SCRS/2012/115 Collect. Vol. Sci. Pap. ICCAT, 69(1): 292-296 (2013) REPORT ON ABUNDANT BLUEFIN LARVAL CONCENTRATIONS THUNNUS THYNNUS) FOUND OFF THE SHELF/SLOPE AREA IN THE SPANISH LEVANTINE COASTS. SIGNS OF RECOVERY? A. García 1, R. Laíz 1, J. Quintanilla 1, F. Corregidor 1 and F. Alemany 2 SUMMARY Since the SCRS at its 2000 meeting in Madrid recommended exploratory research sampling of bluefin larvae and its associated oceanographic conditions in the central Atlantic and in the Mediterranean, the Spanish Institute of Oceanography (IEO) has endeavoured to carry out larval surveys since 2001. The TUNIBAL surveys were undertaken on a yearly basis from 2001-2005. After that period, bluefin larval surveys were more spatially restricted and opportunistic plankton sampling on board fishery acoustic surveys were implemented during 2009-2011. The larval survey of the 2011 MEDIAS acoustic survey yielded 568 bluefin larvae sorted on board off the Spanish Levantine shelf. From the collected data during the bluefin spawning season, it may be reasonable to assume that the spawning area of bluefin may have expanded towards the shelf of the Iberian Peninsula, and thus, providing larval-based evidence on the effects of the Bluefin Recovery Plan. RÉSUMÉ Depuis que le SCRS a recommandé, à sa réunion tenue à Madrid en 2000, que soit réalisé un échantillonnage des larves de thon rouge et de ses conditions océanographiques associées dans l'atlantique central et la Méditerranée dans le cadre de la recherche exploratoire, l'institut espagnol d'océanographie (IEO) mène des prospections larvaires depuis 2001. Les prospections TUNIBAL ont été réalisées tous les ans entre 2001 et 2005. Après cette période, les prospections larvaires de thon rouge ont été davantage restreintes dans l'espace et un échantillonnage opportuniste de plancton a été mis en œuvre entre 2009 et 2011 dans le cadre de prospections acoustiques au sein des pêcheries. La prospection larvaire de la prospection acoustique MEDIAS de 2011 a produit 568 larves de thon rouge triées à bord des embarcations au large du plateau levantin espagnol. À partir des données recueillies pendant la saison de frai du thon rouge, on peut raisonnablement postuler que la zone de frai du thon rouge pourrait s'être étendue vers le plateau de la péninsule ibérique, ce qui fournirait donc des preuves basées sur les larves des effets du programme de rétablissement du thon rouge. RESUMEN Desde que el SCRS recomendó, en su reunión de 2000, que se realizasen muestreos exploratorios de investigación de larvas marinas y de sus condiciones oceanográficas asociadas en el Atlántico central y en el Mediterráneo, el Instituto Español de Oceanografía (IEO) se ha esforzado por realizar prospecciones de larvas desde 2001. Se llevaron a cabo las prospecciones TUNIBAL cada año desde 2001 hasta 2005. Tras dicho periodo, las prospecciones de larvas de atún rojo fueron más restringidas en cuanto a su espacio y se realizaron a bordo muestreos oportunistas de plancton en el marco de prospección acústicas pesqueras durante el periodo 2009-2011. En el marco prospección de larvas de la prospección acústica MEDIAS 2011 se clasificaron a bordo 568 larvas de atún rojo en aguas de la plataforma levantina española. A partir de los datos recopilados durante la temporada de desove de atún rojo, podría ser razonable asumir que la zona de desove del atún rojo podría haberse expandido hacia la plataforma de la Península Ibérica y, de este modo, se proporcionan evidencias basadas en larvas de los efectos del plan de recuperación del atún rojo. KEYWORDS Mediterranean, Thunnus thynnus, larval abundance, spawning areas 1 Instituto Español de Oceanografía, Centro Oceanográfico de Málaga, 29640 Fuengirola, Spain; agarcia@ma.ieo.es 2 Instituto Español de Oceanografía, Muelle Pelaires, Palma de Mallorca, Spain. 292
