Tropical tuna fisheries are among the largest fisheries in the world and interact with a range of bycatch species. Understanding the impacts of these fisheries on bycatch species is important for fisheries management, however the vulnerability of many of these species remains largely unknown given available data. Here, a recent length-based ecological risk assessment method developed for data-limited settings, Ecological Assessment of the Sustainable Impacts of Fisheries (EASI-Fish), was applied to 33 elasmobranch species that interact with western and central Pacific Ocean (WCPO) tropical tuna fisheries. This quantitative, model-based ecological risk assessment provides estimates of vulnerability for these elasmobranchs. In summary, 13 of 33 species assessed were considered 'least vulnerable', 12 'most vulnerable', and eight 'data deficient'. Species considered 'most vulnerable' included several carcharhinids given their low productivity, as well as several Sphyrna species that are particularly susceptible to mortality when interacting with fishing gears. Deep-set longline gear was responsible for most fishing mortality for most species given its vast effort distribution throughout the WCPO and vertically in the water column. A thorough compilation of life history and fisheries information for bycatch species was also undertaken and its reliability qualitatively scored, which can be used to prioritise research. EASI-Fish methods were further developed, including an improved approach to estimating spatial overlap which decreased vulnerability scores. This assessment can assist fisheries administrations to revise their management, stock assessment and research priorities for these elasmobranch bycatch species and provides other practitioners with improved tools to undertake similar assessments going forward.

Humans are exposed to toxic methylmercury mainly by consuming marine fish, in particular top predator species like billfishes or tunas. In seafood risk assessments, mercury is assumed to be mostly present as organic methylmercury in predatory fishes; yet high percentages of inorganic mercury were recently reported in marlins, suggesting markedly different methylmercury metabolism across species. We quantified total mercury and methylmercury concentrations in muscle of four billfish species from the Indian and the Pacific oceans to address this knowledge gap. We found low percentages of methylmercury in blue and black marlins (15 ± 7 %) compared to swordfish and striped marlin (89 ± 13 %), with no significant differences among ocean regions. This illustrates that billfishes exhibit species-specific methylmercury bioaccumulation patterns, likely related to unique selenium-dependent in vivo methylmercury demethylation capacities in muscle. Blue and black marlins therefore appeared generally safer for human consumption than swordfish and striped marlin regarding MeHg toxicological effects. Yet, no matter the species, the frequency of recommended weekly billfish meals decreased with increasing fish size, given that mercury naturally accumulates over time. When assessing potential risks of billfish consumption, we therefore recommend measuring methylmercury, rather than total mercury, and relying on a large number of samples to cover a broad range of fish sizes. This study calls for additional characterization of mercury speciation and bioavailability in billfishes to better understand the mechanisms driving species-specific differences of methylmercury detoxification, and to refine dietary advices associated to marine top predators consumption.

Understanding the movement ecology and resource use of pelagic fishes is essential for their conservation and management. Molids, as predators of gelatinous zooplankton, play important ecological roles in the gelatinous food webs and undertake long-distance migrations across the Pacific Ocean. This study used a novel combination of isotope-based approaches, including isoscapes, compound-specific isotope analysis of amino acids, and Bayesian mixing models, to investigate the migration patterns and habitat uses of molids (Mola mola and M. alexandrini) at a population level in the western Pacific Ocean. Results from isoscapes revealed that most molids in Japan, Taiwan, and New Caledonia might be local residents, with measured δ15N values aligning with predicted isoscape values. However, some larger individuals in Japan and Taiwan showed δ15N values higher than predicted, suggesting recent migrations from isotopically distinct regions. In New Zealand, the measured δ15N values of molids did not overlap with predicted values, indicating non-residency. Source amino acid δ15N values and Bayesian mixing models suggest that local residents in Japan and Taiwan primarily consumed local prey, whereas recent migrants with higher δ15N values may originate from the warm pool region, potentially linked to spawning behavior. Molids in New Caledonia and New Zealand likely migrate from regions with isotopic values distinct from the local food web base. The cross-regional movements of molids highlight the importance of understanding connectivity within the western Pacific Ocean. Identifying migratory corridors and key habitats, such as spawning and feeding grounds, is essential for developing regional and international conservation strategies that ensure the long-term sustainability of molid populations.

