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Marine functional connectivity (MFC) refers to the dynamic spatial exchange of biomass, individuals, genes, and energy via the collective movements of all marine organisms during their lifetimes. In addition to controlling the distribution and resilience of marine biodiversity and exploited stocks, MFC plays a key role in the structure and functioning of ecosystems, at sea and at the land-sea interface. As marine ecosystems and their species face climate change and unprecedented multiple anthropogenic pressures, rapid action is needed to comprehend MFC patterns and their changes in order to anticipate the fates of ocean services to humanity. Despite many advances in techniques to measure or infer marine species’ distributions and spatial dynamics, significant progress is still necessary. A full understanding of MFC requires better knowledge of the relationships between marine communities and their habitats, quantification of fluxes of matter and energy, and the capacity to forecast how the many services provided by the ocean may change. The 17 papers in this Theme Section showcase the range of approaches and scales applied in contemporary MFC studies and encompass the diversity of ecosystems and taxa investigated worldwide. The innovative approaches presented here to advance MFC science pave the way to enrich current understanding of MFC’s role in ecosystem functioning, and flag how MFC knowledge can be better applied to protect marine resources and manage marine and littoral habitats.

This study aimed to evaluate the importance of inshore estuarine habitats for the maintenance of offshore exploited stocks of the horse-eye jack Caranx latus, a marine tropical species of high economic value. Otoliths of 143 yearlings were used to develop a data set of multi-elemental fingerprints for different juvenile habitats of C. latus in northeastern Brazil, including 7 estuaries and the coastal zone. This data set was then compared to the signatures in the juvenile part of the otolith of 40 sub-adult and adult fish caught at sea to identify their origin. Although otolith multi-elemental concentrations overlapped for some of the estuaries investigated, the maximum overall discrimination accuracy between them was only 58%. However, grouping several estuaries according to their similarity in elemental signatures increased accuracy to 80%. In both cases, correct re-assignment rates for the coastal zone were above 94%, and strontium (Sr) was the most significant element in juvenile habitat discrimination. The remaining elements (B, Ba, Co, P, Rb, and Zn) allowed us to distinguish between 3 distinct groups of estuaries, for which correct re-assignment rates were 70-88%. Juvenile fingerprints in sub-adult and adult otoliths revealed that most of the fish in the local, exploited stock (75%) originate from an inshore estuarine habitat, particularly from larger estuaries under constant marine influence (32.5%). This biological connectivity between continental and coastal waters should be considered for the conservation of tropical marine coastal stocks in general, as it is likely to apply to other exploited marine species, in Brazil and elsewhere.

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.

Le projet INPOLAG (2019-2023) s’inscrit dans le contexte réglementaire de la Directive Cadre sur l’Eau (DCE) et vise à développer un indicateur "poissons DCE-compatible" adapté au contexte français. Basé sur un échantillonnage à deux saisons (printemps et automne) de 122 stations réparties sur un panel de 8 lagunes, ce travail a permis de décrire les peuplements de poissons, principalement les juvéniles ou les espèces peu mobiles, en parallèle des conditions environnementales. Les données collectées mettent en évidence les variabilités spatiale (entre habitats) et saisonnière (printemps vs. automne) de la densité et de la composition des peuplements. L’analyse spatio-temporelle des communautés bentho-démersales a montré néanmoins une influence relativement limitée de la saison dans nos données (2% de l’effet temporelle pour 18% de l’effet spatial de la lagune) avec cependant des différences marquées dans les captures au printemps avec l’arrivée d’espèces marines migrantes telles que Sparus aurata, Liza ramada et Solea solea. L'occurrence et la diversité des poissons dans les habitats lagunaires seraient donc fortement influencée par la saisonnalité du recrutement, la dynamique des populations et les schémas de migration comme cela a pu être démontré dans des études antérieures. Au terme de ce travail, de nombreuses métriques de diversité ont été calculées mais aujourd'hui il n'est pas possible de définir un indicateur "poissons DCE-compatible" en raison du faible nombre de couple de données Métrique-Pression. Ce travail a permis néanmoins de valider une stratégie d'acquisition et de traitement des données qui pourra être appliquée à large échelle dans les lagunes méditerranéennes françaises.

Truly sustainable development in a human-altered, fragmented marine environment subject to unprecedented climate change, demands informed planning strategies in order to be successful. Beyond a simple understanding of the distribution of marine species, data describing how variations in spatio-temporal dynamics impact ecosystem functioning and the evolution of species are required. Marine Functional Connectivity (MFC) characterizes the flows of matter, genes and energy produced by organism movements and migrations across the seascape. As such, MFC determines the ecological and evolutionary interdependency of populations, and ultimately the fate of species and ecosystems. Gathering effective MFC knowledge can therefore improve predictions of the impacts of environmental change and help to refine management and conservation strategies for the seas and oceans. Gathering these data are challenging however, as access to, and survey of marine ecosystems still presents significant challenge. Over 50 European institutions currently investigate aspects of MFC using complementary methods across multiple research fields, to understand the ecology and evolution of marine species. The aim of SEA-UNICORN, a COST Action within the European Union Horizon 2020 framework programme, is to bring together this research effort, unite the multiple approaches to MFC, and to integrate these under a common conceptual and analytical framework. The consortium brings together a diverse group of scientists to collate existing MFC data, to identify knowledge gaps, to enhance complementarity among disciplines, and to devise common approaches to MFC. SEA-UNICORN will promote co-working between connectivity practitioners and ecosystem modelers to facilitate the incorporation of MFC data into the predictive models used to identify marine conservation priorities. Ultimately, SEA-UNICORN will forge strong forward-working links between scientists, policy-makers and stakeholders to facilitate the integra

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).

