Study on the Eastern Atlantic and Mediterranean Bluefin Tuna Stock Using the Spanish Traps As Scientific Observatories
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SCRS/2011/029 Collect. Vol. Sci. Pap. ICCAT, 67(1): 331-343 (2012) STUDY ON THE EASTERN ATLANTIC AND MEDITERRANEAN BLUEFIN TUNA STOCK USING THE SPANISH TRAPS AS SCIENTIFIC OBSERVATORIES J.M. de la Serna1, D. Macías, J.M. Ortiz de Urbina, E. Rodriguez-Marín, Francisco Abascal SUMMARY The aim of this paper is to describe the preliminary results of the study on bluefin tuna, using the traps as a scientific observatory that the “Secretaría General del Mar” (SGM) entrusted to the IEO in collaboration with the “Organización de Productores Pesqueros de Almadrabas” (OPP/51). RÉSUMÉ Le présent document a pour objet de décrire les résultats préliminaires de l’étude portant sur le thon rouge, en ayant recours aux madragues en tant qu’observatoire scientifique, que le Secretaría General del Mar (SGM) a confiée à l’IEO en collaboration avec l'Organización de Productores Pesqueros de Almadrabas (OPP/51). RESUMEN El objetivo de este documento es describir los resultados preliminares del estudio sobre atún rojo, utilizando las almadrabas como observatorio científico, que la Secretaría General del Mar encargó al IEO en colaboración con la “Organización de Productores Pesqueros de Almadrabas” (OPP/51). KEY WORDS Bluefin tuna, abundance index, trap fishery 1. Introduction Bluefin tuna have been historically exploited in a sustainable way in the eastern Atlantic and Mediterranean Sea. Trap is the most traditional fishing gear targeting bluefin tuna, with catches declining since the second half of 20th century. The assessment of eastern bluefin tuna stock indicates high levels of overexploitation. In order to improve this situation, ICCAT recommended a 15-year recovery plan starting in 2007 and continuing through 2022. Traps are the source for one of the relative abundance indexes used during stock assessment and could supply information on the effect of the ICCAT recovery plan on the reproductive fraction of the population. The “Secretaria General del Mar” (SGM) entrusted a study on bluefin tuna, using the traps as a scientific observatory, to the IEO in collaboration with the “Organización de Productores Pesqueros de Almadrabas” (OPP/51). The main objective of this study was obtaining the relative abundance index of breeding stock in the same view that in the years previous to the implementation of the bluefin tuna recovery plan, and monitoring the catches in order to detect any trends in the abundance of bluefin tuna. Other particular objectives include studies on stock structure and migratory behaviour, age and growth, reproduction, feeding, biometric relationships, associated species and by-catch, historical data collecting (data mining), comparative studies with bluefin tuna caught from other areas- fleets and trap technical description. 1 Instituto Español de Oceanografía. C.O. Málaga. Pto Pesquero s/n, 29640, Fuengirola, España. E-mail: [email protected] 331 2. Material and methods Catch and effort data for traps fisheries were collected by the Spanish Oceanographic Institute (IEO) on-board observers. In addition, data on number and size of fish released alive were recorded in order to estimate the bluefin tuna and by-catch species abundance indexes. For each fishing operation, data were recorded on location, time, environmental data (sea surface temperature, distance to the coast, depth and weather conditions, moon phase, etc.), species composition; and corresponding biological information (size/weight). Data on marine mammals and sea birds sighting were also recorded. Biological sampling consisted of: gonads, muscle, heart, spines, otoliths, gills and stomachs. Conservation and processing of each type of biological samples were made following conventional protocols. General lineal models (GLM) were used to estimate the relative abundance index, assuming a negative binomial error distribution. Historical information was recovered from trap companies’ archives. 3. Results and discussion During 2010, a total of 49 fishing operations with 5218 bluefin tuna were observed. In addition, 9550 individuals were released. 3.1 Stock structure − Tagging Fifteen (15) pop-up were deployed. In addition, 100 bluefin tuna were tagged with ICCAT conventional tags. To date, data on pop-up have not been finished, and recapture of conventional tags have not been yet reported. − Parasites as biological tags Preliminary results in terms of species composition and their locations are given in Table 1. Figure 1 shows the various species of parasites found. For the first time, trematodes of the subfamily didimozoidos Nematobothriinae were found in the eye socket of bluefin tuna. This species is probably unknown to date due to host specificity and habitat. Our early results indicate that there is less diversity and prevalence of parasitological fauna in adult bluefin tuna caught by traps, in comparison to juvenile tuna caught in the Bay of Biscay, in spite of size differences found. Stomach contents showed two types of parasites: (i) trematodes of the genus Hirudinella (21% of the stomachs analyzed); and (ii) nematodes anisakids (8% of the stomachs analyzed). 