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To clarify potential trans-oceanic connectivity and variation in the natal origin of albacore tuna (Thunnus alalunga) from the southwest Indian Ocean (SWI) and the southeast Atlantic (SA), lifetime otolith elemental signatures were assessed from 46 adults sampled from Reunion Island, and 26 juveniles(group 2+) sampled from two locations along the Atlantic coast of South Africa. LA-ICP-MS analysis was used to assess the multi-elemental composition in B, Ba, Mg, P, Sr, and Zn along the otolith edge (chemical signatures of the capture area), but also near the otolith primordium (spawning origin) and in an area located at 1400–1600 µm from it (nursery origin). Two groups of distinct near-primordium multi-elemental signatures, denoting potentially discrete spawning origins (SpO), were identified using hierarchical clustering. Each of the two SpO was found to contribute to the albacore stocks from all the areas sampled, suggesting a common spawning origin in some fish from the SWI and from the SA, and complex migrations between the two oceans. Three potentially discrete primary nursery sites were identified, each contributing to SA juvenile and SWI adult capture sites differently. The timing for the trans-oceanic movements observed for each albacore capture zone and its implications for local stock management are discussed.

Variation in otolith elemental fingerprints was investigated in albacore tunas (Thunnus alalunga) sampled in the southwest Indian Ocean (SWI) and along the Atlantic coast of South Africa (SA). A total of 72 otoliths were selected, from 46 adult fish captured around the Reunion Island (SWI) and 26 juvenile and sub-adults sampled at two locations off the South African coast (SA-N and SA-S, n = 13 per location). LA-ICP-MS was used to assess the signatures in 15 chemical elements at all otolith cores (to investigate potential differences in fish spawning origin among regions) and along all otolith edges (to characterize the chemical signatures of fish capture areas). Among the 15 chemical elements analysed, only Mg, P, Zn, Sr, Ba, B and Cu were above detection limits and significantly contributed to the variation in otolith composition. Based on differences in these elements, two groups of distinct multi-elemental signatures, denoting potentially discrete spawning origins (SpO), were identified at the otolith cores using hierarchical clustering based on Euclidian distances. Each of the two potential SpO contributed to the tuna sampled in all three areas, suggesting a common origin in some fish caught in the Atlantic and in the Indian Ocean and important trans-oceanic migrations between these two Oceans. The possible location of the two spawning areas is discussed based on the signatures recorded on the otolith edges before the final capture of the fish, in both oceans. This study was part of a collaborative project on the population structure of tuna, billfish and sharks of the Indian Ocean (PSTBS-IO).

Yellowfin tuna of the Indian Ocean is overfished, and a better understanding of the stock structure is needed to enable sustainable management. Here, otolith δ18O values of young-of-the-year fish from known nursery areas of the equatorial Indian Ocean (West, Central and East) were used to establish a reference isotopic signature to predict the origin of sub-adult and adult individuals. Sub-adult tuna otolith δ18O values from Reunion Island were similar to the West nursery signature, but otolith δ18O values of sub-adults from Pakistan were unlike any of the nurseries sampled. Therefore, δ18O values from the Pakistan area samples were considered an additional nursery source for predicting the origin of adult tuna, using a multinomial logistic regression classification method. The western equatorial area was the most productive nursery for three fishing grounds sampled, with a minor contribution of Pakistan-like origin fish. Contribution of Central or East nurseries to the adult population was negligible. One adult otolith was analysed by secondary ion mass spectrometry along the otolith growth transect and results were compared with an isoscape approach to infer lifetime movements. This study is an important first step towards understanding the spatial structure and connectivity of the species

