Abstract
The waters surrounding the Balearic archipelago are considered to be prime spawning habitats of the bluefin tuna (Thunnus thynnus) and other tuna species such as albacore (T. alalunga) in the Mediterranean. During the 2003 bluefin and albacore spawning season, a tuna larval survey (TUNIBAL 0703) was carried out to assess the distribution of bluefin larvae in relation to hydrographic mesoscale features. Tuna larvae were collected by means of surface plankton tows with a bongo gear with a quadrangular mouth measuring 1 m diagonally. The otolith microstructure of 157 and 71 field-captured larvae of Mediterranean bluefin and albacore, respectively, were analyzed to estimate the daily growth pattern. Furthermore, biochemical analysis to estimate DNA, RNA and protein content was done on another batch of 114 bluefin and 132 albacore larvae. The size range included pre-flexion to post-flexion larvae, from 2.8 to 8.6 mm in body length. bluefin and albacore larvae grew linearly in standard length (0.35 and 0.33 mm/day, respectively) and potentially with respect to dry weight. No significant differences between species were observed in the size-weight relationship, or in the allometric relationship of body size and weight to otolith radius. However, the relationship of DNA and RNA to body size and weight showed a significant difference between species. Bluefin tuna larvae showed a greater nucleic acid content than albacore larvae. Protein content showed no significant differences with respect to body length.
Highlights
The high economic value of the North Atlantic bluefin tuna (BFT), (Thunnus thynnus, Linnaeus, 1758) has led the species to a major decline, during the 1990s, possibly due to fishery overexploitation (Anon., 2003)
Most BFT larval samples were caught at two locations: a larval patch found south of Ibiza and a FIG. 2. – Size-frequency distribution of bluefin and albacore larval samples for the daily growth and condition analysis
The stages of the tuna larvae varied from pre-flexion larvae to post-flexion larvae
Summary
The high economic value of the North Atlantic bluefin tuna (BFT), (Thunnus thynnus, Linnaeus, 1758) has led the species to a major decline, during the 1990s, possibly due to fishery overexploitation (Anon., 2003) This led the Spanish Institute of Oceanography (IEO) to carry out a yearly sampling programme (TUNIBAL) aimed at characterising the BFT spawning habitat off the Balearic Sea, historically known as an important tuna spawning area (Dicenta, 1975, 1977). The encounter of two distinct water masses, the lighter inflowing Atlantic waters and the saltier southward Mediterranean water mass, is responsible for the formation of intense frontal structures and for the intense geostrophic circulation of water masses (Pinot et al, 1995; LópezJurado et al, 1995; Vélez-Belchí and Tintoré, 2001) This hydrographic scenario is suitable for spawning of BFT, and of a number of apex fish species, among which frigate tuna (Auxis rochei, Risso 1810) and albacore (T. alalunga, Bonnaterre, 1788) are the most abundant. Other tuna species, such as Sarda sarda (Bloch, 1793), Euthynnus alleteratus (Rafinesque, 1810) and Katsuwonus pelamis (Linnaeus, 1758), have been recorded among tuna larval species, as well as a species of the Xiphidae family, the swordfish (Xiphias gladius, Linnaeus, 1758) (García, IEO unpublished data)
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