SCRS/2007/038 Collect. Vol. Sci. Pap. ICCAT, 62(4): (2008)

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1 SCRS/2007/038 Collect. Vol. Sci. Pap. ICCAT, 62(4): (2008) BLUEFIN TUNA (THUNNUS THYNNUS) CONVENTIONAL TAGGING CARRIED OUT BY THE SPANISH INSTITUTE OF OCEANOGRAPHY (IEO) IN 2005 AND RESULTS AND ANALYSIS INCLUDING PREVIOUS TAGGING ACTIVITIES E. Rodríguez-Marín 1, C. Rodríguez-Cabello 1, J.M. de La Serna 2, E. Alot 2, J.L. Cort 1, J.M. Ortiz de Urbina 2 and M. Quintans 3 SUMMARY This paper describes conventional tagging carried out by the Spanish Institute of Oceanography (IEO) in the Mediterranean and the Bay of Biscay in 2005 and Tagging activities were performed on board professional bait boat fishing vessels and throughout opportunistic tagging on board commercial vessels. A total of 1694 and 349 bluefin tuna were tagged in 2005 and 2006, respectively, representing a 15% of the total tagged bluefin since 1977 in both areas. In the Mediterranean the bluefin tuna tagged were individuals aged 1 year or less, meanwhile in the Bay of Biscay most individuals tagged were 1 or 2 years old. The recaptured fish in 2005 were 67 and 19 in 2006.Relative recapture rates based on tagged individuals and 702 recaptures indicate a noteworthy exchange of juveniles between the west Mediterranean and the East Atlantic. Large numbers of 0 and 1-year-old specimens, tagged in Balearic Island and Alboran Sea area, have been recaptured in the Bay of Biscay with more than 1 year at liberty. Thus, indicating that the relative abundance index of juveniles in the Bay of Biscay can be representative of the Mediterranean western bluefin tuna population. No relationship was found between fishing activity and recaptures in the Bay of Biscay, but a relationship was seen between number of fish tagged and recaptured within the same year. Mean displacements of fish tagged and recaptured in the Bay of Biscay during the same year, showed that probably fish remain inside this feeding area during the fishing season. This study also summarizes the application of conventional tagging data for obtaining useful information for the assessment of the bluefin tuna population; these data were used to estimate migration rates between the Mediterranean and the East Atlantic, growth parameters and mortality rates. RÉSUMÉ Le présent document décrit le marquage réalisé avec des marques conventionnelles par l Institut Espagnol d Océanographie (IEO) en Méditerranée et dans le Golfe de Gascogne en 2005 et Les activités de marquage ont été effectuées à bord de canneurs professionnels ainsi que par marquage opportuniste à bord de navires commerciaux. Au total, et 349 thons rouges ont été marqués en 2005 et 2006, respectivement, soit 15% de l ensemble des thons rouges marqués depuis 1977 dans ces deux zones. En Méditerranée, les thons rouges marqués étaient des spécimens âgés de 1 an ou moins, alors que dans le Golfe de Gascogne la plupart des spécimens marqués avait 1 ou 2 ans. En 2005 et 2006, 67 et 19 poissons, respectivement, ont été recapturés. Les taux relatifs de recapture, basés sur spécimens marqués et 702 recaptures, indiquent un fort échange des juvéniles entre la Méditerranée occidentale et l Atlantique Est. De nombreux spécimens âgés de 0 et 1 an marqués aux Iles Baléares et en Mer d Alboran ont été recapturés dans le Golfe de Gascogne après plus d un an en liberté, ce qui indique que l indice d abondance relative des juvéniles dans le Golfe de Gascogne peut être représentatif de la population de thon rouge de la Méditerranée occidentale. Aucun rapport n a été établi entre les activités de pêche et les recaptures dans le Golfe de Gascogne, mais un rapport a été établi entre le nombre de poissons marqués et recapturés au cours de la même année. Les déplacements moyens des poissons marqués et recapturés dans le Golfe de Gascogne dans la même année indiquent qu il est probable que les poissons restent à l intérieur de cette zone trophique pendant toute la saison de pêche. Cette étude résume également l application des données issues du marquage conventionnel en vue d obtenir des informations utiles pour l évaluation de la population de thon rouge ; ces données ont été utilisées pour estimer les taux de migration entre la Méditerranée et l Atlantique Est, les paramètres de croissance et les taux de mortalité. 1 Instituto Español de Oceanografía. Centro oceanográfico de Santander. Apdo Santander. Spain. rodriguez.marin@st.ieo.es 2 Instituto Español de Oceanografía. Centro Oceanográfico de Fuengirola. Apdo Malaga, Spain. 3 Instituto Español de Oceanografía. Centro Oceanográfico de A Coruña. Apdo A Coruña. Spain. 1182