Micronekton organisms are a central component of the trophic organization in the pelagic ecosystem, being prey to top predators and participating in the export of carbon from the surface to the deep layers. Despite their importance, the abundance estimates and species distribution of micronekton remain largely uncertain. This study aimed to compare and assess two sampling methods classically used for the estimation of micronekton abundance in mesopelagic acoustic scattering layers: scientific echosounder and trawl sampling. Measurements of 38 and 70 kHz hull-mounted echosounders were examined with biological trawl samples from 8 surveys in the South-West tropical Pacific. A model of acoustic observation was built from the trawl sampling species composition and forward scattering models. Predicted and observed acoustic responses are compared to assess the variability and the difference between the acoustic and trawl sampling methods in various scattering layers, for day and night periods. The difference between methods decreased with depth and with increasing abundance of fish with swimbladders caught in trawls. Notably, this difference was found to be minimal in the nocturnal deep scattering layer (mesopelagic zone, depth>200 m). This study emphasizes potential lower estimates of organisms’ abundance by trawling and a bias towards mesopelagic fish. Understanding the differences between methods and their variability within different scattering layers is essential for studying micronekton and improving the precision of biomass estimates.

Oceanic eddies are recognized as pivotal components in marine ecosystems, believed to concentrate a wide range of marine life spanning from phytoplankton to top predators. Previous studies have posited that marine predators are drawn to these eddies due to an aggregation of their forage fauna. In this study, we examine the response of forage fauna, detected by shipboard acoustics, across a broad sample of a thousand eddies across the world's oceans. While our findings show an impact of eddies on surface temperatures and phytoplankton in most cases, they reveal that only a minority (13%) exhibit significant effects on forage fauna, with only 6% demonstrating an oasis effect. We also show that an oasis effect can occur both in anticyclonic and cyclonic eddies, and that the few high-impact eddies are marked by high eddy amplitude and strong water-mass-trapping. Our study underscores the nuanced and complex nature of the aggregating role of oceanic eddies, highlighting the need for further research to elucidate how these structures attract marine predators. This study explores alternative stable states in microbial communities. Focusing on a respiratory tract community of 6 species, the authors identified four distinct stable states that are predicted to be driven by cooperative growth. The findings contrast with the common association between competitive interactions and multistability in microbial communities. Using a sample of a thousand eddies across the world's oceans, this study examines the response of forage fauna detected by shipboard acoustics. The findings show that a minority of eddies exhibit significant effects on forage fauna, but that an oasis effect can sometimes occur. This highlights the need for further research to elucidate how eddies might attract marine predators.

Humans are exposed to toxic methylmercury mainly by consuming marine fish. While reducing mercury emissions and releases aims to protect human health, it is unclear how this affects methylmercury concentrations in seawater and marine biota. We compiled existing and newly acquired mercury concentrations in tropical tunas from the global ocean to explore multidecadal mercury variability between 1971 and 2022. We show the strong inter-annual variability of tuna mercury concentrations at the global scale, after correcting for bioaccumulation effects. We found increasing mercury concentrations in skipjack in the late 1990s in the northwestern Pacific, likely resulting from concomitant increasing Asian mercury emissions. Elsewhere, stable long-term trends of tuna mercury concentrations contrast with an overall decline in global anthropogenic mercury emissions and deposition since the 1970s. Modeling suggests that this limited response observed in tunas likely reflects the inertia of surface ocean mercury with respect to declining emissions, as it is supplied by legacy mercury that accumulated in the subsurface ocean over centuries. To achieve measurable declines in mercury concentrations in highly consumed pelagic fish in the near future, aggressive emission reductions and long-term and continuous mercury monitoring in marine biota are needed.

