Phylogenetic relationships among Thunnus species inferred from rDNA ITS1 sequence

Intra and interspecific nucleotide sequence variation of rDNA first internal transcribed spacer (ITS1) was analysed using all eight species of the genus Thunnus plus two out‐group species within the same family, skipjack tuna Katsuwonus pelamis and striped bonito Sarda orientalis . Intraspecific nucleotide sequence variation in ITS1, including intra‐genomic variation, was low, ranging from 0·003 to 0·014 [Kimura's two parameter distance (K2P)], whereas variation between species within the genus Thunnus ranged from 0·009 to 0·05. The Atlantic and Pacific northern bluefin tunas Thunnus thynnus thynnus and Thunnus thynnus orientalis , recently proposed to be distinct species, were found to share nearly identical ITS1 sequences (mean K2P = 0·006) well within the range of intraspecific variation. The northern bluefin tuna appeared to be a sister group to albacore Thunnus alalunga , with all other Thunnus species in a distinct clade. The ITS1 phylogeny was consistent with mtDNA phylogeny in clustering the three tropical Thunnus species ( T. albacares , T. atlanticus and T. tonggol ). Southern bluefin Thunnus maccoyii and bigeye Thunnus obesus tunas showed a closer affinity to this tropical tuna group than to the northern bluefin tuna and albacore. The molecular data supported mitochondrial introgression between species and contradicted morphological subdivision of the genus into two subgenera Neothunnus and Thunnus .     

Mitochondrial DNA sequence variation within and between tuna Thunnus species and its application to species identification

Restriction analysis detected two types of bigeye tuna (α and β); the α type was in the majority in the Atlantic but nearly absent in the Indo-Pacific. The β type shared a larger number of restriction sites with other species than the conspecific β type, but bigeye-specific nucleotide substitutions with a novel diagnostic restriction profile were found. Although the nucleotide sequence difference between Atlantic and Pacific sub-species of the northern bluefin tuna was nearly the largest among species, individuals possessing the Atlantic type of mtDNA were found at very low frequency in the Pacific and vice versa. Previous RFLP markers were found to be diagnostic for the other five species (albacore, blackfin, longtail, southern bluefin and yellowfin tunas). Genetic information is provided to discriminate all Thunnus species regardless of their origin and to identify the ocean of capture in the northern bluefin and bigeye tunas.     

Orthodox and unorthodox phylogenetic relationships among tunas revealed by the nucleotide sequence analysis of the mitochondrial DNA control region

A phylogeny of all eight recognized taxa of the genus Thunnus was constructed from approximately 400 base pairs of sequence of the mitochondrial DNA (mtDNA) control region. The PCR-amplified control region I segment studied contained a total of 186 variable sites and 159 phylogenetically informative sites. Diagnostic sequences for every taxon were identified. Neighbour-joining phylogenies supported monophyletic origins of the temperate subgenus Thunnus and of the tropical subgenus Neothunnus . Similar results were obtained by maximum parsimony analyses except that there was no support for a monophyletic origin of the subgenus Thunnus . Bigeye tuna, which have been difficult to place in either subgenus using conventional morphological data, was identified as the sister species of Neothunnus . Within the subgenus Thunnus , the Atlantic bluefin and Southern bluefin tunas were shown to be sister taxa of the highly divergent monophyletic clade formed by the Pacific northern bluefin and the Albacore tunas. The conspecific Atlantic ( T. thynnus thynnus ) and Pacific ( T. t. orientalis ) northern bluefin tunas were more divergent (Tamura-Nei distance 0·145 ± 0·019) from each other than the average distance separating most species-pairs within the genus. Thus, a re-examination of their status as subspecies of T. thunnus is warranted.     