2 RESUMEN Este trabajo describe el marcado convencional llevado a cabo por el Instituto Español de Oceanografía (IEO) en el Mediterráneo y en el Golfo de Vizcaya en 2005 y El marcado se realizó de forma dirigida en barcos profesionales de cebo vivo y de forma oportunista en barcos comerciales. Se marcaron un total de 1694 y 349 atunes rojos en 2005 y 2006, respectivamente, representando el 15% del total de individuos marcados desde 1977 en ambas áreas. En el Mediterráneo los atunes rojos marcados tenían 1 año o menos de edad, mientras que en el Golfo de Vizcaya la mayoría de los individuos marcados tenían 1 o 2 años. El número de ejemplares recapturados fue de 67 en 2005 y 19 en Las tasas relativas de recaptura, basadas en ejemplares marcados y 702 recapturas, indican un notable intercambio de juveniles entre la parte occidental del Mar Mediterráneo y el Atlántico este. Un gran número de ejemplares de 0 y 1 año de edad, marcados en las zonas de Islas Baleares y Mar de Alboran, han sido recuperados en el Golfo de Vizcaya después de un año de libertad, lo que indica que el índice de abundancia relativa del Golfo de Vizcaya puede ser representativo de la población del Mediterráneo oriental de atún rojo. No se ha encontrado relación entre la actividad pesquera y las recapturas de ejemplares marcados en el Golfo de Vizcaya, pero esta relación si existe entre el número de peces marcados y recapturados el mismo año. Los desplazamientos medios de atunes marcados y recapturados en el Golfo de Vizcaya durante el mismo año, muestran que probablemente los atunes permanecen en esta área de alimentación durante toda la temporada de pesca. Este estudio también sintetiza la aplicación del marcado convencional para obtener información útil para la evaluación de la población de atún rojo, estos datos se han utilizado para obtener tasas de migración entre el Mediterráneo y el Atlántico Este, parámetros de crecimiento y tasas de mortalidad. KEYWORDS Thunnus thynnus, conventional tagging, migrations, bluefin tuna juveniles 1. Introduction The Instituto Español de Oceanografía (IEO) has been systematically carrying out conventional tagging of buefin tuna in the Bay of Biscay and the Mediterranean since The series of tagging surveys in the Bay of Biscay was interrupted in 1991, to be later resumed thanks to funding from the European Tuna Tagging Program (European Data Collection Program). The tagging activity in the Mediterranean began in 1983, but was not continuous until 1990, and has since then been maintained thanks to the collaboration of recreational vessels and opportunistic tagging on board commercial vessels. In recent years opportunistic tagging has also been performed on board recreational vessels in the Bay of Biscay by AZTI Tecnalia. This study presents the results of the surveys funded by the European Data Collection Program in 2005 and 2006 and these data are analysed together with information from previous tagging activities. 2. Material and methods 2.1 Bay of Biscay Two conventional bluefin tuna tagging surveys were performed in the Bay of Biscay in July of 2005 and The surveys were conducted on board professional tuna-fishing vessels dedicated to bluefin tuna baitboat fishing. External spaghetti tags were used, also known as conventional or streamer tags. Three types of streamer tags were placed, which differed in the system of attachment to the fish (Figure 1). The tags were placed in the dorsal flank at the level of the first dorsal fin. Specimens damaged during the tagging procedure were put down and used for biological sampling. The straight fork length (FL) of specimens was measured to the lower centimetre. Small specimens, those under 10 kg, were tagged using tagging cradles (Figure 2); medium ones, between 10 to 15 kg were tagged on board using foam mattress over the deck (Figure 2) and those over 15 kg were tagged in the sea using tagging poles (Figure 3). In some cases the length of fish of over 75 cm was estimated since it was not possible to carry out the measurement. The date and location of the catch and the state of the sea, the wind and surface water temperature were recorded. 1183

