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Bigeye tuna (Thunnus obesus, Lowe, 1839) is one of the eight recognized species of the genus Thunnus. It is considered a tropical species distributed in the Atlantic, Pacific and Indian Oceans. To date, no validated presence of this species has been reported inside the Mediterranean Sea. This study, however, confirms, for the first time, the presence of three young individuals of this species within the Mediterranean Sea., (© 2021 The Authors. Journal of Fish Biology published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.)

Albacore tuna (Thunnus alalunga) is an important target of tuna fisheries in the Atlantic and Indian Oceans. The commercial catch of albacore is the highest globally among all temperate tuna species, contributing around 6% in weight to global tuna catches over the last decade. The accurate assessment and management of this heavily exploited resource requires a robust understanding of the species' biology and of the pattern of connectivity among oceanic regions, yet Indian Ocean albacore population dynamics remain poorly understood and its level of connectivity with the Atlantic Ocean population is uncertain. We analysed morphometrics and genetics of albacore (n = 1,874) in the southwest Indian (SWIO) and southeast Atlantic (SEAO) Oceans to investigate the connectivity and population structure. Furthermore, we examined the species' dispersal potential by modelling particle drift through major oceanographic features. Males appear larger than females, except in South African waters, yet the length-weight relationship only showed significant male-female difference in one region (east of Madagascar and Reunion waters). The present study produced a genetic differentiation between the southeast Atlantic and southwest Indian Oceans, supporting their demographic independence. The particle drift models suggested dispersal potential of early life stages from SWIO to SEAO and adult or sub-adult migration from SEAO to SWIO.

Yellowfin tuna, Thunnus albacares, is one of the most important seafood commodities in the world. Despite its great biological and economic importance, conflicting evidence arises from classical genetic and tagging studies concerning the yellowfin tuna population structure at local and global oceanic scales. Access to more powerful and cost effective genetic tools would represent the first step towards resolving the population structure of yellowfin tuna across its distribution range. Using a panel of 939 neutral Single Nucleotide Polymorphisms (SNPs), and the most comprehensive data set of yellowfin samples available so far, we found genetic differentiation among the Atlantic, Indian and Pacific oceans. The genetic stock structure analysis carried out with 33 outlier SNPs, putatively under selection, identified discrete populations within the Pacific Ocean and, for the first time, also within the Atlantic Ocean. Stock assessment approaches that consider genetic differences at neutral and adaptive genomic loci should be routinely implemented to check the status of the yellowfin tuna, prevent illegal trade, and develop more sustainable management measures.

Genetic structure of skipjack tuna Katsuwonus pelamis from the Indian region was investigated using sequence data of mitochondrial DNA (mtDNA) D-loop region. A total of 315 individuals were sampled from six major fishing grounds around the east and west coasts of India including the Andaman (Port Blair) and Lakshadweep (Minicoy) Islands. Nucleotide and gene diversities were high in all the sample collections. Significant genetic heterogeneity was observed for the mtDNA sequence data among sites (φ(ST) = 0·0273, P < 0·001). Analysis of molecular variance (AMOVA) showed significant genetic variation among four groups (φ(CT) = 0·0261, P < 0·05) which was also supported by spatial AMOVA results. The null hypothesis of single panmictic population of K. pelamis along the Indian coast can thus be rejected. Phylogenetic analysis of the mtDNA sequence data showed the presence of four clades of K. pelamis in the Indian waters. There was no clear pattern, however, of haplotypes and geographic location among samples. The results of this study suggest the occurrence of four genetically differentiated groups of K. pelamis across the coastal waters of India., (© 2012 The Authors. Journal of Fish Biology © 2012 The Fisheries Society of the British Isles.)