The chemical composition of otoliths (earbones) can provide valuable information about stock structure and connectivity patterns among marine fish. For that, chemical signatures must be sufficiently distinct to allow accurate classification of an unknown fish to their area of origin. Here we have examined the suitability of otolith microchemistry as a tool to better understand the spatial dynamics of skipjack tuna (Katsuwonus pelamis), a highly valuable commercial species for which uncertainties remain regarding its stock structure in the Indian Ocean. For this aim, we have compared the early life otolith chemical composition of young-of-the-year (<6 months) skipjack tuna captured from the three main nursery areas of the equatorial Indian Ocean (West, Central and East). Elemental (Li:Ca, Sr:Ca, Ba:Ca, Mg:Ca and Mn:Ca) and stable isotopic (δ13C, δ18O) signatures were used, from individuals captured in 2018 and 2019. Otolith Sr:Ca, Ba:Ca, Mg:Ca and δ18O significantly differed among fish from different nurseries, but, in general, the chemical signatures of the three nursery areas largely overlapped. Multivariate analyses of otolith chemical signatures revealed low geographic separation among Central and Eastern nurseries, achieving a maximum overall random forest cross validated classification success of 51%. Cohort effect on otolith trace element signatures was also detected, indicating that variations in chemical signatures associated with seasonal changes in oceanographic conditions must be well understood, particularly for species with several reproductive peaks throughout the year. Otolith microchemistry in conjunction with other techniques (e.g., genetics, particle tracking) should be further investigated to resolve skipjack stock structure, which will ultimately contribute to the sustainable management of this stock in the Indian Ocean.

Skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna are the target species of tropical tuna fisheries in the Indian Ocean, with high commercial value in the international market. High fishing pressure over the past three decades has raised concerns about their sustainability. Understanding life history strategies and stock structure is essential to determine species resilience and how they might respond to exploitation. Here we provide a comprehensive review of available knowledge on the biology, ecology, and stock structure of tropical tuna species in the Indian Ocean. We describe the characteristics of Indian Ocean tropical tuna fisheries and synthesize skipjack, yellowfin, and bigeye tuna key life history attributes such as biogeography, trophic ecology, growth, and reproductive biology. In addition, we evaluate the available literature about their stock structure using different approaches such as analysis of fisheries data, genetic markers, otolith microchemistry and tagging, among others. Based on this review, we conclude that there is a clear lack of ocean basin-scale studies on skipjack, yellowfin and bigeye tuna life history, and that regional stock structure studies indicate that the panmictic population assumption of these stocks should be investigated further. Finally, we identify specific knowledge gaps that should be addressed with priority to ensure a sustainable and effective management of these species.