3 Tagging activities dissemination has continued to be provided to ports, fishermen associations and first sale fish markets through distribution of posters (Figure 4). Money, t-shirts and caps have been used as reward. An agreement was established with the main bluefin tuna landing port in the Bay of Biscay so that the fishermen association gave directly the reward to the fishermen when presenting the recapture form. This method motivates fishermen since they obtain a quick reply and is easy to follow. 2.2 Mediterranean Sea The tagging survey with conventional tags in the Alboran Sea took place in autumn 2006 on board a commercial vessel based in the port of Algeciras, which used baitboat gear in the Strait of Gibraltar fishing ground. In addition to the rods used for catching bluefin tuna, the vessel fished using trolling. All of the specimens tagged on the survey were caught using this gear. Also tagging activities were performed in the Balearic area on board four commercial vessels which fished using surface long line. These surveys took place between July and December 2006, although the greatest intensity of tagging was in November and December. To estimate the age of the fish tagged the Von Bertalanffy growth function (VBGF) obtained by Cort (1991) was used. 3. Results and discussion 3.1 Bay of Biscay tagging surveys A total of 1736 bluefin tuna were tagged in 44 tagging operations, the locations of which are shown in Figure 5 and Figure 6 for the 2005 and 2006 surveys respectively. The tagging locations in 2006 are found in areas of greater depth and more distant from the Cap Breton Canyon than in Bluefin tuna tagged during the 2005 survey ranged between 54 cm and 105 cm FL corresponding to well defined age groups of one and two years. Length and age distribution of bluefin tuna tagged in 2005 appears in Figure the specimens of age 1 were estimated, 423 of age 2 and just one of age 3. A total of 42 tunas were tagged in 2006, most of which were over 4 years old and were an average of 155 cm straight fork length and 70 kg in weight. The length distribution of the bluefin tuna tagged is shown in Figure 8. Four corresponded to juveniles of 2 and 3 years, while most of the fish were between 5 and 7 years old. The estimated length of the largest specimen tagged was 218 cm, which correspond to a fish of 11 years. 3.2 Mediterranean Sea tagging surveys A total of 25 bluefin tuna and 3 frigate tuna (Auxis thazard) were tagged in the Alboran Sea area. Their degree of vitality after tagging was very good for all the specimens. The locations of tagging operations were spread throughout the Spanish Levantine coast from Cape Gata to the area of Denia-Villajoyosa (Figure 9). All the bluefin tuna tagged corresponded to age 0 specimens. The length distribution of the bluefin tuna tagged is shown in Figure 9. Length ranged from 37 to 49 cm FL. The median and the mode coincided at 41 cm FL. The location of the specimens tagged in the Balearic area on board commercial long liners is shown in Figure 10. A total of 283 bluefin tuna were tagged. The length distribution of the whole catch is shown in Figure 10. Lengths range between 37 and 125 cm FL, with a mean of 65 cm, a median of 68 cm and a mode of 70 cm FL. 3.3 Recaptures During the 2005 fishing season a total of 67 specimens of bluefin tuna have been recovered in the Bay of Biscay, 51 were tagged in the same year. Most of the recaptures occurred close to the tagging area with the exception of one specimen, which covered 193 miles in one month (Figure 11). The time at liberty varied between 8 and 107 days. Forty-four of the recaptures belonged to 1-year-old fish and seven corresponded to 2-year-old fish. Nearly 50% of the recaptures were at liberty for less than 3 months and only four were recovered within the first month. One year-old specimen tagged in November 2004 in the Strait of Gibraltar was recovered in 2005 in the Bay of Biscay (86 cm). 1184