Context Understanding tuna diet and its drivers can provide valuable insights into the dynamics of pelagic ecosystems and their capacity to adapt to environmental and human-induced stressors.Aims To reconstruct tropical tuna dietary histories by using different metrics of stomach fullness and to assess their association with fisheries-related, environmental and biological covariates.Methods We examined stomachs from 8229 skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye tuna (T. obesus) captured in the western and central Pacific Ocean between 2001 and 2021, classified each on the basis of categorical and continuous fullness metrics, then built statistical models to gauge covariate effects.Key results Only models for the binary fullness metric (i.e. empty or non-empty) performed well. Tuna with empty stomachs were more likely to be caught on active gear (e.g. purse seine) and earlier in the day. Skipjack and bigeye tuna with empty stomachs were more likely to be associated with floating objects (e.g. fish-aggregating devices).Conclusions Our results add to the evidence that tropical tunas forage more effectively during the day and more actively when not associated with floating objects. At the individual level, tuna stomach fullness is highly variable.Implications Further research is needed to understand the factors governing this promising indicator of ecosystem change. In this study, we sought to reconstruct tropical tuna dietary histories by using different metrics of stomach fullness and to assess their association with fisheries-related, environmental and biological covariates. Our results add to the evidence that tropical tunas forage more effectively during the day and more actively when not associated with floating objects. Further research is needed to understand the factors governing this promising indicator of ecosystem change.

Minimising the unintended capture of fish, marine mammals, reptiles, seabirds and other marine organisms is an important component of responsible fisheries management and for stabilising declines and rebuilding populations of threatened species. The analyses presented were designed to establish the first quantitative baseline of historical catches, catch rates and species composition for the dominant tuna fisheries operating in the western and central Pacific, the world's largest in terms of tuna catch. Using records from 612,148 fishing events collected by independent 'at sea' observers, estimates for finfish, billfish, elasmobranchs, marine mammals and sea turtles show that the composition and magnitude of catches varied considerably by fishery type and practice for the period 2003-2019. Simulations indicated that precision in longline estimates would be improved by monitoring a proportion of fishing sets from all fishing trips rather than full coverage from a proportion of all fishing trips. While attributing reasons for temporal trends in estimated bycatch was difficult due to the confounding impacts of changing abundances and fishing practices, the trends identified the nature of potential relationships for species that are not accurately quantified, or not covered, by fishing vessel logbooks. The trends in catch estimates, and the catch rate models, have utility in identifying species which may require targeted additional analyses and management interventions, including species of conservation interest (either due to their threatened status or vulnerability to fishing) such as elasmobranchs and sea turtles. Moreover, the estimates should support future evaluations of the impact of these industrial-scale fisheries on bycatch species.

Pacific Ocean tuna is among the most-consumed seafood products but contains relatively high levels of the neurotoxin methylmercury. Limited observations suggest tuna mercury levels vary in space and time, yet the drivers are not well understood. Here, we map mercury concentrations in skipjack tuna across the Pacific Ocean and build generalized additive models to quantify the anthropogenic, ecological, and biogeochemical drivers. Skipjack mercury levels display a fivefold spatial gradient, with maximum concentrations in the northwest near Asia, intermediate values in the east, and the lowest levels in the west, southwest, and central Pacific. Large spatial differences can be explained by the depth of the seawater methylmercury peak near low-oxygen zones, leading to enhanced tuna mercury concentrations in regions where oxygen depletion is shallow. Despite this natural biogeochemical control, the mercury hotspot in tuna caught near Asia is explained by elevated atmospheric mercury concentrations and/or mercury river inputs to the coastal shelf. While we cannot ignore the legacy mercury contribution from other regions to the Pacific Ocean (e.g., North America and Europe), our results suggest that recent anthropogenic mercury release, which is currently largest in Asia, contributes directly to present-day human mercury exposure.

Understanding the relationship between mercury in seafood and the distribution of oceanic methylmercury is key to understand human mercury exposure. Here, we determined mercury concentrations in muscle and blood of bigeye and yellowfin tunas from the Western and Central Pacific. Results showed similar latitudinal patterns in tuna blood and muscle, indicating that both tissues are good candidates for mercury monitoring. Complementary tuna species analyses indicated species- and tissue- specific mercury patterns, highlighting differences in physiologic processes of mercury uptake and accumulation associated with tuna vertical habitat. Tuna mercury content was correlated to ambient seawater methylmercury concentrations, with blood being enriched at a higher rate than muscle with increasing habitat depth. The consideration of a significant uptake of dissolved methylmercury from seawater in tuna, in addition to assimilation from food, might be interesting to test in models to represent the spatiotemporal evolutions of mercury in tuna under different mercury emission scenarios.