Microsatellite DNA markers for population‐genetic studies of Atlantic bluefin tuna (Thunnus thynnus thynnus) and other species of genus Thunnus

Twenty‐five microsatellites from Atlantic bluefin tuna (Thunnus thynnus thynnus) were characterized. All 25 microsatellites were polymorphic; the number of alleles among up to 56 individuals surveyed ranged from two to 23. Atlantic bluefin tuna are highly exploited and major questions remain as to stock structure and abundance in the eastern and western North Atlantic. The microsatellites will be useful in testing stock‐structure hypotheses and in generating estimates of effective population size. The polymerase chain reaction primer sets developed also amplified identifiable alleles in three other species of genus Thunnus: T. albacares (yellowfin tuna), T. alalunga (albacore tuna) and T. obesus (bigeye tuna).     

Comparative phylogeography of Atlantic bluefin tuna and swordfish: the combined effects of vicariance, secondary contact, introgression, and population expansion on the regional phylogenies of two highly migratory pelagic fishes

Comparative phylogeography has revealed remarkable patterns of concordance in the maternal phylogenies of many species. The phylogeography and historical demography of the mitochondrial control region I for 607 Atlantic bluefin tuna (Thunnus thynnus) and 275 swordfish (Xiphias gladius) were analyzed to clarify the complex phylogenetic signals in the North Atlantic-Mediterranean region where they are sympatric. Atlantic bluefin tuna mtDNA is polyphyletic, and includes rare sequences sister to Pacific bluefin tuna (Thunnus orientalis) and introgressed albacore (Thunnus alalunga) sequences. There is no geographic partitioning between Atlantic and Mediterranean samples of Atlantic bluefin tuna (Phi(ST)=0.002). In contrast, Atlantic and Mediterranean swordfish are differentiated (Phi(ST)=0.091) due to the combined effects of vicariance, secondary contact, and dissimilar regional demographic histories. Mediterranean swordfish has substantially less variation, and a more recent history (tau=2.42) than that of Atlantic swordfish (tau=7.02). In spite of the discordant phylogenetic and phylogeographic signals, the demographic history of Atlantic swordfish and Atlantic bluefin tuna (tau=7.51) suggests concordance in the timeline of population expansion. Possible scenarios of cladogenesis, expansion, and contraction, influenced by glacial cycles during the Pleistocene, are formulated.     

Fast versus slow growing tuna species: age,growth, and implications for population dynamics and fisheries management

Growth models describe the change in length or weight as a function of age. Growth curves in tunas can take different forms from relatively simple von Bertalanffy growth curves (Atlantic bluefin, albacore tunas) to more complex two- or three-stanza growth curves (yellowfin, bigeye, skipjack, southern bluefin tunas). We reviewed the growth of the principal market tunas (albacore, bigeye, skipjack, yellowfin and the three bluefin tuna species) in all oceans to ascertain the different growth rates among tuna species and their implications for population productivity and resilience. Tunas are among the fastest-growing of all fishes. Compared to other species, tunas exhibit rapid growth (i.e., relatively high K) and achieve large body sizes (i.e., high L _∞ ). A comparison of their growth functions reveals that tunas have evolved different growth strategies. Tunas attain asymptotic sizes (L _∞ ), ranging from 75 cm FL (skipjack tuna) to 400 cm FL (Atlantic bluefin tuna), and reach L _∞ at different rates (K), varying from 0.95 year^?1 (skipjack tuna) to 0.05 year^?1 (Atlantic bluefin tuna). Skipjack tuna (followed by yellowfin tuna) is considered the “fastest growing” species of all tunas. Growth characteristics have important implications for population dynamics and fisheries management outcomes since tunas, and other fish species, with faster growth rates generally support higher estimates of Maximum Sustainable Yield (MSY) than species with slower growth rates.     