The population genetic structure and phylogeography of wahoo, Acanthocybium solandri, were investigated on a global scale with intron six of lactate dehydrogenase-A (ldhA6, 8 locations, N = 213) and mtDNA cytochrome b (Cytb, 10 locations, N = 322). Results show extensive sharing of haplotypes across the wahoo's entire global range, and analyses were unable to detect significant structure (nuclear F(ST) = 0.0125, P = 0.106; mtDNA Phi(ST) < 0.0001, P = 0.634). Power analyses indicated 95% confidence in detecting nuclear F(ST) > or = 0.0389 and mtDNA Phi(ST) > or = 0.0148. These findings appear unique, as most other tunas, billfishes, and oceanic sharks exhibit significant population structure on the scale of East-West Atlantic, Atlantic vs. Indian-Pacific, or East-West Pacific. Overall nuclear heterozygosity (H = 0.714) and mtDNA haplotype diversity (h = 0.918) are both high in wahoo, while overall mtDNA nucleotide diversity (pi = 0.006) and nuclear nucleotide diversity (pi = 0.004) are uniformly low, indicating a recent increase in population size. Coalescence analyses yield an estimate of effective female population size (NeF) at approximately 816,000, and a population bottleneck approximately 690,000 years ago. However, conclusions about population history from our Cytb data set are not concordant with a control region survey, a finding that will require further investigation. This is the first example of a vertebrate with a single globally distributed population, a finding we attribute to extensive dispersal at all life stages. The indications of a worldwide stock for wahoo reinforce the mandate for international cooperation on fisheries issues.

Background: Large pelagic fishes are generally thought to have little population genetic structuring based on their cosmopolitan distribution, large population sizes and high dispersal capacities. However, gene flow can be influenced by ecological (e.g. homing behaviour) and physical (e.g. present-day ocean currents, past changes in sea temperature and levels) factors. In this regard, Atlantic bigeye tuna shows an interesting genetic structuring pattern with two highly divergent mitochondrial clades (Clades I and II), which are assumed to have been originated during the last Pleistocene glacial maxima. We assess genetic structure patterns of Atlantic bigeye tuna at the nuclear level, and compare them with mitochondrial evidence., Results: We examined allele size variation of nine microsatellite loci in 380 individuals from the Gulf of Guinea, Canary, Azores, Canada, Indian Ocean, and Pacific Ocean. To investigate temporal stability of genetic structure, three Atlantic Ocean sites were re-sampled a second year. Hierarchical AMOVA tests, RST pairwise comparisons, isolation by distance (Mantel) tests, Bayesian clustering analyses, and coalescence-based migration rate inferences supported unrestricted gene flow within the Atlantic Ocean at the nuclear level, and therefore interbreeding between individuals belonging to both mitochondrial clades. Moreover, departures from HWE in several loci were inferred for the samples of Guinea, and attributed to a Wahlund effect supporting the role of this region as a spawning and nursery area. Our microsatellite data supported a single worldwide panmictic unit for bigeye tunas. Despite the strong Agulhas Current, immigration rates seem to be higher from the Atlantic Ocean into the Indo-Pacific Ocean, but the actual number of individuals moving per generation is relatively low compared to the large population sizes inhabiting each ocean basin., Conclusion: Lack of congruence between mt and nuclear evidences, which is also found in other species, most likely reflects past events of isolation and secondary contact. Given the inferred relatively low number of immigrants per generation around the Cape of Good Hope, the proportions of the mitochondrial clades in the different oceans may keep stable, and it seems plausible that the presence of individuals belonging to the mt Clade I in the Atlantic Ocean may be due to extensive migrations that predated the last glaciation.

Two mitochondrial DNA segments of the bigeye tuna (Thunnus obesus) were amplified by polymerase chain reaction (PCR), and restriction fragment length polymorphism (RFLP) analyses of these segments were used for the genetic stock study. The variation in a segment flanking the ATPase and COIII genes was low; only two genotypes (alpha and beta) were detected by RsaI digestion. Yet a large difference in the genotype distribution was observed between ocean basin samples. The alpha type predominated in four Atlantic samples, where 178 of 244 individuals were the alpha type. In contrast, only one of 195 individuals collected in the Indo-Pacific was the alpha type? The frequency of the alpha type varied considerably from 0 to 80% among seven samples collected off the Cape of Good Hope. The variation found in the other segment, containing the D-loop region, was much higher; two endonucleases (DpnII and RsaI) detected five genotypes each and 15 composite genotypes. A highly significant difference in genotype frequencies was observed between the Atlantic and Indo-Pacific samples, but no heterogeneity was observed among the four Atlantic or among four Indo-Pacific samples. These results clearly indicate that not only gene flow, but also fish migration, between the Atlantic and Indian Oceans are severely restricted, and that fishes from these distinct stocks are intermingling around South Africa. The simple and diagnostic genetic marker found in this study can be used to estimate mixing ratios between Atlantic and Indian stocks around South Africa.