INTRODUCTION The state of bluefin resources menacing depletion during the recent past led ICCAT to establish a multi-annual recovery plan intended to attain sustainable stock levels in future assessments. A recommendation by ICCAT during 2010 set 12,900 t as the total allowable catch for the Mediterranean. To assess progress on the implementation of the management measures larval surveys may provide important clues on the effectiveness of such measures. Within the Data Collection Framework (DCF) of the EU, annual MEDIAS surveys (MEDiterranean Acoustic Surveys) are carried out for the assessment of pelagic resources off the Spanish Mediterranean coasts covering from Cape Creus in the NW Mediterranean to the neighbouring waters bordering the Alboran Sea. Within the acoustic sampling design, an ichthyoplankton sampling program was applied from 2009-2011. The MEDIAS surveys were undertaken in June-July during the Mediterranean peak spawning season of anchovy and optimal for sampling bluefin larvae in the Balearic Sea. During the 2011 MEDIAS survey, considerable numbers of bluefin larvae were collected along the Levantine coasts of Spain by means of Bongo plankton tows. Bluefin larvae were concentrated along the shelf/slope waters from Cape La Nao to the Gulf of Vera, south of Cape Palos. MATERIALS AND METHODS An ichthyoplankton sampling scheme was implemented during 2009 to 2011 within the acoustic survey MEDIAS which evaluates the small pelagic resources off the Spanish Mediterranean coasts, concretely from Cape Creus to the bordering waters of the Alboran Sea. The joint undertaking of the planktonic sampling scheme with the acoustic tracking was easily feasible because the acoustic operations and fishing sampling manoeuvres normally ended at 20:00h GMT. Consequently, night hours were reserved for planktonic sampling. The MEDIAS survey was carried out on board the R/V Cornide de Saavedra from June 22 to July 21, 2011. The acoustic track follow a series of transects perpendicular to the coastline (Figure 1). The distance between transects varied from 4 nautical miles in of narrow shelf areas to 8 nm where the shelf was wider. Night time plankton sampling back-tracked the acoustic course carried out during the day and plankton stations were selectively carried out in each transect. The plankton gear used consisted in a Bongo of 60cm mouth opening equipped with a 300 m mesh and a Bongo 90cm squared mouth equipped with black tinted netting measuring ~ 1mm mesh. The latter plankton net was the standard net used in the TUNIBAL surveys (García et al., 2005). The Bongo 60 was used for oblique tows targeting a maximum depth of 100 m where possible. The Bongo 90 tows were superficial mainly filtering over the upper 10m layer over a mean duration of 20 minutes. The objective of these sampling tows were not focused on quantifying larval abundance but to acquire larval samples to do a regional and interannual comparative study on early life growth and trophic characterization. Thus, abundance data presented is relative and should not be considered quantitative sensu stricto. Once the plankton sample was on board, larvae were sorted and conserved in liquid nitrogen for future analysis in the laboratory as described in Garcia et al. 2006. RESULTS AND DISCUSSION A total of 568 bluefin larvae (Thunnus thynnus) (Table 1) were sorted from the Bongo plankton samples, among which 42.6% corresponded to catches occurring in 9 stations of Bongo 60 tows distributed over 7 of the acoustic transects. The remaining fraction of the bluefin larvae were captured in 21 tows of Bongo 90 tows located over 4 transects, 2 of which overlapped with the Bongo 60 tows to assure greater and larger-sized bluefin larvae. Bullet tuna larvae (Auxis rochei) also showed similarly fairly large catches. Largest catches of bullet larvae were observed in the Bongo 90 tows (Table 1). Bluefin and bullet tuna larvae began to appear in the plankton samples when reaching the Levantine coasts, south of Cape La Nao. In general, a spatial overlapping of both species is observed. Nevertheless, bluefin tended to produce larger catches in the outer edges of the sampled area near the shelf/slope break while bullet tuna 293