Population genetics of tunas

Population genetic studies on tunas are reviewed. These studies have focused on phylogenetic reconstructions, species identifications and stock delineation, and have used tools ranging from blood group and allozyme analysis to PCR-aided examination of mitochondrial DNA variation. Both allozyme and mtDNA approaches show tunas in the genus Thunnus to be very closely related to one another, but also indicate that the two presently recognized subspecies of northern bluefin tuna, Thunnus thynnus thynnus and T. t. orientalis , in fact may be worthy of species status. These techniques also permit the unequivocal recognition of specimens, which is not always possible on morphological grounds. However, it is arguable that, until recently, tunas have not received their due attention from geneticists given their commercial significance and the need for information on stock structure to ensure sustainable management. This may be because tunas are known to be highly vagile and therefore levels of population differentiation are expected to be low. None the less, population subdivision has been recorded in several species (skipjack, yellowfin, albacore), although this tends to be on a broad (intra- or inter-oceanic) rather than on a more local scale. New molecular tools, including the PCR-based analyses of nuclear genes and microsatellite loci, are yielding new, highly polymorphic markers, and will enable more powerful analyses of stock structure than have hitherto been possible.     

Single nucleotide polymorphism discovery in albacore and Atlantic bluefin tuna provides insights into worldwide population structure

The optimal management of the commercially important, but mostly over‐exploited, pelagic tunas, albacore (Thunnus alalunga Bonn., 1788) and Atlantic bluefin tuna (BFT; Thunnus thynnus L., 1758), requires a better understanding of population structure than has been provided by previous molecular methods. Despite numerous studies of both species, their population structures remain controversial. This study reports the development of single nucleotide polymorphisms (SNPs) in albacore and BFT and the application of these SNPs to survey genetic variability across the geographic ranges of these tunas. A total of 616 SNPs were discovered in 35 albacore tuna by comparing sequences of 54 nuclear DNA fragments. A panel of 53 SNPs yielded F_ST values ranging from 0.0 to 0.050 between samples after genotyping 460 albacore collected throughout the distribution of this species. No significant heterogeneity was detected within oceans, but between‐ocean comparisons (Atlantic, Pacific and Indian oceans along with Mediterranean Sea) were significant. Additionally, a 17‐SNP panel was developed in Atlantic BFT by cross‐species amplification in 107 fish. This limited number of SNPs discriminated between samples from the two major spawning areas of Atlantic BFT (F_ST = 0.116). The SNP markers developed in this study can be used to genotype large numbers of fish without the need for standardizing alleles among laboratories.     

Isolation and characterization of seven tetranucleotide microsatellite loci from Atlantic northern bluefin tuna Thunnus thynnus thynnus

Microsatellite‐enriched genomic libraries were obtained from the Atlantic northern bluefin tuna, Thunnus thynnus thynnus and seven tetranucleotide markers were successfully isolated and characterized from this library. These markers were found to have between 1 and 17 alleles in Atlantic northern bluefin tuna and heterozygosity ranged from 0 to 0.85. No deviations from the expectation of Hardy‐Weinburg equilibrium were found for any marker. Several of these markers amplify reliably in other tuna species.     

Large‐scale distribution of tuna species in a warming ocean

Tuna are globally distributed species of major commercial importance and some tuna species are a major source of protein in many countries. Tuna are characterized by dynamic distribution patterns that respond to climate variability and long‐term change. Here, we investigated the effect of environmental conditions on the worldwide distribution and relative abundance of six tuna species between 1958 and 2004 and estimated the expected end‐of‐the‐century changes based on a high‐greenhouse gas concentration scenario (RCP8.5). We created species distribution models using a long‐term Japanese longline fishery dataset and two‐step generalized additive models. Over the historical period, suitable habitats shifted poleward for 20 out of 22 tuna stocks, based on their gravity centre (GC) and/or one of their distribution limits. On average, tuna habitat distribution limits have shifted poleward 6.5 km per decade in the northern hemisphere and 5.5 km per decade in the southern hemisphere. Larger tuna distribution shifts and changes in abundance are expected in the future, especially by the end‐of‐the‐century (2080–2099). Temperate tunas (albacore, Atlantic bluefin, and southern bluefin) and the tropical bigeye tuna are expected to decline in the tropics and shift poleward. In contrast, skipjack and yellowfin tunas are projected to become more abundant in tropical areas as well as in most coastal countries' exclusive economic zones (EEZ). These results provide global information on the potential effects of climate change in tuna populations and can assist countries seeking to minimize these effects via adaptive management.     

