SCRS/2008/042 Collect. Vol. Sci. Pap. ICCAT, 64(7): 2184-2191 (2009) DISTRIBUTION AND ABUNDANCE OF BULLET TUNA LARVAE (AUXIS ROCHEI) OFF THE BALEARIC SEA DURING THE 2003-2005 SPAWNING SEASONS A. García, F. Alemany, J.M. Rodríguez, D. Cortes, F. Corregidor, E. Ceballos, L. Quintanilla and P. Velez-Belchí SUMMARY The waters surrounding the Balearic archipelago are characterized by the confluence of Atlantic and Mediterranean water masses. Consequently, the water masses’ convergence imparts on the area an intense geostrophic circulation of water masses and hydrographic processes, particularly suitable for the spawning of a number of tuna species, among which frigate tuna (Auxis rochei) is most abundant. The TUNIBAL project whose main aim was focused on bluefin (Thunnus thynnus) larval ecology allowed acquiring information on its related species during the 2003-2005 spawning seasons. This document presents information on the interannual variability of bullet tuna larval abundance and on their distribution pattern observed throughout these years. Overall, the 2003 Mediterranean heat wave appears to have affected the strong interannual differences observed of bullet tuna larval abundance. Although, their larvae are widely spread over the area, major concentrations tend to be closer to the coast than the other tuna species spawning in the area, particularly bluefin and albacore (T. alalunga). Main larval concentrations of bullet larvae were located between the islands of Mallorca and Menorca shelf and slope regions. RÉSUMÉ Les eaux aux alentours de l’archipel des Baléares se caractérisent par la confluence des masses d’eau atlantiques et méditerranéennes. Par conséquent, la convergence des masses d’eau confère à cette région une intense circulation géostrophique des masses d’eau et des processus hydrographiques, particulièrement propices au frai de nombreuses espèces de thonidés, dont le bonitou (Auxis rochei) qui est l’espèce la plus abondante. Le projet TUNIBAL dont le principal objectif visait à l’écologie larvaire du thon rouge (Thunnus thynnus) a permis d’obtenir des informations sur cette espèce voisine au cours des saisons de frai de 2003 à 2005. Ce document présente des données sur la variabilité interannuelle de l’abondance larvaire du bonitou et sur son schéma de distribution observé au cours de ces années. Globalement, la vague de chaleur qui s’est abattue sur la Méditerranée en 2003 semble avoir affecté les fortes différences interannuelles observées dans l’abondance des larves de bonitou. Bien que leurs larves soient répandues dans toute la zone, les plus grandes concentrations ont tendance à se trouver plus près de la côte que les autres espèces thonières qui fraient dans la zone, notamment le thon rouge et le germon (T. alalunga). Les principales concentrations de larves de bonitou se situaient entre les îles de Majorque et le plateau de Minorque et les régions du versant. RESUMEN Las aguas que rodean al archipiélago balear se caracterizan por la confluencia de masas de agua del Mediterráneo y del Atlántico. Por consiguiente, la convergencia de masas de agua confiere a la zona una intensa circulación geotrófica de masas de agua y procesos hidrográficos, particularmente adecuados para la reproducción de una serie de especies de túnidos, de las cuales la melva (Auxis rochei) es la más abundante. Con el proyecto TUNIBAL, que se centró en la ecología de las larvas de atún rojo (Thunnus thynnus), se adquirió información sobre las especies relacionadas durante las temporadas de desove de 2003 a 2005. En este documento se presenta información sobre la variabilidad estacional de la abundancia de larvas de melva y su patrón de distribución observado durante estos años. En general, la ola de calor mediterráneo de 2003 parece haber afectado a las marcadas diferencias interanuales en la abundancia de larvas de la melvera. Aunque sus larvas tienen una amplia distribución en toda la zona, las mayores concentraciones tienden a producirse en zonas más cercanas a la costa en comparación con las demás especies que se reproducen en la zona, sobre todo el atún rojo y el atún blanco (Thunnus alalunga). Las principales concentraciones de larvas de melvera 2184 se sitúan entre la plataforma de las islas de Mallorca y Menorca y las regiones del talud continental. KEYWORDS Auxis rochei, Balearic archipelago, larvae, distribution, abundance 1. Introduction The TUNIBAL project of the Spanish Institute of Oceanography (IEO) focused on bluefin (Thunnus thynnus) larval ecology demonstrated the importance of the Balearic Sea as one of the most important spawning grounds of bluefin and its associated species (García et al., 2002a,b). From the hydrographic standpoint, the Balearic Sea is the transition area between the northern, and denser, Mediterranean waters, and the southern, and lighter Atlantic waters. (López-Jurado et al., 1995; López-Jurado, 2002). As a result, the Balearic oceanic ecosystem is characterized by a front that separates both waters masses, and