One of the most common problems in fisheries is the definition of management units. Albacore Thunnus alalunga is an important species for commercial fisheries. Its population structure is still partially unknown; however, on the basis of fisheries data, tagging experiments, and morpho-ecological studies, 6 management units are currently accepted for this species. The main objective of this study was to define genetic entities within T. alalunga and to discuss the appropriateness of current management units. For this purpose, 13 microsatellite loci were applied to 551 albacore samples collected worldwide, and the population genetic structure was assessed. The most relevant differences between management and genetic units were that (1) Atlantic and Indian Ocean samples are genetically indistinguishable, and (2) possible differentiation exists within the Pacific Ocean and also within the Mediterranean Sea. Thus, this study provides genetic information to clarify albacore population delimitation, which is a key factor to reach the demanded sustainable management of this resource.

Stock heterogeneity was investigated in albacore tuna (Thunnus alalunga, Bonnaterre 1788), a commercially important species in the North Atlantic Ocean and Mediterranean Sea. Twelve polymorphic microsatellite loci were examined in 581 albacore tuna from nine locations, four in the northeast Atlantic Ocean (NEA), three in the Mediterranean Sea (MED) and two in the southwestern Pacific Ocean (SWP). Maximum numbers of alleles per locus ranged from 9 to 38 (sample mean, 5.2-22.6 per locus; overall mean, 14.2 ± 0.47 SE), and observed heterozygosities per locus ranged from 0.44 to 1.00 (overall mean: 0.79 ± 0.19 SE). Significant deficits of heterozygotes were observed in 20% of tests. Multilocus F ST values were observed ranging from 0.00 to H = 0.036 and H 0 = 0.253, with a mean of H = 0.013 and H 0 = 0.079. Pairwise F ST values showed that the SWP, NEA and MED stocks were significantly distinct from one another, thus corroborating findings in previous studies based on mitochondrial DNA, nuclear DNA (other than microsatellites) and allozyme analyses. Heterogeneity was observed for the first time between samples within the Mediterranean Sea. GENELAND indicated the potential presence of three populations across the NEA and two separate populations in the Mediterranean Sea. Observed genetic structure may be related to migration patterns and timing of movements of subpopulations to the feeding grounds in either summer or autumn. We suggest that a more intensive survey be conducted throughout the entire fishing season to ratify or refute the currently accepted genetic homogeneity within the NEA albacore stock. Communicated by T. Reusch.

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