larvae were more abundant in the closer to shore waters stations (Figure 2). Similarly, a significant preference for open waters was observed in bluefin and for inner shelf waters in bullet larvae was observed in the analysis of the TUNIBAL survey series data (Alemany et al., 2010). The greatest catches occurred just south of Cape La Nao where in a sole Bongo 60 and 90 tow yielded 116 and 140 bluefin larvae, respectively. As the survey tracking progressed southward over the Gulf of Vera catches declined considerably. Nonetheless, bullet larvae were still found in the Bay of Almería in the Alboran Sea where the survey reached its maximum spatial coverage. There are some indications that such high catches in these areas may have hopeful signs of bluefin tuna stock recovery. Firstly, during the first MEDIAS survey carried out in 2009 there was no reporting on bluefin larval catches, while in 2010 some occasional bluefin larvae were reported to have occurred (Alemany, pers. com.). Furthermore, during the summer of 2011, a smaller scale oceanographic and plankton survey than the TUNIBAL surveys was undertaken south of the Balearic archipelago which produced significantly large numbers of bluefin larvae (Alemany, pers. com.). Thus, it would seem reasonable to assume that the distribution of bluefin larvae found in the Levantine coasts correspond to the expansion of the distributional area of bluefin spawning. Furthermore, during the Spanish Research Council project, ATAME carried out a bluefin larval survey last July 2012 in the Balearic archipelago which reported a significant increase of bluefin larval catches and the occurrence of single plankton haul which produced a massive amount of bluefin eggs never observed before (Alemany, pers. com.). The rearing on board of this catch verified that these corresponded to bluefin. A comparative examination of the distribution of small pelagic larvae (Figure 3) may also provide some other sources of evidence that may explain bluefin distributional spawning range. The northern waters off the Gulf of Valencia and the Catalonian coasts are highly influenced by the Northern current propitiating the fertilization of waters off the Ebro river coasts where anchovy (Engraulis encrasicolus) have their spawning core (Palomera et al., 2007). The area is characterized by high productivity under the influence of the Mediterranean Northern current which certainly would not correspond to bluefin spawning habitat preferences (Alemany et al., 2010; García et al., 2005). Southward of Cape La Nao anchovy abundance decrease and practically disappear from the samples. These are replaced by larvae of Sardinella aurita which show a significant preference for warmer waters in temperatures over 23ºC (Palomera et al., 2007) which would correspond to the preferential temperature range of bluefin spawning. Moreover, from the bottom topographic point of view the southwestern promontory of the Balearic archipelago is linked to continental platform south of Cape La Nao (Banda et al., 1980), which divides the northern waters of the Gulf of Valencia from the South Mediterranean waters. In conclusion, the unexpected catches of bluefin larvae off the South Mediterranean shelf may be a hopeful sign of the recovery of the stock. The unofficial reports on the increase of bluefin larvae corroborate the increase of bluefin larval concentrations. If bluefin stock assessments are in the same line of stock increase, it would be a demonstration that larval surveys may be a useful tool to assess the state of the resource. 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García, A., Cortés, D., Ramírez, T., Fehri-Bedoui, R., Alemany, F., Rodríguez, J.M., Carpena, A. and Álvarez, J.P. 2006, First data on growth and nucleic acid and protein content of field-captured Mediterranean bluefin (T. thynnus) and albacore (T. alalunga) larvae: a comparative study. Sci. Mar., 70S2: 67-78. Palomera, I., Olivar, M.P., Salat, J., Sabatés, A., Coll, M., García, A., Morales-Nin, B. 2007, Small pelagic fish in the NW Mediterranean Sea: An ecological review. Progress in Oceanography, 74: 377 396. 294
Table 1. Relative catches of bluefin and bullet larvae by Bongo gear. Bongo 60 Bongo 90 Total Tows 9 21 30 T. thynnus 242 326 568 A. rochei 74 653 727 Figure 1. Acoustic transect track designed for the MEDIAS Survey series. 295
Figure 2. Bluefin (T. thynnus) and bullet tuna (A. rochei) larval catches of the 2011 MEDIAS survey. Figure 3. Anchovy (E. encrasicolus) and round sardinella (S. aurita) larval catches of the 2011 MEDIAS survey. 296