This report provides the presentations, discussions and conclusions from the ICCAT bluefin tuna workshop for direct ageing held in Santander, Spain, in April 2006. The report summarizes the ageing criteria used in the past and the agreements on future age determination based on otoliths, vertebrae and spines. Advantages and disadvantages of each calcified structure for ageing and border interpretation were discussed. It was considered that bluefin tuna age interpretation becomes very difficult from age ten onwards using the whole vertebra and the spine sections methods, but this last technique continues to be useful for older ages. Otolith sections can be used for the whole age range. Participants agreed that none of these three structures could be excluded from routine ageing because otoliths are not easily available. Age estimations within the same structure and between different structures of the same specimen were compared for several readers. Better precision was found between spine readers compared to vertebra and otolith readers. Good age agreement was also achieved between readers of spines and vertebrae from the same bluefin for ages less than 12 years. Preliminary results from radiocarbon assays on otoliths were presented at the workshop and gave promising outcomes for bluefin tuna age validation. Also, these suggested that bluefin tuna can live longer than had previously been established and that a review is needed of the currently used asymptotic size and growth rate for both stocks. Another important contribution of the workshop was a manual for age interpretation., Le présent rapport recueille les présentations, discussions et conclusions de l’Atelier de l’ICCAT chargé de la détermination directe de l’âge du thon rouge, tenu à Santander (Espagne) au mois d’avril 2006. Le rapport résume les critères employés par le passé pour interpréter l’âge et les accords pour la détermination future de l’âge à partir des otolithes, vertèbres et épines. L’Atelier a discuté des avantages et des inconvénients de chaque structure calcifiée pour déterminer l’âge et l’interprétation du type de bord. On a abordé la difficulté de l’interprétation de l’âge des thons de plus de 10 ans au moyen de la vertèbre entière et des sections des épines, bien que cette dernière méthode continue d’être utile pour les âges avancés. Les sections d’otolithes peuvent être employées pour toute la gamme d’âges. Les participants ont convenu qu’aucune de ces trois structures ne doit être exclue pour l’interprétation de l’âge parce qu’il n’est pas toujours possible d’obtenir des otolithes. On a comparé les lectures de l’âge à l’intérieur de la même structure et entre différentes structures du même exemplaire pour divers lecteurs. On a obtenu une plus grande précision parmi les lecteurs d’épines que parmi les lecteurs de vertèbres et d’otolithes. On a également obtenu un bon accord entre les lecteurs d’épines et de vertèbres originaires du même exemplaire pour les âges inférieurs à 12 ans. Les résultats préliminaires des essais de radiocarbone dans les otolithes ont été présentés à l’Atelier, offrant de bonnes perspectives pour son utilisation dans la validation de l’âge. Ces résultats indiquent aussi que le thon rouge a une plus grande longévité que ce qui avait été auparavant établi et qu’il est nécessaire de réviser la longueur asymptotique et le taux de croissance actuellement utilisés. L’élaboration d’un manuel aux fins de l’interprétation de l’âge a constitué une autre contribution importante de l’Atelier., Este informe recoge las presentaciones, discusiones y conclusiones del congreso de ICCAT para la determinación directa de la edad de atún rojo, celebrado en Santander, España, en abril de 2006. El informe resume los criterios empleados en el pasado para interpretar la edad y los acuerdos para la determinación futura de la edad a partir de otolitos, vértebras y espinas. Se discutieron las ventajas y los inconvenientes de cada estructura calcificada para determinar la edad y la interpretación del tipo borde. Se planteó la dificultad en la interpretación de la edad de atunes mayores de 10 años utilizando la vértebra entera y las secciones de espinas, no obstante este último método continúa siendo útil para edades mayores. Las secciones de otolitos pueden ser empleadas para todo el rango de edades. Los participantes acordaron que ninguna de estas tres estructuras deben excluirse para la interpretación de la edad porque no siempre es posible obtener los otolitos. Se compararon las lecturas de edad dentro de la misma estructura y entre diferentes estructuras del mismo ejemplar para varios lectores. Se obtuvo una mayor precisión entre lectores de espinas comparada con las obtenidas por los lectores de vértebras y otolitos. También se obtuvo un buen acuerdo entre lectores de espinas y vértebras procedentes del mismo ejemplar para edades menores de 12 años. Los resultados preliminares de las pruebas de radiocarbono en otolitos fueron presentados en el congreso, proporcionando buenas expectativas para su uso en la validación de la edad. Estos resultados también indican que el atún rojo es más longevo de lo que se consideraba y que es necesaria una revisión de la longitud asintótica y de la tasa de crecimiento empleadas actualmente. Otra importante contribución del congreso fue la elaboración de un manual para la interpretación de la edad.

Biological parameters such as age, growth and age (or size) at maturity are vital for accurate stock assessments and management plans to ensure that fisheries develop sustainably. Despite this, very few validated age studies have been conducted for large tropical pelagic species within the Australian region. Age and growth parameters were estimated for bigeye tuna, Thunnus obesus (Lowe, 1839), sampled from longline fisheries in the Australian region using validated techniques based on counts of annual increments. Poor increment clarity reduced the number of otoliths included in the final analysis to only 50% of the 3200 selected for reading (39–178-cm fork length). Microincrement analysis confirmed the position of the first two annual increments in these otoliths. A maximum age of 16 years was obtained, but over 80% of fish in the Australian catch were <5 years old. Growth is most rapid in the first few years of life and asymptotic length is reached at about age 9 to 10 years. The von Bertalanffy growth parameters were estimated at L∞ = 169.09, k = 0.238, and to = –1.706 for the south-west Pacific Ocean and L∞ = 178.41, k = 0.176, and to = –2.500 for the eastern Indian Ocean. These parameters were significantly different, suggesting that there is little mixing between populations in the Pacific and Indian Oceans. Length at 50% maturity for females sampled in northern Queensland was estimated to be 102.4-cm fork length.