This preliminary study used DNA metabarcoding to test whether the stomach content and gut microbiome of tuna could be a viable near real-time monitoring tool for detecting composition and change in oceanic ecosystems. The gut content of skipjack (Katsuwonus pelamis, n=55) and yellowfin tuna (Thunnus albacares, n=46) captured in the Pacific Ocean during El Niño Southern Oscillation events (ENSO) between 2015-2017 were examined by high throughput sequencing and complemented by morphological assessments to identify fishes, crustaceans and cephalopods in the stomach content. Gut microbiome was examined solely by high throughput sequencing. Stomach content and gut microbiome were compared between tuna species, ENSO events and capture location using generalised linear models. The full model (tuna species, capture location and interaction with ENSO) best explained fish prey composition, while capture location and ENSO weakly explained the composition of crustaceans and cephalopods. Skipjack and yellowfin tuna captured near coastal areas (Longitude<170°W) showed a greater diversity of prey compared to fish captured in oceanic regions of the Pacific, while Thunnus albacares showed a much more diverse stomach content than K. pelamis (21 fish, eight cephalopods and six crustaceans). Fish captured during La Niña events showed higher prey diversity compared to fish captured during El Niño. Tuna species best explained differences in gut microbiome to the Phylum level, while no model explained gut microbiome differences to the Order or Family level. This preliminary study shows that capture location and ENSO events explained differences in stomach content of K. pelamis and T. albacares, while tuna species best explained gut microbiome assemblages to the Phylum level.

Ecosystem models forced by future climate simulations outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulate a substantial decline of tropical marine animal biomass over the course of the 21st century. Regional projections are however far more uncertain because of well-known biases common to most CMIP5 historical simulations that propagate within the food web. Moreover, the model outputs for high trophic levels marine fauna suffer from lack of validation based on in situ data. In this study, we implement a "bias-mitigation" strategy to reduce the physical oceanography and biogeochemical biases simulated by three CMIP5 models under the future RCP8.5 scenario. We force two very different micronekton models with these "bias-mitigated" outputs to infer the future micronekton changes in the Coral Sea: a 3-D deterministic population dynamics model; and a 3-D statistical model based on in situ hydro-acoustic data. These two models forecast a consistent pattern of micronekton abundance changes in the epipelagic layer (0-150 m) by 2100 for three different climate forcing used, with a marked decrease south of 22 degrees S and a smaller increase further north mostly related to temperature and chlorophyll changes. In contrast, changes in the vertical patterns of micronekton predicted by the two models considerably differ in the upper mesopelagic layers (150-450 m) and lower mesopelagic layer (450-1000 m), highlighting the structural sensitivity in model type. Since micronekton are prey of all larger marine predators, those discrepancies in vertical structures of micronekton may hamper our potential to predict how top predators may evolve in the future.

Nitrogen and carbon stable isotope data sets are commonly used to assess complex population to ecosystem responses to natural or anthropogenic changes at regional to global spatial scales, and monthly to decadal timescales. Measured in the tissues of consumers, nitrogen isotopes (δ15 N) are primarily used to estimate trophic position while carbon isotopes (δ13 C) describe habitat associations and feeding pathways. Models of both δ15 N and δ13 C values and their associated variance can be used to estimate likely dietary contributions and niche width and provide inferences about consumer movement and migration. Stable isotope data have added utility when used in combination with other empirical data sets (e.g., stomach content, movement tracking, bioregionalization, contaminant, or fisheries data) and are increasingly relied upon in food web and ecosystem models. While numerous regional studies publish tables of mean δ15 N and δ13 C values, limited individual records have been made available for wider use. Such a deficiency has impeded full utility of the data, which otherwise would facilitate identification of macroscale patterns. The data provided here consist of 4,498 records of individuals of three tuna species, Thunnus alalunga, T. obesus, and T. albacares sampled from all major ocean basins from 2000 to 2015. For each individual tuna, we provide a record of the following: species name, sampling date, sampling location, tuna length, muscle bulk and baseline corrected δ15 N values, and muscle bulk and, where available, lipid corrected δ13 C values. We provide these individual records to support comparative studies and more robust modeling projects seeking to improve understanding of complex marine ecosystem dynamics and their responses to a changing environment. There are no copyright restrictions for research and/or teaching purposes. Users are requested to acknowledge their use of the data in publications, research proposals, websites, and other outlets following the citation instructions in Class III, Section B.