an associated jet that runs eastward and an intense ageostrophic circulation responsible in the formation of mesoscale features that confer a suitable spawning habitat for several scombroid top predator species (Alemany, 1997; Alemany et al., 2006). Forming part of the top predators within the trophic web, scombrids play an important role in marine ecosystems (Scheffer et al., 2005) which cannot be disregarded. As such, bullet tuna certainly must play an important role in the trophic food chain because although their commercial interest is much lower than bluefin or albacore, their larvae predominate in the tuna spawning grounds of the Balearic Sea (García et al., 2000a; Sabatés and Recasens, 2001). Their importance within the tuna spawning ecosystem was highlighted in a CLIOTOP Workshop of Working Group 1 on Early Life History of Top Predators emphasizing on the need of developing Auxis-based ecological indices (García, A. et al., 2006) in view of their competition for larval fish preys with other tuna species (Morote et al., 2008; Bakun, 2006) during their early developmental stages. The spawning seasonality of bullet tuna extends primarily from June to September, attaining its maximum when water temperature is highest (Sabatés and Recasens, 2001). Their larvae are mainly distributed over the surface layers, practically over the first 20m of the water column (Morote et al., 2008). 2. Materials and methods Tuna larval samples were collected in the annual surveys carried out in the Balearic archipelago from 2003-2005 (Table 1). The sampling design consisted in establishing in the study area a previously defined reference grid of 5x5 nautical miles. Plankton surface tows with a squared mouth Bongo 90 frame equipped with a 500m mesh were carried out at 10x10 nautical mile intervals. Under certain circumstances, which depended on bluefin larval abundance, sampling intensity was intensified. All plankton tows were carried out at surface for approximately 10 minutes. The number of plankton tows used for this study is shown in Table 1. Due to the change of mesh aperture of the Bongo 90 after undertaking a series of hauls with a different net mesh, the 2003 survey has substantially less number of stations than the 2004-2005 surveys. One of the cod-ends of the Bongo 90 samples was conserved 4% seawater-buffered formalin. The samples were sorted in the laboratory and all the scombroidei species were identified and counted. Hydrographic data of the water column was collected by means of CTD (Sbe9+) casts over the whole trajectory of the surveys, as well as, by means of a continuous temperature and salinity profiler (Sbe) whose sensors were at 5 m depth. 3. Results and discussion 3.1 Relative bullet larval catch and abundance by survey The relative catch of Auxis rochei by year in relation to other scombroid species and to the rest of the larval fish species is shown in Table 2 (Figure 1). The scombroidei species mainly composed of Auxis rochei, T. thynnus and T. alalunga comprise a large share of the total larval catch, varying from 29% of the total catch (2004) to a minimum of 25% (2005), indicating the suitable spawning conditions of these waters for these species. However, Auxis rochei larvae represented the greatest fraction of the top predator scombroid species comprising as much as 16% of the total ichthyoplankton catch in 2003 and 11% in 2004-2005 (Figure 2). Out of a total of 65,839 2185 estimated larval catch in 2003, 10,630 corresponded to bullet tuna larva. In the 2004-2005 surveys, total larval catch and bullet tuna larvae decreased significantly, in spite of the fact of their greater number of stations (Figure 1). Bullet tuna larval abundance estimates (larvae/1000m3) shown in Table 3 demonstrate from the number of positive tows for this species that their larvae are widespread over the study area. Positive tows for Auxis rochei ranged from a maximum of 82% of the total tows in 2003 to 64% in 2005. Maximum larval abundance was observed in 2003, where 3,461 larvae/1000m3 was recorded for a single tow (Table 3). The environmental conditions that predominated during 2003 were strongly influenced by the 2003 heat wave that struck the Mediterranean which caused a significant increase of surface temperatures (Sparnocchia et al., 2006). This climatic warming anomaly is considered among the highest of historical records (Levinson and Waple, 2004). The overall average temperature at 10m depth of all the TUNIBAL survey series carried out from 2001-2005 recorded an increase of 2.7ºC. It is believed that this climatic anomaly may have altered the normal environmental regime off the Balearic Sea ecosystem. Higher surface temperatures influenced an increased daily growth potential in bluefin larvae (García et al., 2006). Bullet tuna attains its peak spawning with highest temperatures (Uchida, 1981; Sabatés and Recasens, 2001). Thus, warming of surface temperature may be held responsible for the substantial increase of bullet tuna larvae during 2003.