Distribution and abundance of bullet tuna larvae (Auxis rochei) off the Balearic Sea during the 2003-2005 spawning seasons García, A., 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. INTRODUCTION KEYWORDS Auxis rochei, Balearic archipelago, larvae, distribution, abundance 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, F., 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). 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 500μm 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% seawaterbuffered formalin. The samples were sorted in the laboratory and all the scombroidei species were identified and counted. SURVEY VESSEL DATES STATIONS TUNIBAL 0703 R/V Cornide de Saavedra July 4 30, 2003 140 TUNIBAL 0604 R/V Cornide de Saavedra June 16 - July 12, 2004 197 TUNIBAL 0605 R/V Cornide de Saavedra June 24 - July 27, 2005 192 Table.1- General information of the 2003-2005 TUNIBAL survey series. 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. 2
RESULTS AND DISCUSSION 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 (Fig. 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 70000 60000 50000 Auxis rochei species comprising as much as 16% of 40000 the total ichthyoplankton catch in 2003 30000 and 11% in 2004-2005 (Fig. 2). Out of a total of 65,839 estimated larval catch in 2003, 10,630 corresponded to bullet tuna larva. In the 2004-2005 surveys, total larval catch and bullet tuna larvae 20000 10000 0 decreased significantly, in spite of the fact of their greater number of stations 2003 2004 2005 (Fig. 1). Fig. 1. Relative larval composition by survey. Other Scombroidei Other larval species Species 2003 2004 2005 Auxis rochei 10630 2626 2378 Euthynnus alleteratus 42 21 Katsuwonus pelamis 63 1 Sarda sarda 3 Thunnus alalunga 3960 851 1945 Thunnus thynnus 625 3300 866 Xiphias gladius 40 24 35 Other larvae 50476 16558 15250 Total larval catch 65839 23381 20474 Scombroidei Table 2. Larval catch by species, groups and survey years. Bullet tuna larval abundance estimates (larvae/1000m 3 ) 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/1000m 3 was recorded for a single tow (Table 3). 3
Variables 2003 2004 2005 Total stations 140 197 199 Positive stations 115 137 128 Mean abundance 140,19 23,59 22,78 Maximum abundance 3,461.61 729.78 569,12 St Dv 428,61 76,19 60,58 Table 3. Bullet tuna larval abundance estimates by surveys. 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 16000 Thunnus thynnus 14000 Thunnus alalunga its peak spawning with highest temperatures 12000 Auxis rochei (Uchida, 1981; Sabatés 10000 and Recasens, 2001). 8000 Thus, warming of surface temperature may be held 6000 4000 responsible for the 2000 substantial increase of 0 bullet tuna larvae during 2003 2004 2005 2003. During this year, maximum bullet tuna Fig. 2. Relative larval composition of scombroidei species by survey. catch from a single plankton haul was 2,200 larvae, whereas, during 2004 and 2005, maximum larval catch was 418 and 263, respectively. 2003-2005 Bullet tuna larval abundance distribution 2003 TUNIBAL Survey Bullet tuna larval abundance distribution expressed as larvae/1000m 3 during 2003 was mainly concentrated in the northern part of the survey area (Fig. 3). Dense larval patches were found in the areas near the shelf and slope regions off the islands of Mallorca and Menorca. Nevertheless, bullet larvae were also found spread all Fig. 3. Frigate tuna larval abundance distribution (larvae/1000m 3 ) in 2003. 4
over the oceanic waters of the survey area. Bullet tuna larvae were mostly distributed over the northern Mediterranean water masses where highest surface salinities and temperatures were observed, indicating the species preference for Mediterranean waters and for a higher temperature regime (Fig. 4). The transition region where Atlantic water masses encounter Mediterranean waters was mainly located south of the Ibiza channel and spreading in a north- Fig. 4. Spatial distribution of temperature and salinity eastward direction. at 10m depth in 2003. Spatial segregation between the bullet larval distribution in regards to the bluefin larval distribution was observed in this survey because most bluefin larval concentrations occurred in the southeastern transitional waters of the study area (García et al., 2005). 2004 TUNIBAL Survey Although bullet larvae were regularly found spread over the whole study area, major concentrations were found off the shelf between the islands of Mallorca and Menorca (Fig. 5), as observed in the previous 2003 survey. Patches of bullet larvae were found within the northern border of an inflowing Atlantic tongue of water that impinged the shelf/slope contour south of Menorca (Fig. 6) causing the formation of an anticyclonic gyre. To the north and east of this anticyclonic gyre, water masses were significantly warmer than the western water masses located south of the island of Ibiza and the channel of Mallorca, where bullet larvae were much scarcer. Fig. 5. Frigate tuna larval abundance distribution (larvae/1000m 3 ) in 2004. 5