Projections of climate‐driven changes in tuna vertical habitat based on species‐specific differences in blood oxygen affinity

Oxygen concentrations are hypothesized to decrease in many areas of the ocean as a result of anthropogenically driven climate change, resulting in habitat compression for pelagic animals. The oxygen partial pressure, pO₂, at which blood is 50% saturated (P₅₀) is a measure of blood oxygen affinity and a gauge of the tolerance of animals for low ambient oxygen. Tuna species display a wide range of blood oxygen affinities (i.e., P₅₀ values) and therefore may be differentially impacted by habitat compression as they make extensive vertical movements to forage on subdaily time scales. To project the effects of end‐of‐the‐century climate change on tuna habitat, we calculate tuna P₅₀ depths (i.e., the vertical position in the water column at which ambient pO₂ is equal to species‐specific blood P₅₀ values) from 21st century Earth System Model (ESM) projections included in the fifth phase of the Climate Model Intercomparison Project (CMIP5). Overall, we project P₅₀ depths to shoal, indicating likely habitat compression for tuna species due to climate change. Tunas that will be most impacted by shoaling are Pacific and southern bluefin tunas—habitat compression is projected for the entire geographic range of Pacific bluefin tuna and for the spawning region of southern bluefin tuna. Vertical shifts in P₅₀ depths will potentially influence resource partitioning among Pacific bluefin, bigeye, yellowfin, and skipjack tunas in the northern subtropical and eastern tropical Pacific Ocean, the Arabian Sea, and the Bay of Bengal. By establishing linkages between tuna physiology and environmental conditions, we provide a mechanistic basis to project the effects of anthropogenic climate change on tuna habitats.     

PCR primers for fish G6PD gene intron and characterization of intron length variation in the albacore Thunnus alalunga

Polymerase chain reaction (PCR) primers to amplify the fourth intron of glucose‐6‐phosphate dehydrogenase (G6PD) gene were designed. A large length variation of amplified fragment was observed in the Atlantic albacore sample with a moderate level of heterozygosity (H_E = 0.488). Nucleotide sequence analysis revealed deletion or insertion of a large nucleotide block (110 base pairs) to be responsible for the length difference. Successful amplification of single or two fragments was confirmed in the northern bluefin tuna and Pacific saury, indicating the wide cross‐species applicability.     

Longtail tuna,Thunnus tonggol (Bleeker, 1851): a global review of population dynamics,ecology, fisheries,and considerations for future conservation and management

Longtail tuna (Thunnus tonggol) is a neritic species that supports commercial, artisanal and recreational fisheries throughout the Indo-Pacific region. Historically receiving little attention by commercial fisheries, the global annual catch of longtail tuna has steadily risen from around 30,000 t in the early 1980s to exceeding 200,000 t since 2004, reaching a peak of 291,264 t in 2007, and was 281,613 t in 2017. Catches of longtail tuna in the Indian Ocean now exceed catches of principal commercial target species, such as albacore and bigeye tunas. A sequence of stock assessments undertaken throughout the species’ range since the late 1980s persistently indicated that at least three of the four stocks defined in this paper are likely to have been, and most likely are currently, subject to overfishing and overfished as a result of excess fishing effort on this relatively slow-growing and long-lived tuna species. As the spawning biomass of principal tuna target species continue to decline in both the Indian and western and central Pacific Oceans, the increasing catches of longtail tuna, other neritic tunas, and seerfishes is worrisome. Few conservation and management measures (CMMs) are currently in place specifically for longtail tuna, although in recent years some coastal States, Regional Fishery Bodies, and tuna Regional Fisheries Management Organisations have begun to develop initiatives to improve the catch and biological data quality for longtail tuna and sympatric species of neritic tunas and tuna-like species. This paper provides a global review of biological, ecological and fishery information to provide researchers, fishery managers and policy makers with the most current information from which to begin to guide future stock assessment and the development of CMMs for longtail tuna.