4 During the 2006 fishing season, a total of 19 recaptures of bluefin tuna were reported in the Bay of Biscay. One had been tagged in the Strait of Gibraltar in November 2004 and had been recovered 19 months later in the Bay of Biscay. It measured 66 cm at tagging and had increased by 27 cm. Eleven of the recaptures were specimens tagged in the previous tagging season, thus most of them had been at liberty for nearly one year, whereas only three of the recaptures corresponded to the same tagging survey. A trawler caught one specimen the following day, the distance covered by this fish in one day was 48 miles in a NW direction, more or less the same as the other two fish recovered (Figure 12). The last four year recaptures of bluefin tuna tagged in the Mediterranean are shown in Figure 13. Both displacements out of the Mediterranean to the Bay of Biscay and towards the Gulf of Lyon in the Mediterranean can be observed. 4. Analysis including previous tagging activities 4.1 Relative recapture rates Nearly 15,000 individuals have been tagged with spaghetti tags around the Iberian Peninsula since the middle 1970s (Figure 14). Almost exclusively scientist from the IEO conducted this tagging from late 1970s to the present in the Bay of Biscay and in the Alboran Sea and Balearic Islands areas. Some tagging surveys were also conducted in Moroccan Atlantic waters from 1972 to 1978, a total of 195 bluefin tuna were tagged (Lamboeuf, 1975; Anon. 1978; Brethes and Mason 1979; Brethes, 1979). For this study tagged fish and 702 recaptures reported to date have been considered (Figure 14). Tagging activity mainly occurred at the beginning of summer in Moroccan Atlantic waters, in mid-summer in the Bay of Biscay, at the end of summer and beginning of autumn in the Balearic Islands area and in the Alborán Sea tagging took place well into autumn. Table 1 shows the number of tagged bluefin tuna by age, geographic area and tagging strategy. The tagging effort in the Bay of Biscay and in the Mediterranean is nearly the same and the tagging activities funded by the Data Collection Program in 2005 and represent 15 % of the total number of specimens tagged. In the Mediterranean, practically all the bluefin tuna tagged were individuals aged 1 year or less. In the Bay of Biscay most individuals tagged were 1 or 2 years of age. Relative recapture rate (recaptured/tagged*100) by tagging area and time at liberty were analysed for less and more than one year (Table 2). The majority of the recaptures with less than one year at liberty were caught in the same area where individuals were tagged. Some bluefin tuna tagged in de Bay of Biscay move to the West Atlantic coast within the first year after release (0.7%) and some of them visit the South Atlantic Iberian Peninsula Area. In addition, young of the year and few one-year-old specimens tagged in the Mediterranean moved across the Strait of Gibraltar to adjacent waters in the Atlantic and to the Bay of Biscay. These Mediterranean specimens were mainly tagged at the end of the year, thus confirming the movements leaving the Mediterranean and wintering along the Atlantic coast of Morocco until, on attaining age 1, the next summer, migrating into the Bay of Biscay (Aloncle, 1964; Cort, 1990; Mather et al., 1995). Similar Trans-Atlantic relative recapture rates from west to east were found by Rooker et al. (2007) for bluefin tuna juveniles at large less than one year. Recaptures that were at liberty more than 1 year show a different relative recapture rates picture. In the case of fish tagged in Alboran Sea a high percentage of recaptures were recovered in the Bay of Biscay (81%) which supports the movement between both areas and also it is significant the rate of recaptures in the Mediterranean sea (not so much previously reported), thus supporting the theory that some fish remain inside the tagging area. When looking at the Balearic waters tagging area, large number of the recaptures (40%) were also found in the Bay of Biscay (not so much previously reported), some moved outside the Strait of Gibraltar (40%) and few remained in the tagging area. These results should be analyzed with caution since recovery rates in the Mediterranean are lower than in the Bay of Biscay (Rodríguez-Marín et al., 2005). For tags deployed in the Bay of Biscay, most of the fish were recaptured after one year in the same area, supporting the theory that they repeat the migration route each year to this feeding area since bluefin tuna only appears in the Bay of Biscay from June to November. Also some proportion of fish moved to the south Atlantic and Mediterranean Sea corresponding to adult fish. Significant relative recapture rates from the Bay of Biscay towards the West Atlantic were also found (10%). All this information coming from release and recapture data together with the knowledge of fisheries and spawning and feeding areas allows us to infer bluefin tuna migrations and the close relationship between the juveniles from the west Mediterranean and the north east Atlantic. A significant fraction of bluefin tuna born in 1185