Fig. 6. Spatial temperature and salinity distribution at 10 m depth in 2004. 2005 TUNIBAL Survey Bullet larvae showed a greater dispersion over the area. Their larvae were found distributed in southern and northern oceanic areas of the study area, as well as, around the islands of Mallorca and Menorca. A concentrated patch was located west off Mallorca shelf. Lower larval concentrations were found in waters surrounding the island of Ibiza. Fig. 7. Frigate tuna larval abundance distribution (larvae/1000m 3 ) in 2005. Surface waters were relatively warmer than the previous survey carried out in 2004. This fact may have influenced the distribution of bullet larvae since larval distribution was rather widely spread over the study area. Fig. 8. Spatial temperature and salinity distribution at 10 m depth in 2005. Water masses of Atlantic origin were mostly distributed over the southernmost part of the study area in which a strong frontal system was observed with its associated jet running between the northern side of Ibiza and the southern region off Mallorca and further south than the previous years off Menorca. 6
CONCLUSIONS Though bullet tuna larvae show a wide spread distribution pattern in the Balearic Sea, major larval concentrations are inclined shelf/slope water masses off the Balearic islands, where highest larval patches were found in this three year study. Strong interannual differences in larval abundances were observed, consequent with the significantly higher surface thermic regime of the 2003 survey. The Mediterranean heat wave seems to have influenced the increased bullet tuna larval abundance in that year, representing more than a three-fold increase of their larval catch. REFERENCES Alemany, F. 1997. Ictioplancton del Mar Balear. PhD thesis, Univ. de les Illes Balears. Alemany, F., S. Deudero, B. Morales-Nin, J.L. Lopez-Jurado, J. Jansa, M. Palmer and I. Palomera. 2006. Influence of physical environmental factors on the composition and horizontal distribution of summer larval fish assemblages off Mallorca island (Balearic archipelago, western Mediterranean). J. Plankton Res., 8: 471-487. Bakun, A. 2006. Fronts and eddies as key structures in the habitat of marine fish larvae: opportunity, adaptive response and competitive advantage. Scientia Marina, 70S2, 105-122. García, A., A. Bakun and D. Margulies. 2006. Report of CLIOTOP Workshop of Working Group 1 on Early Life History of Top Predators. ICCAT, SCRS/2006/123. García, A., F. Alemany, P. Velez-Belchí, J.L. López Jurado, J.M. de la Serna, C. González Pola, J.M. Rodríguez and J. Jansá. 2002b. Bluefin tuna and associated species spawning grounds in the oceanographic scenario of the Balearic archipelago during June 2001. ICCAT, SCRS/2002/041. García, A., F. Alemany, P. Velez-Belchí, J.M. Rodríguez, J.L. López Jurado, C. González Pola and J.M. de la Serna. 2002a. Bluefin and frigate tuna spawning off the Balearic archipelago in the environmental conditions observed during the 00 spawning season. ICCAT, SCRS/2002/165. García, A., J. Quintanilla, I. Alvárez, A. Carpena, D. Cortés, F. Alemany and J.M. Rodríguez. 2006. Interannual variability of bluefin larval growth observed during the spawning seasons 2003-2005. ICCAT, SCRS/2006/122. García, A., F. Alemany, P. Velez-Belchí, J.L. López Jurado, D. Cortés, J.M. de la Serna, C. González Pola, J.M. Rodríguez, J. Jansá and T. Ramírez. 2005. Characterization of the bluefin tuna spawning habitat off the Balearic Archipelago in relation to key hydrographic features and associated environmental conditions. Coll. Vol. Sci. Pap., ICCAT 58:535 549. Levinson, D.H, and A.M.Waple. 2004. State of climate in 2003. Bulletin of the American Meteorological Society, 85(6), S1-S72. 7
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