DISTRIBUTION OF TUNA LARVAE OFF THE BALEARIC SEA: PRELIMINARY RESULTS OF THE TUNIBAL 0600 LARVAL SURVEY

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1 SCRS/2001/129 DISTRIBUTION OF TUNA LARVAE OFF THE BALEARIC SEA: PRELIMINARY RESULTS OF THE TUNIBAL 0600 LARVAL SURVEY A. García 1, F. Alemany 2 and J.M. Rodríguez 3 ABSTRACT The present document describes de distribution of bluefin tuna larvae (Thunnus thynnus), frigate tuna larvae (Auxis rochei) and albacore larvae (T. alalunga) off the Balearic islands as a result of the preliminary analysis of the samples taken during the tuna larval survey, TUNIBAL 06/01 on board the R/V Vizconde de Eza from 15 th June-10 th July, The plankton samples analysed are from surface tows carried out with a Bongo net of 90 cm mouth opening and a mesh of 1mm that facilitated the sorting and identification of tuna larval specimens on board. RÉSUMÉ Le présent document décrit la distribution des larves de thon rouge (Thunnus thynnus), d=auxide (Auxis rochei) et de germon (T. alalunga) au large des îles Baléares d=après l=analyse préliminaire des échantillons prélevés pendant la prospection de larves de thon, TUNIBAL 06/01 à bord du N/O Vizconde de Eza entre le 15 juin et le 10 juillet Les échantillons de plancton analysés proviennent de traits de surface de filets Bongo de 90 cm d=ouverture avec une maille de 1 mm qui a facilité le tri et l=identification à bord des spécimens de larves de thon. RESUMEN El documento describe la distribución de larvas de atún rojo (Thunnus thynnus), de melva (Auxis rochei) y de bonito (T. alalunga) en aguas de las islas Baleares como resultado de los análisis preliminares de las muestras planctónicas tomadas durante la campaña TUNIBAL 06/01 a bordo del B/O Vizconde de Eza durante los días 15 de junio al 10 de julio de Las muestras planctónicas fueron realizadas en superficie por medio de una red Bongo de 90 cm de apertura de boca y de un 1mm de malla. Las larvas fueron triadas e identificadas a bordo inmediatamente después del arrastre. KEYWORDS Thunnus thynnus, Auxis rochei, Thunnus alalunga, fish larvae, abundance, geographical distribution, spawning grounds INTRODUCTION At its 2000 meeting in Madrid (Spain), the SCRS endorsed several research activities considered essential to the future of the BYP. The SCRS recommended exploratory research sampling of larvae and spawning sized bluefin tuna and the associated oceanographic conditions in the central Atlantic as outlined in SCRS/00/125, as well as, in the Balearic Islands region. At the BYP Coordination Meeting for Sampling Survey Research held in the NOAA headquarters of Miami, during April 2001, a research proposal formulated by the Instituto Español de Oceanografía (IEO) focusing on a combined 1 Centro Oceanográfico de Málaga, Puerto Pesquero, Fuengirola, Spain 2 Centro Oceanográfico de Baleares, Muelle Pelaires, Palma de, Spain 3 Centro Oceanográfico de Gijón, Camino del Arbeyal, s/n, 33212, Gijón, Spain