The European COST Action 'Unifying Approaches to Marine Connectivity for improved Resource Management for the Seas' (SEA-UNICORN, 2020-2025) is an international research coordination initiative that unites an interdisciplinary community of scientists and policymakers from over 100 organizations across Europe and beyond. It is establishing a globally harmonized framework to deliver actionable, transdisciplinary knowledge of marine functional connectivity, promoting a sustainable blue economy and ocean conservation. Planning sustainable development in rapidly changing oceans requires a thorough comprehension of marine biodiversity and the processes underpinning the functioning of ecosystems. Connectivity among marine populations and habitats facilitates the persistence and resilience of vulnerable species and ecosystems and controls the spread of invasive species. Constructing effective networks of restoration or conservation areas and promoting sustainable harvesting requires knowledge of connectivity. SEA-UNICORN advances worldwide collaboration by coordinating the collection, sharing, and application of knowledge on species, community, and ecosystem connectivity at sea and at the land-sea interface. It engages scientists from diverse areas and early-career researchers and creates a stronger match between natural and social science and policy needs to better address key environmental issues that challenge the future of our planet.

Les lagunes méditerranéennes sont aujourd’hui décrites comme des habitats essentiels au cycle de vie de nombreuses espèces de poissons côtiers qu’elles soient sédentaires ou migratrices. Ces masses d’eau, relativement confinées, présentes en effet des caractéristiques environnementales permettant de maximiser la croissance des juvéniles et d’optimiser la survie des adultes (abondance en nourriture, température plus élevée, prédation limitée…) mais celles-ci varient spatialement et temporellement au sein même d’une lagune sous l’influence de facteurs naturels et anthropiques. Comprendre le lien entre les habitats et les poissons nécessite donc de qualifier et quantifier ces interactions à ces échelles de temps et d’espace afin d’être en mesure d’adapter in fine la gestion de ces milieux pour renforcer leurs rôles dans le cycle de vie des espèces de poissons lagunaires et côtiers. Au stade juvénile, l’attractivité des habitats lagunaires variraient selon l’espèce et le stade de vie, aussi bien pour les juvéniles des espèces migratrices que pour ceux des espèces résidentes. La présence de macrophytes et en particulier de macroalgues sur le fond jouerait un rôle essentiel en structurant d’une part les habitats pour les juvéniles (zone de refuge, abrits de proies) et en contribuant d’autre part au régime trophique de manière directe ou indirecte. Ces résultats permettent de mieux définir la notion de nurserie en considérant une mosaïque d’habitats qui assure des fonctions complémentaires pour les juvéniles de poissons. Au stade adulte, la dynamique spatiale de trois espèces modèle (Dicentrarchus labrax, Salpa salpa et Sparus aurata) en relation avec les habitats que ces lagunes suivie par télémétrie acoustique illustre le rôle clés de ces habitats lagunaire probablement en fournissant un habitat d’alimentation répondant à leurs besoins en termes de croissance. Pour ces trois espèces, un effet saisonnier important dans sa fréquentation, essentiellement durant la phase hiv

The European COST Action “Unifying Approaches to Marine Connectivity for improved Resource Management for the Seas” (SEA-UNICORN, 2020–2025) is an international research coordination initiative that unites an interdisciplinary community of scientists and policymakers from over 100 organizations across Europe and beyond. It is establishing a globally harmonized framework to deliver actionable, transdisciplinary knowledge of marine functional connectivity, promoting a sustainable blue economy and ocean conservation. Planning sustainable development in rapidly changing oceans requires a thorough comprehension of marine biodiversity and the processes underpinning the functioning of ecosystems. Connectivity among marine populations and habitats facilitates the persistence and resilience of vulnerable species and ecosystems and controls the spread of invasive species. Construct-ing effective networks of restoration or conservation areas and promoting sustainable harvesting requires knowledge of connectivity. SEA-UNICORN advances worldwide collaboration by coordinating the collection, sharing, and application of knowledge on species, community, and ecosystem connectivity at sea and at the land–sea interface. It engages scientists from diverse areas and early-career researchers and creates a stronger match between natural and social science and policy needs to better address key environmental issues that challenge the future of our planet.

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.