     

Cardicola opisthorchis n. sp. (Trematoda: Aporocotylidae) from the Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel, 1844), cultured in Japan

A new aporocotylid blood fluke is described, based on specimens from the ventricle of the Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel), cultured in Wakayama and Nagasaki Prefectures, Japan. The new species is morphologically similar to the members of the genus Cardicola Short, 1953, but shows distinct differences in the body form, location of the testis and the orientation of the ootype. The body of the new species is long and slender, whereas other Cardicola species are small and generally lanceolate. The testis is mostly located posterior to the caeca and anterior to the ovary, occupying 31–45% of body length, in contrast to the known Cardicola species, whose testis is typically intercaecal. The ootype is oriented anteriorly, while in most congeners, it is directed posteriorly or horizontally. Phylogenetic analyses of this aporocotylid, together with Cardicola orientalis Ogawa, Tanaka, Sugihara et Takami, 2010 from the same host, were conducted based on DNA sequences of the ITS2 rDNA and the 28S region of ribosomal RNA. The analyses revealed that the new blood fluke belongs to the genus Cardicola despite the marked morphological differences. Thus, this aporocotylid is named Cardicola opisthorchis n. sp. and the generic diagnosis is emended in this paper. In addition, 100% identity among the ITS2 sequences from the present species, Cardicola sp. from T. orientalis in Mexico and Cardicola sp. from the northern bluefin tuna, Thunnus thynnus (Linnaeus) in Spain suggests that C. opisthorchis n. sp. has a broad geographical distribution and that it infects both the Pacific and northern bluefin tuna.     

Phylogeography and phylogeny of the epineritic cosmopolitan bonitos of the genus Sarda (Cuvier): inferred patterns of intra‐ and inter‐oceanic connectivity derived from nuclear and mitochondrial DNA data

Aim To reconstruct the phylogenetic relationships of the four species of the genus Sarda (Sarda sarda, Sarda orientalis, Sarda australis and Sarda chilensis) and their phylogeographic history in the context of historical and ecological biogeography. Also, to reconstruct within‐species phylogenetic relationships to test whether the North Atlantic and Mediterranean populations of Atlantic bonito (S. sarda) warrant subspecies status, and the validity of the allopatric northern and southern populations of eastern Pacific bonito (S. chiliensis), recognized as S. chiliensis lineolata and S. chiliensis chiliensis. Location Representative samples of all four Sarda species collected world‐wide were analysed. Methods Phylogenetic inference was carried out with neighbour‐joining, maximum parsimony and maximum likelihood, employing nucleotide sequences of the mitochondrial DNA (mtDNA) control region I (CR‐I) and of the single‐copy nuclear DNA (nDNA) Tmo‐4c4 gene. Analysis of molecular variance was used on the mtDNA data to estimate the extent of geographic population structuring. Results Gene trees derived from mtDNA and nDNA data yielded concordant phylogenies that support the monophyly of the genus Sarda. The following sibling pairs received strong statistical support: striped bonito (S. orientalis) with Australian bonito (S. australis), and Atlantic bonito (S. sarda) with eastern Pacific bonito (S. chiliensis). Furthermore, the origin of S. sarda mtDNA is paraphyletic with respect to S. chiliensis, and these results are indicative of introgression. The analysis of Tmo‐4c4 sequences corroborates the ancestral hybridization between these allopatric species. Comparisons of north‐west Atlantic and Mediterranean populations of S. sarda using mtDNA CR‐I data revealed substantial genetic differentiation. By contrast, no differences between the putative northern and southern allopatric subspecies of S. chiliensis were detected. Main conclusions The monophyly of the genus Sarda as indicated by morphology is corroborated using both molecular markers. However, molecular phylogenies depicted a paraphyletic relationship between S. sarda and S. chiliensis. This phylogeographical relationship is better explained by an ancestral introgression facilitated by trans‐Arctic contact during the Pleistocene. The pronounced genetic differentiation between S. sarda samples from the north‐west Atlantic and the Mediterranean is consistent with the differentiation of these two regions, but not with the amphi‐Atlantic speciation hypothesis. Finally, the S. chiliensis lineolata and S. chiliensis chiliensis subspecies status is not supported by the molecular data.     