5 June in the western Mediterranean (possibly in the spawning area around Balearic Islands) move across the Strait of Gibraltar in autumn towards the area bounded by the Atlantic coasts of Morocco, Canary Islands and Azores Islands, to winter. In the following spring, with almost one year of life, bluefin migrate to the Bay of Biscay, before returning to Moroccan waters in autumn. Juveniles make these trophic migrations between these wintering and summer feeding areas up to age 4-5, and then as they reach sexual maturity, enter the Mediterranean Sea going to the spawning areas where the were born. This contribution of bluefin tuna juveniles to the eastern Atlantic from the nursery areas in the Mediterranean and the exchange between the Atlantic coast of Morocco and the Bay of Biscay has been described in the bibliography (Sella, 1929; Furnestin and Dardignac, 1962; Aloncle, 1964; 1973; Rodríguez-Roda, 1964; Cort and Rey, 1985; Cort, 1990; Cort and De la Serna, 1994; Cort and Liorzou, 1995; Mather et al., 1995; de la Serna et al., 2001; Rodríguez-Marín et al., 2005), but it has not been quantified as in the present paper because the volume of tagging data analyzed has increased and inferences are based in other time and area stratifications. The significant contribution of small bluefin tuna from the western Mediterranean to the Bay of Biscay indicate that the relative abundance index of juveniles in the Bay of Biscay (Rodríguez-Marín et al., 2003) can be representative of the Mediterranean western bluefin tuna population. Movements from the West Atlantic to the Bay of Biscay, of juveniles at large more than one year, show lower relative recapture rates than our estimations in the opposite direction, and even lower for the entire transatlantic migration from the west to the east for this small bluefin (Rooker et al., 2007). 4.2 Fishing activity and recaptures To check the relationship between fishing activity and recaptures in the Bay of Biscay, fishing effort and number of recaptured fish that were tagged in the same fishing season (recaptures with less than half a year at liberty) were plotted (Figure 15). No relationship was found, but a relationship was seen between numbers of fish tagged and recaptured within the same year (Figure 16). 4.3 Distance traveled in the feeding area In order to the measure the bluefin tuna tagged displacements in the fishing area during the fishing season, mean distance was calculated for fish tagged and recaptured in the Bay of Biscay during the same tagging season (Table 3). The mean distance did not exceed 45 nautical miles showing that probably fish remain inside the feeding area during the fishing season. Estimated daily movements do not usually exceed more than 3 nautical miles a day, although they are able to move more than 60 nm a day. 5. Utility of conventional tagging data for stock assessment Conventional tagging experiments can give us a lot of information, although they can only tell us about the location of the fish at the time of release and recapture, conventional tagging can produce valuable information on fish movements, migratory patterns, and statistics such as migration, mortality, and growth rates. The historical series of tagging surveys carried out by the Spanish Institute of Oceanography have allowed the estimation of some of these parameters. Conventional tagging data were used to obtain migration rates between the Mediterranean and the East Atlantic and to assess the optimum number of fish that would need to be tagged to estimate migration rates with precision (Arregi et a., 2006). Data set was fit under different model structures (fishing mortality, reporting rates, and migration in the same year of tagging). Results show that transatlantic migration rate estimates were similar to previous studies: 0.4%-4.9% from East to West and 2.9%-11.8% from West to East. Migration rates between the East Atlantic and the Mediterranean ranged considerably depending on model definition and reporting rate assumptions. Under most of the proposed scenarios, practically no gain in precision and accuracy occurred when tagging more than 10,000-15,000 fish. Estimation of growth parameters from tagging data was also achieved (Rodríguez-Cabello et al., 2006). The data set, which included recaptures after more than six months at liberty (n=179), was selected after exploring the residuals in order to eliminate the possible effect tagging might have on short-term growth and to minimize the possible effects of seasonal fluctuations in growth on parameter estimation. The estimates of the growth parameters in this study are rather imprecise, perhaps as a result of various sources of uncertainty such as small sample size, inaccurate size measurements at the time of tagging and/or recapture, preponderance of short times at 1186