Sharks exhibit varied demographic strategies depending on both the species and the population location, which make them more or less vulnerable to fishing. Accurate evaluation of local age and growth parameters is therefore fundamental for the sustainable management of their stocks. Although demographic parameters have been assessed for bull shark (Carcharhinus leucas) populations in several locations of the world, this information was missing so far around the Reunion Island, in the South West Indian Ocean. To fill this gap of knowledge, age and growth data was gathered from the vertebrae of 140 individuals of C. leucas (77 females and 63 males, mostly adults) fished around the island between 2012 and 2019. After verification of the annual deposition of growth band pairs on these structures using relative marginal increment analysis on 40 individuals, band pairs were counted along the vertebral centrum for each individual. Thanks to this approach, growth was shown to significantly differ between male and female C. leucas around the reunion island, with respective von Bertalanffy growth model equations of Lt = and Lt = . Indeed, the females of the species fished in this area were significantly (p < 0.001) larger than local males, with an estimated difference in size of ~ 16.1 cm at 20 years old. They also apparently reach older ages, with an estimated maximum age of 33.50 years, against 29.75 years only for the males. The estimated size at birth around the island is larger than elsewhere in the world, varying from 92.30 to 100.00 cm depending on the method used. These results confirm that the population of C. leucas around the Reunion Island exhibits a K-selected strategy, which makes it highly vulnerable to fishing pressure.

Recurrent jellyfish blooms in the coastal zone call for understanding the impacts of jelly-falls on the functioning of benthic communities, especially in shallow enclosed ecosystems where their biomass can affect local carbon cycling and productivity. Each year, blooms of the jellyfish Aurelia coerulea appear and collapse in a semi-enclosed coastal Mediterranean lagoon (the Thau lagoon, south of France). Although the lagoon is shallow, large accumulations of dead jellyfish are never observed on its bottom, so it was hypothesized that decaying jellyfish were rapidly consumed by local macrobenthic organisms. The current work aimed to test this hypothesis, by estimating the impact of the presence of dead A. aurelia medusae on local macrobenthic community composition and assessing their biomass loss rates under different scenarios of accessibility by the macrobenthos. Unexpectedly, our results revealed a limited role of macrobenthic scavengers in the disappearance of dead medusae, although this later was particularly fast (19–78h). Only one taxon (Tritia sp., Nassariidae family) showed a significant response to the presence of dead A. coerulea medusae on the seabed. Thus, our results suggest that the fast disappearance of dead jellyfish biomass in Thau results from its rapid degradation and consumption by local microorganisms, likely due to the combined effects of high local temperatures and the small size of A. coerulea medusae. Thus, the important biomass produced during A. aurelia blooms in Thau might essentially boost its microbial food web. The potential role of jellyfish blooms in controlling biogeochemical cycles and food web functioning in shallow lagoons is discussed, underlying the need to include this process in ecosystem-based models.

Understanding the genetic underpinnings of fitness trade-o s across spatially variable environments remains a major challenge in evolutionary biology. In Mediterranean gilthead sea bream, first-year juveniles use various marine and brackish lagoon nursery habitats characterized by a trade-o between food availability and environmental disturbance. Phenotypic di erences among juveniles foraging in di erent habitats rapidly appear after larval settlement, but the relative role of local selection and plasticity in phenotypic variation remains unclear. Here, we combine phenotypic and genetic data to address this question. We first report correlations of opposite signs between growth and condition depending on juvenile habitat type. Then, we use single nucleotide polymorphism (SNP) data obtained by Restriction Associated DNA (RAD) sequencing to search for allele frequency changes caused by a single generation of spatially varying selection between habitats. We found evidence for moderate selection operating at multiple loci showing subtle allele frequency shifts between groups of marine and brackish juveniles. We identified subsets of candidate outlier SNPs that, in interaction with habitat type, additively explain up to 3.8% of the variance in juvenile growth and 8.7% in juvenile condition; these SNPs also explained significant fraction of growth rate in an independent larval sample. Our results indicate that selective mortality across environments during early-life stages involves complex trade-o s between alternative growth strategies

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Partial migration, where migrant and resident organisms coexist within the same population, has been found in many fishes. Although it seems obvious that different life cycles exploit habitats and food webs differently, few assessments about the trophic consequences of partial migration are available. To unveil part of this complexity, we combined otolith chemistry with stable isotope analyses data for hind-casting Merluccius australis habitat use and diet composition at age. By providing detailed information about lifetime variability in diet, trophic position, and prey demand of four M. australis life-cycle types, we show that these groups feed differentially in estuarine and oceanic habitats throughout their ontogeny. Although trophic positions were similar between habitats for juvenile and subadults, substantial differences between life-cycle types were found regarding lifetime diet and trophic demand. Thus, the more abundant and heavily exploited oceanic stock of M. australis was heavily dependent of estuarine habitats within the Patagonian Fjords System, where it consumes large biomasses of Macruronus magellanicus, Pasiphaea, Sprattus fuegensis, and Euphausiidae at earlier stages. We show ignoring trophic consequences of partial migration and life-cycle diversity may produce highly biased results, both in terms of prey and habitat use, which appears critical for multispecies and ecosystem management approaches.