balearic Sea precisely on the dates when spawning SKJ were found in this study. For more details on the oceanography of the western Mediterranean Sea, see Vargas-Yañez et al. (2010Vargas-Yañez M., García M.C., Moya F., et al. 2010. Cambio Climático en el Mediterráneo español. Segunda edición. Instituto Español de Oceanografía. 176 pp., 2019Vargas-Yañez M., García M.C., Moya F., et al. 2019. Estado de los ecosistemas marinos en el Mediterráneo español en un contexto de cambio climático. Instituto Español de Oceanografía. 284 pp.). Our results confirm the preliminary study carried out by Saber et al. (2012)Saber S., Gómez-Vives M.J., García-Barcelona S., et al. 2012. Recreational catch rates and biology of Skipjack tuna from the western Mediterranean Sea. In: Morris E.P., Mañanes R., Fernández M.C., Gómez J. (eds), Libro de resúmenes del III Simposio Internacional de Ciencias del Mar (ISMS12), Cádiz, ISBN: 978-84-695-1394-1, p. 112. showing spawning-stage fish in the Balearic Sea.

SKJ from the Balearic Sea were capable of spawning every day (spawning frequency=1.03 d) during the spawning season. This result is slightly less than that found by Schaefer and Fuller (2019)Schaefer K.M., Fuller D.W. 2019. Spatiotemporal variability in the reproductive dynamics of skipjack tuna (Katsuwonus pelamis) in the eastern Pacific Ocean. Fish. Res. 209: 1-13. https://doi.org/10.1016/j.fishres.2018.09.002 in the eastern Pacific Ocean (spawning frequency=1.18 d) and by Ashida (2020)Ashida H. 2020. Spatial and temporal differences in the reproductive traits of skipjack tuna Katsuwonus pelamis between the subtropical and temperate western Pacific Ocean. Fish. Res. 221: 105352. https://doi.org/10.1016/j.fishres.2019.105352 in the western Pacific Ocean (spawning frequency ranged from 4.38 d in the Kuroshio-Oyashio transition area to 2.16 d in the Nasei Islands area).

Given the subtropical and tropical distribution of the species and its high plasticity, the increasing presence of SKJ in the western Mediterranean and the new spawning area detected in the Balearic Sea could be related to the trend towards an increase in SST in the region. Vargas-Yañez et al. (2010Vargas-Yañez M., García M.C., Moya F., et al. 2010. Cambio Climático en el Mediterráneo español. Segunda edición. Instituto Español de Oceanografía. 176 pp., 2019Vargas-Yañez M., García M.C., Moya F., et al. 2019. Estado de los ecosistemas marinos en el Mediterráneo español en un contexto de cambio climático. Instituto Español de Oceanografía. 284 pp.) analysed a long-term time series of temperature generated by the monitoring programmes supported by the Spanish Oceanography Institute in Spanish Mediterranean Waters, complemented with meteorological information from the Spanish Meteorological Agency (AEMET), satellite data from the NOAA and data from meteorological stations to assess the possible impact of climate change. All the areas analysed by these authors, including the Alboran Sea and the Balearic Sea, show a trend to increase in temperature. Analysing the intermediate and deep waters, the same authors detected an increase since the mid-20th century of between 0.13 and 0.3°C/100 years. The increasing temperature trend was confirmed in surface waters, and at a higher rate since 1970. According to Ashida (2020)Ashida H. 2020. Spatial and temporal differences in the reproductive traits of skipjack tuna Katsuwonus pelamis between the subtropical and temperate western Pacific Ocean. Fish. Res. 221: 105352. https://doi.org/10.1016/j.fishres.2019.105352 , the reproductive investment of spawning fish in high-latitude areas of the western Indian Ocean may be lower than in tropical and subtropical zones because of shorter durations of favourable oceanographic conditions. These results agree with our results in the western Mediterranean, where the reproductive season only occurs in the summer months.

The specimens caught in the Balearic Sea were reproductively active, while those captured in the Alboran Sea were reproductively inactive and would be migrating towards the Atlantic at the end of the summer once the spawning season was over and SST in the western Mediterranean was decreasing. The exceptional SST conditions in the Alboran Sea in August 2018 could explain the unusual presence of spawning females in Area II. Using NOAA data, we saw that a few days before the championship there was an SST peak maintaining higher temperatures than expected for these dates. However, it must be taken into consideration that the SST in 2018 were above the historical monthly means, a phenomenon that may be related to global warming (Vargas-Yañez M., pers. com. 13/03/2022). This information could be related to changes in the oceanographic conditions (mainly temperature) or changes in the migratory behaviour of the species. Varela et al. (2019)Varela J.L., Cañavate J. P., Medina A. Mourente G. 2019. Inter-regional variation in feeding patterns of skipjack tuna (Katsuwonus pelamis) inferred from stomach content, stable isotope and fatty acid analyses. Mar. Env. Res. 152: 104821. https://doi.org/10.1016/j.marenvres.2019.104821 found that SKJ feeds actively between the two areas in our study. This result supports the “income breeding strategy” described for the species. Spawning individuals can adjust their reproductive investment in response to environmental factors, indicating that higher investments in reproduction can be made when food resources are available (McBride et al. 2015McBride R.S., Somarakis S., Fitzhugh G.R., et al. 2015. Energy acquisition and allocation to egg production in relation to fish reproductive strategies. Fish Fish. 16: 23-57. https://doi.org/10.1111/faf.12043 ) and when they have optimal environmental conditions.

As shown above, SST is of high importance for the highly migratory species, as it impacts spawning activity (Medina et al. 2002Medina A., Abascal F.J., Megina C., García A. 2002. Stereological assessment of the reproductive status of female Atlantic northern bluefin tuna during migration to Mediterranean spawning grounds through the Strait of Gibraltar. J. Fish Biol. 60: 217-230. https://doi.org/10.1111/j.1095-8649.2002.tb02398.x ), and climate change will directly affect the physiology and spatial distribution of marine organisms, and is expected to affect SKJ. Dueri et al. (2014)Dueri S., Bopp L., Maury O. 2014. Projecting the impacts of climate change on skipjack tuna abundance and spatial distribution. Global Change Biol. 20: 742-753. https://doi.org/10.1111/gcb.12460 projecting the impacts of climate change on SKJ abundance and spatial distribution, found that under present climatic conditions, the biomass peaks at the Equator. In addition, the model projected an increase of biomass at the latitudes 10°N and 10°S for 2050 and a displacement of the biomass peak from the Equator to 10°N. The reproduction rate showed a temporal trend similar to biomass abundance. Our study confirms that the Balearic Sea is a new spawning ground for this species in the western Mediterranean Sea and agrees with the expanding habitats projected for the species in the future.

The presence of a new reproductive species in the study area could be affecting the larval ecology and recruitment of local tuna species. This issue must be studied in the near future to assess its implications for tuna management and conservation in the region.