3.2 Age and growth Our results show that most of the catches of the Spanish Atlantic traps consist of ages 7 to 10 years (Figure 2). These results are consistent with previous annual demographic composition of the traps, which shows that ages 7 and up are well represented in the capture and that from the age 10 are fully recruited to the gear. Figure 3 shows that the Tarifa trap caught the older specimens, mostly ages 9 to 10 years. However, the trap of Conil caught younger specimens of 6 to 7 years. 3.3 Reproduction Of the 61 females examined, all were classified as inactive mature. Only pre-vitellogenic or early vitellogenic oocytes (with little yolk) plus alpha and/or beta atresic follicles (Figure 4) were observed in their ovaries. The size at first maturity estimated from the data was 108,004 cm. The L50 estimated from the bootstrap was slightly lower: 107,928 cm. The estimated L50 corresponds with tuna for 3-4 years and an estimated average 332 weight of 26 kg. Nevertheless and due to the low number of samples around the size at first maturity used in the study this value could be underestimating the size at first maturity (Figure 5). 3.4 Feeding − Stomach contents Ninety-one (91) stomachs of bluefin tunas caught in the traps of Barbate, Conil and Zahara were analysed. 100% of the stomachs were empty, although 10% of the stomachs had some species consumed inside the trap and 9% presented skeletal remains from the digestion of their prey (fish otoliths, cephalopod beaks and pincers of crustaceans). − Isotopic analysis Our preliminary results indicate that the relative abundances of Delta N (15N/14N) and Delta C (13C/12C) of the samples from the traps do not differ from samples from other sampling sites, as Larache (traps) and Balearic Islands (Purse seine). These results indicate that the prey species are very similar. Table 2 shows the main results in terms of prey composition from the isotopic analysis. 3.5 Biometric relationships Size composition is shown in Figure 6. Our data showed average values of LH of 223.37 cm for males and 204.16 cm for females. With regard to the total weight, the average values were, 199.88kg for males and for females 152.20kg. Biometric values were found higher for males than for females. We estimated length-weight relationships for round weigh-fork length, and round weigh-first dorsal fin length (LD1). Round Weight = 0.00029 x FL2.48515 Round Weight = 1.9655 x LD12.784 3.6 Comparative studies with other fleets In general, most of the gear targets the adult fraction of the stock, except for the baitboat fishery which that part of the year targets the juvenile fraction. The largest average catch size corresponded to the hand line fishery of the Strait of Gibraltar, followed by traps, surface longline and purse seine. 3.7 Standardized index of relative abundance Forty-nine (49) fishing operations with 5218 bluefin tuna were recorded. In addition, 9550 individuals were released. A total of 14 768 individuals were caught, which is a value higher than the average catch for the last decade. As for the relative abundance index used in stock assessment, it is observed a stable trend for the last years, with some variability between years (Figure 7). 3.8 Environmental factors Sea surface temperature was in the range 16ºC- 20ºC (Figure 8). Sea surface temperature was variable throughout the fishing period and across traps. Turbidity was in the range 2.5m- 28m, depending on the bottom of the trap (Figure 9). Wind: 51 % W, 37 % E y 22 % other and/or variable and/or calm (Figure 10). 3.9 Associated species The main associated species were small tuna and swordfish, as well as very low frequencies of other species. Table 3 shows the summary of the main associated species caught by the traps during 2010. 3.10 Data mining Bluefin tuna catch series, in number of fish and weight by fishing operation for the period 1975- 2010, were collected for the four operative traps. 333 4. Acknowledgements We are grateful to the skippers and crews of fishing vessels who worked in collaboration with the research team of the IEO, and to our colleagues of the Large Pelagic Fisheries Department in Malaga. 5. References Alves, D.R., Luque, J.L. 2006, Ecologia das comunidades de metazoários parasitos de cinco espécies de escombrídeos (Perciformes: Scombridae) do litoral do estado do Rio de Janeiro, Brasil. Rev Bras Parasitol 15:167-181. Bush, A.O., Lafferty, K.D., Lotz, J.M., Shostak, A.W. 1997, Parasitology meets ecology on its own terms: Margolis et al. revisited. J Parasitol 83:575-583. Bussiéras, J., Baudin-Laurencin, F. 1973, Les helminthes parasites des thons tropicaux. Communication aux VIIIe journées médicales de Dakar du 9-14 Avril 1973. Rev Elev Med Veto Pays Trop 26:13-19.