Assessment and management of Atlantic bluefin tuna Thunnus thynnus populations is hindered by our lack of knowledge regarding trans-Atlantic movement and connectivity of eastern and western populations. Here, we evaluated migratory and homing behaviors of bluefin tuna in several regions of the North Atlantic Ocean and Mediterranean Sea using chemical tags (δ 13 C and δ 18 O) in otoliths. Significant emigration of bluefin tuna from their place of origin was inferred from otolith δ 13 C and δ 18 O, with both eastern and western bluefin tuna commonly 'crossing the line' (45° W management boundary) in the Central North Atlantic Ocean and mixing with the other population. Several western migrants were also detected in Moroccan traps off the coast of Africa, indicating that trans-Atlantic movement occurs for members of the western population; however, the degree of mixing declined with proximity to the eastern spawning area (Mediterranean Sea). The origin of bluefin tuna collected at the entrance to the Strait of Gibraltar and from several regions within the Mediterranean Sea (Balearic Islands, Malta, and Sardinia) was essentially 100% eastern fish, demonstrating that natal homing is well developed by the eastern population, with western migrants rarely entering the Mediterranean Sea.
Effective sustainable management of marine fisheries requires that assessed management units (that is, fish stocks) correspond to biological populations. This issue has long been discussed in the context of Atlantic bluefin tuna (ABFT, Thunnus thynnus) management, which currently considers two unmixed stocks but does not take into account how individuals born in each of the two main spawning grounds (Gulf of Mexico and Mediterranean Sea) mix in feeding aggregations throughout the Atlantic Ocean. Using thousands of genome‐wide molecular markers obtained from larvae and young of the year collected at the species’ main spawning grounds, we provide what is, to the best of our knowledge, the first direct genetic evidence for “natal homing” in ABFT. This has facilitated the development of an accurate, cost‐effective, and non‐invasive tool for tracing the genetic origin of ABFT that allows for the assignment of catches to their population of origin, which is crucial for ensuring that ABFT management is based on biologically meaningful stock units rather than simply on catch location.
Uncertainty regarding the movement and population exchange of Atlantic bluefin tuna (Thunnus thynnus) from the two primary spawning areas (Gulf of Mexico, Mediterranean Sea) is increasingly implicated as a major impediment for the conservation of this species. Here, two mixture methods were applied to natural chemical markers (δ 18 O and δ 13 C) in otoliths (ear stones) to comprehensively investigate the nature and degree of transoceanic movement and mixing of eastern and western populations in several areas of the North Atlantic Ocean that potentially represent mixing hotspots. Areas investigated occurred on both sides of the 45 • W management boundary as well as waters off the coast of Africa (Morocco, Canary Islands) where both populations are known to occur. Projections of population composition (i.e., natal or nursery origin) from a multinomial logistic regression (MLR) classification method with different probability thresholds were generally in agreement with maximum likelihood estimates from the commonly used mixed-population program HISEA; however, predicted contributions for the less abundant population were occasionally higher for MLR estimates. Both MLR and HISEA clearly showed that mixing of Atlantic bluefin tuna in the Central North Atlantic Ocean was highly variable from year to year with expatriates of eastern or western origin commonly crossing into the other management area. Pronounced transoceanic movement and mixing of western migrants was also present off the coast of Africa, with the occurrence of western migrants in the Canary Islands and Morocco ranging from zero to the majority of the individuals assayed for the years examined. Results indicate highly variable rates of movement and population exchange for Atlantic bluefin tuna, highlighting the need for temporally resolved estimates of natal origin in mixing hotspots to improve population models used to evaluate the status of this threatened species.
The Atlantic bluefin tuna is a highly migratory species emblematic of the challenges associated with shared fisheries management. In an effort to resolve the species' stock dynamics, a genomewide search for spatially informative single nucleotide polymorphisms (SNPs) was undertaken, by way of sequencing reduced representation libraries. An allele frequency approach to SNP discovery was used, combining the data of 555 larvae and young-of-the-year (LYOY) into pools representing major geographical areas and mapping against a newly assembled genomic reference. From a set of 184,895 candidate loci, 384 were selected for validation using 167 LYOY. A highly discriminatory genotyping panel of 95 SNPs was ultimately developed by selecting loci with the most pronounced differences between western Atlantic and Mediterranean Sea LYOY. The panel was evaluated by genotyping a different set of LYOY (n = 326), and from these, 77.8% and 82.1% were correctly assigned to western Atlantic and Mediterranean Sea origins, respectively. The panel revealed temporally persistent differentiation among LYOY from the western Atlantic and Mediterranean Sea (F = 0.008, p = .034). The composition of six mixed feeding aggregations in the Atlantic Ocean and Mediterranean Sea was characterized using genotypes from medium (n = 184) and large (n = 48) adults, applying population assignment and mixture analyses. The results provide evidence of persistent population structuring across broad geographic areas and extensive mixing in the Atlantic Ocean, particularly in the mid-Atlantic Bight and Gulf of St. Lawrence. The genomic reference and genotyping tools presented here constitute novel resources useful for future research and conservation efforts.
spanning most of the known Atlantic and Mediterranean Atlantic bluefin tuna fisheries dating from 1605 to 2011, give L values ranging from L min = 20 cm and L max = 330 cm. The results indicate that the parameter L ∞ = 318.85 cm of the growth equation used by ICCAT's Standing Committee on Research and Statistics Atlantic bluefin tuna assessment group for the eastern stock (Lt = 318.85 [1 -e −0.093 (t + 0.97) ]) lies within the confidence limits of the maximum Ls presented in the study: L max = 319.93 ± 11.3 cm, confirming that this equation perfectly fits the biology of the growth of this species. These conclusions are also valid for the equation for the western stock ]). The ICCAT Atlantic bluefin tuna database contains numerous records of Atlantic bluefin tuna L outside the biological feasibility, and solutions are provided to recognize and remove these outliers based on the application of fixed values of Fulton's condition factor (K) between 1.4 and 2.6 and appropriate L-W relationships to correct this situation in the future.
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