Habitat and behaviour of yellowfin tuna Thunnus albacares in the Gulf of Mexico determined using pop-up satellite archival tags

This study presents the first data on movement, habitat use and behaviour for yellowfin tuna Thunnus albacares in the Atlantic Basin. Six individuals were tracked in the Gulf of Mexico using pop-up satellite archival tags. Records up to 80 days in length were obtained, providing information on depth and temperature preferences as well as horizontal movements. Thunnus albacares in the Gulf of Mexico showed a strong preference for the mixed layer and thermocline, consistent with findings for this species in other ocean basins. Fish showed a diel pattern in depth distribution, remaining in surface and mixed layer waters at night and diving to deeper waters during the day. The vertical extent of T. albacares habitat appeared to be temperature limited, with fish generally avoiding waters that were >6° C cooler than surface waters. The vertical and thermal habitat usage of T. albacares differs from that of bigeye Thunnus obesus and bluefin Thunnus thynnus , Thunnus orientalis and Thunnus maccoyii tunas. These results are consistent with the results of earlier studies conducted on T. albacares in other oceans.     

Use of AFLP Analyses to Assess Genetic Variation in Morone and Thunnus Species

The amplified fragment length polymorphism (AFLP) technique was used to assess genetic variation across the genera of 2 Morone and 3 Thunnus species. The AFLP profiles from 23 primers revealed higher levels of polymorphism in each of 3 Thunnus species than in either of the 2 Morone species. However, extensive variation (20 of 23 primers) was observed between the 2 Morone species, but much less variation was observed among the 3 Thunnus species. In addition, comparisons of banding patterns indicated that albacore is divergent from both Atlantic northern bluefin tuna and yellowfin tuna. This result is consistent with the findings of several previous studies employing either allozymes or mitochondrial DNA. Overall, these results demonstrate that AFLP is a useful technique for the assessment of both intraspecies and interspecies variation of fish. Furthermore, the species-specific patterns produced by AFLP can be used for the identification of closely related species.     

Spawning of bluefin tuna in the black sea: historical evidence, environmental constraints and population plasticity

The lucrative and highly migratory Atlantic bluefin tuna, Thunnus thynnus (Linnaeus 1758; Scombridae), used to be distributed widely throughout the north Atlantic Ocean, Mediterranean Sea and Black Sea. Its migrations have supported sustainable fisheries and impacted local cultures since antiquity, but its biogeographic range has contracted since the 1950s. Most recently, the species disappeared from the Black Sea in the late 1980s and has not yet recovered. Reasons for the Black Sea disappearance, and the species-wide range contraction, are unclear. However bluefin tuna formerly foraged and possibly spawned in the Black Sea. Loss of a locally-reproducing population would represent a decline in population richness, and an increase in species vulnerability to perturbations such as exploitation and environmental change. Here we identify the main genetic and phenotypic adaptations that the population must have (had) in order to reproduce successfully in the specific hydrographic (estuarine) conditions of the Black Sea. By comparing hydrographic conditions in spawning areas of the three species of bluefin tunas, and applying a mechanistic model of egg buoyancy and sinking rate, we show that reproduction in the Black Sea must have required specific adaptations of egg buoyancy, fertilisation and development for reproductive success. Such adaptations by local populations of marine fish species spawning in estuarine areas are common as is evident from a meta-analysis of egg buoyancy data from 16 species of fish. We conclude that these adaptations would have been necessary for successful local reproduction by bluefin tuna in the Black Sea, and that a locally-adapted reproducing population may have disappeared. Recovery of bluefin tuna in the Black Sea, either for spawning or foraging, will occur fastest if any remaining locally adapted individuals are allowed to survive, and by conservation and recovery of depleted Mediterranean populations which could through time re-establish local Black Sea spawning and foraging.     