6 liberty, the potential effects of tagging on growth, etc. Nevertheless, the tagging data used appear to be consistent with the currently adopted growth curve for the stock. Estimation of mortality from tagging data was accomplished (Rodríguez-Marín et al., 2005). Four different models were compared, considering survival (S) and recovery rate (f) constant or time-specific for the Bay of Biscay and the Mediterranean Sea. The results indicate that there is evidence of variability in survival and recapture rates among years in both areas. The recovery rates are low in both areas but particularly those of the Mediterranean Sea. The estimated survival rate obtained by the more simple model with S constant S(.) gave S= and S= for the Bay of Biscay and Mediterranean respectively. Thus, the finite mortality rate would be Z=0.51 and Z= 0.70 respectively. Though many factors could help to explain this difference, one possible explanation is a distinct survival rate between age groups. Whereas in the first area most of the fish tagged correspond to groups of age 1 and 2, in the Mediterranean Sea all the fish tagged correspond to 0 and 1 year old. Acknowledgements We would like to thank all the people who participated and were involved in these tagging surveys, particularly to Santiago Barreiro, Angel Carpena, Enrique de Cárdenas, Roberto Gancedo, Jose Luis Gutierrez-Zabala, Ignacio Olaso, Victoria Ortiz de Zárate, Juan Carlos Rey, Pilar Rioja, Marta Ruiz, Xulio Valeiras and all the skippers and crew who helped to achieve the objectives. Conventional tagging surveys in 2005 and 2006 were supported by the European Tuna Tagging Program (under the European Data Collection Program from the Directorate-General for Fisheries and Maritime Affairs of the European Commission) and the Spanish Institute of Oceanography. References ALONCLE, H Note sur le thon rouge de la Baie Ibéro-Marocaine. Bulletin Institut des Pêches Maritimes du Maroc, 12: ALONCLE, H Marquage de thons rouges dans le Golfe de Gascogne. Collect. Vol. Sci. Pap. ICCAT, 1: ANON Tagging cruise of juvenile bluefin tuna along the Morocco coasts. Collect. Vol. Sci. Pap. ICCAT, 7(2): ARREGUI, I., H. Arrizabalaga, J.M. De la Serna Preliminary approach to the experimental design of tagging campaigns for movement rates estimation of East Atlantic bluefin tuna. Collect. Vol. Sci. Pap. ICCAT, 59(3): BRETHES, J.C On the first catches of tagged bluefin tuna in July 1977 in Moroccan waters. Collect. Vol. Sci. Pap. ICCAT, 8(2): BRETHES, J.C. and J.M. Mason, Jr Bluefin tuna tagging off the Atlantic coast of Morocco in Collect. Vol. Sci. Pap. ICCAT, 8(2): CORT, J.L Biología y pesca del atún rojo, Thunnus thynnus (L.), del mar Cantábrico (Tesis doctoral). Publicaciones Especiales Instituto Español de Oceanografía, 4, 272 pp. CORT, J.L Age and growth of the bluefin tuna, Thunnus thynnus (L.) of the northeast Atlantic. Collect. Vol. Sci. Pap. ICCAT, 35(2): CORT, J.L. and J.M. de la Serna Revision of tag/recapture of bluefin tuna (Thunnus thynnus, L.) in the East Atlantic and the Mediterranean. Collect. Vol. Sci. Pap. ICCAT, 42(1): CORT, J.L. and B. Liorzou Revision on tagging/recovery of bluefin tuna in the eastern Atlantic and Mediterranean Sea. Collect. Vol. Sci. Pap. ICCAT, 44(1):

