Alex Hanke

Determination of Lmax for Atlantic Bluefin Tuna, Thunnus thynnus (L.), from Meta-Analysis of Published and Available Biometric Data

A meta-analysis of the straight fork lengths (herewith abbreviated as L) of 2,458,028 Atlantic bluefin tuna, Thunnus thynnus (L.), taken from 224 scientific publications and unpublished L data from scientific organizations and fishing companies spanning most of the known Atlantic and Mediterranean Atlantic bluefin tuna fisheries dating from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 ICCATs 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 (Lt = 314.90 [1 – e−0.089 (t +1.13)]). 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 Fultons condition factor (K) between 1.4 and 2.6 and appropriate L-W relationships to correct this situation in the future.

Otolith shape variation provides a marker of stock origin for north Atlantic bluefin tuna (Thunnus thynnus)

Two stocks of bluefin tuna (Thunnus thynnus) inhabit the north Atlantic; the western and eastern stocks spawn in the Gulf of Mexico and the Mediterranean Sea respectively. Trans-Atlantic movements occur outside spawning time whereas natal homing maintains stock structure. Commercial fisheries may exploit a mixed assemblage of both stocks. The incorporation of mixing rates into stock assessment is precluded by uncertainties surrounding stock discrimination. Otolith shape descriptors were used to characterise western and eastern stocks of Atlantic bluefin tuna in the present study and to estimate stock composition in catches of unknown origin. Otolith shape varied with length and between locations and years. Within a restricted size range (200–297-cm fork length (FL)) the two stocks were distinguished with an accuracy of 83%. Bayesian stock mixture analysis indicated that samples from the east Atlantic and Mediterranean were predominantly of eastern origin. The proportion assigned to the eastern stock showed slight spatial variation; however, overlapping 95% credible intervals indicated no significant difference (200–297cm FL: central Atlantic, 73–100%; Straits of Gibraltar, 73–100%; Morocco, 50–99%; Portugal 64–100%). Otolith shape could be used in combination with other population markers to improve the accuracy of mixing rate estimates for Atlantic bluefin tuna.

Spatial dynamics and mixing of bluefin tuna in the Atlantic Ocean and Mediterranean Sea revealed using next-generation sequencing

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 (FST = 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.