The Real maccoyii: Identifying Tuna Sushi with DNA Barcodes – Contrasting Characteristic Attributes and Genetic Distances

Background

The use of DNA barcodes for the identification of described species is one of the least controversial and most promising applications of barcoding. There is no consensus, however, as to what constitutes an appropriate identification standard and most barcoding efforts simply attempt to pair a query sequence with reference sequences and deem identification successful if it falls within the bounds of some pre-established cutoffs using genetic distance. Since the Renaissance, however, most biological classification schemes have relied on the use of diagnostic characters to identify and place species.

Methodology/Principal Findings

Here we developed a cytochrome c oxidase subunit I character-based key for the identification of all tuna species of the genus Thunnus, and compared its performance with distance-based measures for identification of 68 samples of tuna sushi purchased from 31 restaurants in Manhattan (New York City) and Denver, Colorado. Both the character-based key and GenBank BLAST successfully identified 100% of the tuna samples, while the Barcode of Life Database (BOLD) as well as genetic distance thresholds, and neighbor-joining phylogenetic tree building performed poorly in terms of species identification. A piece of tuna sushi has the potential to be an endangered species, a fraud, or a health hazard. All three of these cases were uncovered in this study. Nineteen restaurant establishments were unable to clarify or misrepresented what species they sold. Five out of nine samples sold as a variant of “white tuna” were not albacore (T. alalunga), but escolar (Lepidocybium flavorunneum), a gempylid species banned for sale in Italy and Japan due to health concerns. Nineteen samples were northern bluefin tuna (T. thynnus) or the critically endangered southern bluefin tuna (T. maccoyii), though nine restaurants that sold these species did not state these species on their menus.

Conclusions/Significance

The Convention on International Trade Endangered Species (CITES) requires that listed species must be identifiable in trade. This research fulfills this requirement for tuna, and supports the nomination of northern bluefin tuna for CITES listing in 2010.

–The question is not what you look at, but what you see Thoreau

     

Genetic identity of YOY bluefin tuna from the eastern and western Atlantic spawning areas

We used 320 young-of-the-year (YOY) specimens of the highly migratory and overfished Atlantic bluefin tuna, Thunnus thynnus, Linnaeus 1758, to evaluate the hypothesis that Atlantic bluefin tuna comprises 2 stocks with spawning grounds in the Gulf of Mexico and in the Mediterranean Sea. Significant genetic differentiation at 8 nuclear microsatellite loci (F(ST) = 0.0059, P = 0.0005) and at the mitochondrial control region (Phi(ST) = 0.0129, P = 0.0139) was detected among YOY Atlantic bluefin tuna captured on spawning grounds in the Gulf of Mexico (n = 40) versus the western (n = 255) and eastern (n = 25) basins of the Mediterranean Sea. The genetic divergence among spawning populations, combined with the extensive trans-Atlantic movements reported for juvenile and adult Atlantic bluefin tuna, indicates a high degree of spawning site fidelity. Recognition of genetically distinct populations necessitates independent management of Atlantic bluefin tuna on spawning grounds and warrants evaluation of the level of mixing of populations on feeding grounds. The genetic pattern might not have been detected unless juvenile specimens or actively spawning adults had been sampled.