7 CORT, J.L. and J.C. Rey Analysis of tagging data of bluefin (Thunnus thynnus, L.) in the East Atlantic and Mediterranean. Migration, growth and mortality. Collect. Vol. Sci. Pap. ICCAT, 22: DE LA SERNA, J.M., V. Ortiz de Zárate and M.J. Gómez Actualización de los datos de marcadorecaptura de atún rojo (Thunnus thynnus L.) en el Atlático Este y Mediterráneo. Collect. Vol. Sci. Pap. ICCAT, 52(2): FURNESTIN, J. and J. Dardignac Le thon rouge de Maroc Atlantic (Thunnus thynnus). Rev. Trav. Inst. Pêches Marit., 26(4): LAMBOEUF, M Contribution a la connaissance des migrations des jeunes thons rouges a partir du Maroc. Collect. Vol. Sci. Pap. ICCAT, 4: MATHER, F.J., J.M. Mason A.C. and Jones Historical document: life history and fisheries of Atlantic bluefin tuna. NOAA Technical Memorandum, NMFS SEFSC pp. ROOKER, J.R., J.R. Alvarado, B.A. Block, H. Dewar, G. de Metrio, A. Corriero, R.T. Kraus, E.D. Prince, E. Rodríguez-Marín and D.H. Secor Life History and Stock Structure of Atlantic Bluefin Tuna (Thunnus thynnus). Reviews in Fisheries Science, 15(4): RODRÍGUEZ-CABELLO, C., V. R. Restrepo, E. Rodríguez-Marín, J.L.Cort and J. M. de la Serna Estimation of northeast Atlantic bluefin tuna (Thunnus thynnus) growth parameters from tagging data. Collect. Vol. Sci. Pap. ICCAT, 60(4): RODRÍGUEZ-MARÍN, E., H. Arrizabalaga, M. Ortiz, C. Rodríguez-Cabello, G. Moreno and L.T. Kell Standardization of bluefin tuna, Thunnus thynnus, catch per unit effort in the baitboat fishery of the Bay of Biscay (Eastern Atlantic). ICES Journal of Marine Science, 60(6): RODRÍGUEZ-MARÍN, E., C. Rodríguez-Cabello, J.M. De la Serna, J.L. Cort, E. Alot, J.C. Rey, V. Ortiz de Zarate, J.L. Gutierrez and E. Abad A Review of bluefin tuna juveniles tagging information and mortality estimation in waters around the Iberian Peninsula. Collect. Vol. Sci. Pap. ICCAT, 58(4): RODRÍGUEZ-RODA, J Movimientos migratorios del atún, Thunnus thynnus (L.), deducidos por nuestras propias marcaciones en aguas españolas. Publicaciones Técnicas de la Junta de Estudios de Pesca, Dirección General de Pesca Marítima, Madrid 3: SELLA, M Migrazioni e habitat del tonno (Thunnus thynnus, L.) studiati col metodo degli am, con osservazioni su l`accrescimento, sul regime delle tonnare ecc. Memorie, R. Comitato Talassografico Italiano, Venecia 156:

8 Table 1. Bluefin tuna conventionally tagged from 1977 to 2006 in different areas around the Iberian Peninsula (Gibral. Str. refers to Strait of Gibraltar; Alboran to Alboran Sea and Balearic to Balearic Islands area). Bay of Biscay Mediterranaean Sea Surveys (IEO) Opportunistic Surveys (IEO) Gibral. Opportunistic (IEO) (AZTI Tecnalia) Str./Alboran/Balearic Balearic area Year\Age > >1 0 1 >1 Total Total % Total Table 2. Relative recapture rate (recaptured/tagged*100) by tagging area and time at liberty for less and greater than one year. (Number in brackets represents percentage with respect to total recaptured bluefin by area). Recapture area Number of < 1 YEAR AT LIBERTY Bay of Mediterranean South Atlantic tagged bluefin West Atlantic Tagging area Biscay Sea Iberian Penins tuna Bay of Biscay 3.9 (99%) 0.01 (0.3%) 0.03 (0.7%) 7648 Alboran Sea and Gibraltar strait 0.3 (14%) 1.6 (83%) 0.06 (3%) 4699 Balearics Island area 0.2 (4%) 4.2 (96%) 2418 North Atlantic coast of Morocco 7.7 (100%) 195 Recapture area Number of > 1 YEAR AT LIBERTY Bay of Mediterranean South Atlantic tagged bluefin West Atlantic Tagging area Biscay Sea Iberian Penins tuna Bay of Biscay 1.5 (78%) 0.1 (6%) 0.1 (6%) 0.2 (10%) 7648 Alboran Sea and Gibraltar strait 0.4 (81%) 0.09 (19%) 4699 Balearics Island area 0.08 (40%) 0.04 (20%) 0.08 (40%) 2418 North Atlantic coast of Morocco 2.6 (83%) 0.5 (17%)

