Stock structure of tropical tuna in the Indian Ocean: an otolith microchemistry approach

258 p.Entre las especies que desempeñan un papel importante en la economía azul, se encuentran las denominadas como túnidos tropicales; el listado, Katsuwonus pelamis, el rabil Thunnus albacares y el patudo Thunnus obesus. Estas especies son altamente migratorias y habitan las aguas tropicales y subtropicales de los tres océanos principales (Atlántico, Índico y Pacífico). Dentro de su área de distribución geográfica, el océano Índico es un caladero de vital importancia para las capturas de estas tres especies, sin embargo, es del que menos información se posee. Aunque en la actualidad cada una de las especies de túnidos tropicales se gestiona como un sólo stock en el Océano Índico, siguen existiendo dudas sobre la conectividad y dinámica espacial de las mismas. En este contexto, el objetivo principal de esta tesis es investigar la estructura de los stocks de túnidos tropicales en el Océano Índico, mediante el análisis microquímico de los otolitos (pares de piedras que se encuentran en el oído interno de los peces).AZT

Chemical signatures in fin spine edge of Atlantic bluefin tuna (Thunnus thynnus) can serve as habitat markers of geographically distinct marine environments

Chemical fingerprints in otoliths are commonly used as natural habitat markers in fishes. Alternatively, the first dorsal fin spine can provide valuable chemical information and may be more suitable for studying (i) endangered fish species that cannot be sacrificed for their otoliths or (ii) fishes for which otoliths might not be available because of management or commercial reasons. Here, we studied multi-element chemistry of fin spine edges collected from Atlantic bluefin tuna (ABFT; Thunnus thynnus) (Linnaeus, 1758) to investigate the utility of the fin spine edge as a natural habitat marker. We determined stable isotopic δ18O and δ13C ratios, as well as concentrations of the tracer elements Mg, Mn, Li, Ba, and Sr, at the edge of ABFT fin spines, and then we used these measures to discriminate ABFT individuals among capture regions (i.e., the eastern Atlantic Ocean or Mediterranean Sea). Isotope ratios and tracer element concentrations, and especially a combined multi-element approach, were able to effectively discriminate individuals by capture region. The Mg, Mn, Li, and δ18O concentrations were the strongest variables driving this discrimination. Overall, our results demonstrate that chemical signatures are consistently retained in the ABFT fin spine edge and support the use of fin spine edges for discerning habitat use. The fin spine chemistry as a minimally invasive sampling method, combined with otolith chemistry, genetic markers, and tagging efforts can help us to reconstruct fish movements, providing a deeper understanding of the spatial population dynamics of this iconic fish species.The authors also wish to thank the many people who were involved in the collection of the fin spine samples used for this study under the provision of the ICCAT Atlantic Wide Research Programme for Bluefin Tuna (GBYP), which an ICCAT special research program funded by the European Union, several ICCAT CPCs, the ICCAT Secretariat, and other entities (see https://www.iccat.int/gbyp/en/overview.asp). The content of this paper does not necessarily reflect ICCAT's point of view or that of any of the other sponsors, who carry no responsibility. In addition, it does not indicate the Commission's future policy in this area. Special thanks to Pedro Lino and Ruben Muñoz-Lechuga from IPMA - Portuguese Institute for the Ocean and Atmosphere (Portugal), as provider of samples from the South of Portugal. Fulvio Garibaldi from UNIG - University of Genoa, Dept. of Earth, Life and Environment Sciences, for samples collected in the Ligurian Sea (Italy); Piero Addis and Rita Cannas from UNIC - Department of Life Science and Environment, University of Cagliari, for samples collected around Sardinian coast; F. Saadet Karakulak from ISTA - Department of Fisheries Technology and Management, Faculty of Aquatic Sciences, Istanbul University, for provider samples collected in the Levantine sea (Turkey); Antonio Celona from NECT - Necton Marine Research Society, for samples collected around Sicily (Italy), and Leif Nottestad from IMR - Institute of Marine Research, for providing samples collected in the Norwegian waters. We are so grateful for their efforts in collecting biological samples. Femtosecond Laser Ablation (fs-LA) analyses at the Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matériaux, Université de Pau et des Pays de l’Adour/CNRS (Pau, France) were conducted by Gaelle Barbotin as the engineer and under the supervision of Research engineer Dr. Christophe Pécheyran. We thank them for their help and assistance with technical issues. Stable Isotopes Analysis were conducted at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and we are grateful for their assistance. The contents of this manuscript do not necessarily reflect the point of view of ICCAT or of the other funders, neither do they necessarily reflect the views of the funders and in no ways anticipate the Commission's future policy in this area. Editing help was provided by Science Journal Editors, Inc

Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean

Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2–4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available

Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean

Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2–4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available

Discrimination of yellowfin tuna Thunnus albacares between nursery areas in the Indian Ocean using otolith chemistry

International audienceYellowfin tuna Thunnus albacares is a highly exploited species in the Indian Ocean. Yet, its stock structure is still not well understood, hindering assessment of the stock at a suitable spatial scale for management. Here, young-of-the-year (\textless4 mo) yellowfin tuna otoliths were collected in 2018 and 2019, from 4 major nursery areas in the Indian Ocean: Madagascar, Seychelles-Somalia, Maldives and Sumatra. First, direct age estimates were made in a subset of otoliths by visually counting microincrements to identify the portion of the otolith corresponding to the larval stage. We then developed 2-dimensional maps of trace element concentrations to examine spatial distribution of elements across otolith transverse sections. Different distribution patterns were observed among the elements analysed; Li, Sr and Ba were enriched in the portion of the otolith representing early life, whereas Mn and Mg concentrations were heterogeneous across growth bands. Last, we analysed inter-annual and regional variation in otolith chemical composition using both trace elements (Li, Mg, Sr, Ba and Mn) and stable isotopes (δ13C and δ18O). Significant regional variation in otolith chemical signatures was detected among nurseries, except between Madagascar and Seychelles-Somalia. Otolith δ13C and δ18O were important drivers of differentiation between western (Madagascar and Seychelles-Somalia), Maldives and Sumatra nurseries, whereas the elemental signatures were cohort specific. Overall nursery assignment accuracies were 69-71%. The present study demonstrates that baseline chemical signatures in the otoliths of yellowfin tuna are regionally distinct and can be used as a natural tag to investigate the nursery origin of older individuals in the Indian Ocean

Otolith δ18O Composition as a Tracer of Yellowfin Tuna (Thunnus albacares) Origin in the Indian Ocean

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 specie

Otolith microchemistry suggests probable population structuring in the Indian Ocean for the broadbill swordfish Xiphias gladius

Variation in otolith elemental fingerprints was investigated in the broadbill swordfish (Xiphias gladius) to complement genetic data obtained by next generation sequencing in the framework of a collaborative project on population stock structure of tuna, billfish and sharks of the Indian Ocean (PSTBS-IO). Swordfish specimens for this work were sampled in the southwest (SWI), west central (WCI) and southeast (SEI) regions of the Indian Ocean. A total of 70 otoliths (30 from SWI and 20 from each WCI and SEI) were selected and the elemental signatures of their cores were analysed by LA-ICP-MS to investigate potential differences in spawning origin among regions. Among the 15 chemical elements analysed, only Mg, P, Sr, Ba and B were above detection limits and significantly contributed to the variation in otolith core composition. Based on differences in these five elements, three groups of distinct multi-elemental signatures, denoting potentially discrete spawning origins (SpO), were identified using hierarchical clustering based on Euclidian distances. All SpO identified apparently contributed to the swordfish stocks of the three regions sampled, but in different proportions. SpO-1 was the most common spawning source among the fish sampled (49%); it probably corresponds to the swordfish spawning ground located between northeast Australia and Indonesia. SpO-3 was found to provide 34% of the total fish analysed, but mainly in SWI (53%) and WCI (35%). It could correspond to the spawning grounds reported for the species in the central and southwestern Indian Ocean. Lastly, SpO-2, which contributed to only 17% of the total fish analysed (mainly in SEI and WCI), may correspond to the spawning ground previously reported in the northwestern Indian Ocean, off the Somalian coast. Although our results show mixed origins in the fish sampled at each sampling location, the contrast in otolith core fingerprints between SWI and SEI otoliths suggests differences in main spawning origin, at least for the swordfish captured in these two regions of the Indian Ocean. Additional analyses are needed to consolidate these results, as well as information on the spatiotemporal distribution of chemical tracers in the water masses of the Indian Ocean to assign regions to otolith elemental signatures

New Insights in Lifetime Migrations of Albacore Tuna (Thunnus alalunga, Bonnaterre, 1788) between the Southwest Indian and the Southeast Atlantic Oceans Using Otolith Microchemistry

International audienceTo 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

Otolith chemical fingerprints of skipjack tuna (Katsuwonus pelamis) in the Indian Ocean: First insights into stock structure delineation

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

Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean

Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2–4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available