2 hydrographic and bluefin tuna larval survey was presented (García et al., 2001). Previous information collected during the mid-seventies by Dicenta (1975, 1977) demonstrated that the Balearic Islands region showed important spawning of bluefin tuna and other tuna species. As a result of the TUNIBAL project, the IEO carried out during 15 th June-10 th July, 2001, a combined hydrographic and ichthyoplanktonic survey on board the R/V Vizconde de Eza off the Balearic Sea with the objectives of defining bluefin spawning grounds in the region and characterising the oceanographic conditions in which this species spawns. MATERIAL AND METHODS The survey consisted of a hydrographic sampling together with the ichthyoplankton tows. Two kinds of plankton nets were used which defined two towing manoeuvres. Oblique plankton tows were carried out with a Bongo net of 60 cm inlet diameter (250 µm mesh) and surface tows with a Bongo squared mouth opening of 90 cm measured diagonally. The latter net is 1mm mesh and is tinted black so as to lessen the effect of avoidance of larvae, especially during nighttime (picture 1). The net was towed at 2.5 knots at surface during a constant duration of 10 minutes. Immediately, after retrieval of the net the cod ends were placed in buckets for sorting of fish larvae on board. Most surface tows were composed of fish larvae facilitating a rapid sorting of fish larvae. If tuna larvae appeared in the catch, these were measured and identified by means of a Image Analysis system as shown in picture 2. Most of the tuna species larvae were measured fresh on board and the image analysis system provided a complete set of images for future examinations. The larvae captured with this net were conserved in liquid nitrogen on the vessel and upon arriving to land conserved in 80ºC for daily growth and condition analysis by quantification of nucleic acids and protein content. The net was geared with a General Oceanic 2030 flowmeter providing the information of volume filtered. The counts on board and the posterior analysis of the saved images provided the data for a preliminary analysis of the principal species composing the tuna larvae. Surface tow stations were normally placed in a 10 x 10 nm grid, but intensified to 5x5 depending on the intensity of tuna larval abundance (Fig. 1). In some particular regions of high abundance (i.e., south of ), stations distanced two miles apart were placed in a cross like manner. A total of 124 bluefin tuna larvae, 482 frigate tuna larvae and 50 albacore larvae were recorded from the Bongo 90 cod-end samples, preliminary data which has aided in the elaboration of this document. RESULTS Bluefin tuna larvae (Thunnus thynnus) Fig. 2 represents the distribution of tuna larval abundance off the explored region off the Balearic archipelago. The highest abundance was observed south of and along the mid section of the channel. Isolated stations of lower abundance were observed in the northern sector of the island of. Fig. 3 shows the distribution of bluefin tuna larvae by different size classes (<5, 5-7, 7-9 and > 9mm). A greater concentration of smaller bluefin tuna larvae are observed in the channel increasing in size towards the south of. This seems to indicate some drifting mechanisms towards this sector. Nevertheless, taking into account the high daily growth rates of this species and that this latter sector formed part of the last sections sampled during the survey, these larvae could have grown in neighbouring waters. A further analysis of the hydrographic data together with the larval ageing data can aid in clarifying either of the proposed hypotheses.

3 Frigate tuna larvae (Auxis rochei) The distribution of frigate tuna larva (Auxis rochei) offers a similar pattern as that of the previously described species, overlapping the distribution of bluefin tuna larvae (Fig. 4). Similarly, the greatest concentrations occur in the region south of and within the channel. In the distribution of frigate larvae by size class (Fig. 5), the greatest proportion of larger size classes (7-9 and >9mm) occur in the area south of, where the greatest tuna larval concentrations were observed. As in the case of bluefin tuna larvae, this seems to indicate a transport mechanism induced by the current regime in the area. Nevertheless, this possibility will have to be validated in the view of future examination and analysis of the hydrographic data. Albacore tuna larvae (Thunnus alalunga) Although this species showed a much lower abundance, its distribution seems to show a similar pattern to the other two main larval tuna species described. Major concentration was also observed in the area south of and to a lesser importance in the channel area. CONCLUSION As a result of the analysis of distribution pattern of the principal larval tuna species found in the exploration of the Balearic archipelago, it seems that the preferential spawning grounds are located in the areas off the channel, following a path that leads towards the region of greater concentration located in the southern explored sector off. This pattern probably is indicating certain particular conditions of the hydrographic regime influencing their spawning strategy. REFERENCES GARCIA, A., J.M. de la Serna Ernst, J.L. López Jurado F. Alemany and E. Rodríguez Marin, Bluefin tuna egg and larval survey in the Balearic Sea, June 2001 (TUNIBAL 06/01). SCRS/01/82. DICENTA, A., Identificación de algunos huevos y larvas de túnidos en el Mediterráneo. Bol. Inst. Esp. Oceanogr., 198, DICENTA, A., Desove de atún ( Thunnus thynnus) en el Mediterráneo Occidental y evaluación directa del stock de reproductores básico en la abundancia de sus larvas. SCRS, 77/33.

4 Fig. 1. Station grid of the plankton sampling tows. Circle symbols and different circle colors represent Bongo 90 surface tows followed during the different legs of the survey bluefin abundance 1000m Fig. 2- Bluefin tuna larval abundance distribution off the Balearic archipelago.

5 < "5-7 "7-9 > Fig. 3- Bluefin tuna larval abundance distribution by size class Auxis sp. abundance 1000m Fig.4.-- Bluefin tuna larval abundance distribution in the area explored off the Balearic archipelago.

6 Auxis 38.8 " " Abundance (1000m-3) <5 > Fig. 5- Frigate tuna larval abundance distribution by size class off the Balearic Sea T. alalunga abundance 1000m Fig. 6- Albacore tuna larval abundance distribution off the Balearic Sea.

7 Picture 1.- Deployment of Bongo 90 plankton net during the TUN IBAL 0601 survey. Picture 2.- Measuring and identification procedures on board the R/V Vizconde de Eza.