9 Table 3. Total and daily mean distance (nautical miles) traveled by fish tagged and recaptured in the Bay of Biscay during the same tagging season (less than 1 year at liberty). (Max. and Min. represent maximum and minimum values. Nº obs. corresponds to the number of observations). Total nautical miles Speed nm/day Year Mean Max Min Mean Max Min Nº obs

10 Figure 1. Conventional tags and corresponding applicators used in tagging. Figure 2. Tagging cradle and tags prepared for use (left) and foam mattress over the deck used for tagging specimens of 10 to 15 kg (right). Figure 3. Bluefin tuna larger than 15 kg were tagged with a pole. 1191

11 Figure 4. Tagging reward poster for bluefin tuna. 1192

12 55º 500 m 200 m 45º 50º Area enlarged 45º 44º FRANCE 40º 43º SPAIN 35º -15º -10º -5º 0º 5º 10º Figure 5. Tagging area of bluefin tuna in the Bay of Biscay (Spain) in º 3º 2º 1º 0º 500 m 200 m FRANCE SPAIN 5º 4º 3º 2º 1º 0º Figure 6. Tagging area of bluefin tuna in the Bay of Biscay (Spain) in Nº Fish Age 1 n=1270 Age 2 n= FL (cm) Figure 7. Length frequency distribution of bluefin tuna tagged in the 2005 survey. 1193

13 Bluefin tuna tagged in 2006 N = Fork length (cm) 10 Nº Age Figure 8. Length and age distribution of bluefin tuna tagged in the 2006 survey Nº length BFT FRI Figure 9. Location of tagging operations (2 degrees grid from 34ºN,10ºW to 46ºN,10ºE) (left) and length distribution (right) of tagged specimens in the Alboran Sea in Bluefin tuna appear are shown in red and frigate tuna in blue. 1194

14 n Lenght Figure 10. Location of tagging operations (2 degrees grid from 34ºN,10ºW to 46ºN,10ºE) (left) and length distribution of specimens tagged in 2006 on commercial vessels in the Balearic area. 47º 46º 45º 44º FRANCE 43º SPAIN Tag Position (n=1700) Recapture (n=51) 5º 4º 3º 2º 1º 0º Figure 11. Recaptures in In blue those with up to 1 month of time at liberty, in red from 1 to 2 months, in green from 2 to 3 months and in brown more than 3 months. One fish (on land on the map) was recaptured in a bay near the coast by a purse seiner. 1195

15 47º FRANCE 46º 45º 44º 43º SPAIN Tag Position Recapture 5º 4º 3º 2º 1º 0º Figure 12. Recaptures of bluefin tuna during the 2006 fishing season in the Bay of Biscay. Three (in red) correspond to the same year of tagging whereas 11 belong to the previous tagging year in º 45º 40º 35º Recaptures between BFT Tagging areas in the Med. 30º 20º 15º 10º 5º 0º 5º Figure 13. Bluefin tuna recaptures between 2003 and 2006 from tagging areas in the Mediterranean. 1196

16 55º 50º 45º 40º 35º WEST ATLANTIC AREA BAY OF BISCAY AREA SOUTH ATLANTIC IBERIAN PENINSULA AREA BALEARIC IBERIAN ISLANDS PENINSULA 2418 MEDITERRANEAN AREA 4699 ALBORAN SEA NORTH ATLANTIC COAST OF MOROCCO 30º 35º 30º 25º 20º 15º 10º 5º 0º 5º 10º Figure 14. Main tagging and recapture areas defined in the study. Numbers within circles indicate releases. Effort & Tagged Effort (fishing days) Tagged Recaptures <1 year at liberty Recaptures < 1 year at liberty Figure 15. Fishing effort versus tagged and recaptured fish with less than half a year at liberty in the Bay of Biscay. nº recaptured y = 0,0328x - 0,0494 R 2 = 0, nº tagged Figure 16. Regression line between the number of fish tagged and the recaptures of fish tagged in the same year in the Bay of Biscay. 1197