Maintaining healthy, productive ecosystems in the face of pervasive and accelerating human impacts including climate change requires globally coordinated and sustained observations of marine biodiversity. Global coordination is predicated on an understanding of the scope and capacity of existing monitoring programs, and the extent to which they use standardized, interoperable practices for data management. Global coordination also requires identification of gaps in spatial and ecosystem coverage, and how these gaps correspond to management priorities and information needs. We undertook such an assessment by conducting an audit and gap analysis from global databases and structured surveys of experts. Of 371 survey respondents, 203 active, long-term (>5 years) observing programs systematically sampled marine life. These programs spanned about 7% of the ocean surface area, mostly concentrated in coastal regions of the United States, Canada, Europe, and Australia. Seagrasses, mangroves, hard corals, and macroalgae were sampled in 6% of the entire global coastal zone. Two-thirds of all observing programs offered accessible data, but methods and conditions for access were highly variable. Our assessment indicates that the global observing system is largely uncoordinated which results in a failure to deliver critical information required for informed decision-making such as, status and trends, for the conservation and sustainability of marine ecosystems and provision of ecosystem services. Based on our study, we suggest four key steps that can increase the sustainability, connectivity and spatial coverage of biological Essential Ocean Variables in the global ocean: (1) sustaining existing observing programs and encouraging coordination among these; (2) continuing to strive for data strategies that follow FAIR principles (findable, accessible, interoperable, and reusable); (3) utilizing existing ocean observing platforms and enhancing support to expand observing along coasts of developing countries, in deep ocean basins, and near the poles; and (4) targeting capacity building efforts. Following these suggestions could help create a coordinated marine biodiversity observing system enabling ecological forecasting and better planning for a sustainable use of ocean resources.

Understanding the ecological mechanisms underpinning distribution patterns is vital in managing populations of mobile marine species. This study is a first step towards an integrated description of the habitats and spatial distributions of marine predators in the Natural Park of the Coral Sea, one of the world’s largest marine-protected areas at about 1.3 million km 2 , covering the entirety of New Caledonia’s pelagic waters. The study aims to quantify the benefit of including a proxy for prey abundance in predator niche modelling, relative to other marine physical variables. Spatial distributions and relationships with environmental data were analysed using catch per unit of effort data for three fish species (albacore tuna, yellowfin tuna and dolphinfish), sightings collected from aerial surveys for three cetacean guilds (Delphininae, Globicephalinae and Ziphiidae) and foraging locations identified from bio-tracking for three seabird species (wedge-tailed shearwater, Tahiti petrel and red-footed booby). Predator distributions were modelled as a function of a static covariate (bathymetry), oceanographic covariates (sea surface temperature, chlorophyll- a concentration and 20 °C-isotherm depth) and an acoustically derived micronekton preyscape covariate. While distributions were mostly linked to bathymetry for seabirds, and chlorophyll and temperature for fish and cetaceans, acoustically derived prey abundance proxies slightly improved distribution models for all fishes and seabirds except the Tahiti petrel, but not for the cetaceans. Predicted spatial distributions showed that pelagic habitats occupied by predator fishes did not spatially overlap. Finally, predicted habitats and the use of the preyscapes in predator habitat modelling were discussed.

Delineating the stock structure of highly-mobile, wide-ranging fishes subject to exploitation is a challenging task, yet one that is fundamental to optimal fisheries management. A case in point are stocks of skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus) and albacore tuna (Thunnus alalunga) in the Pacific Ocean, which support important commercial, artisanal, subsistence, and recreational fisheries, and contribute roughly 70 % of global commercial tuna catches. Although some spatial and temporal structuring is recognised within these stocks, growing evidence from a range of approaches suggests that the stock structure of each tuna species is more complex than is currently assumed in both stock assessment and climate change models, and in management regimes. In a move towards improving understanding of the stock structure of skipjack, yellowfin, bigeye and South Pacific albacore tunas in the Pacific Ocean, an international workshop was held in Nouméa, New Caledonia, in October 2018 to review knowledge about their movement and stock structure in the region, define and discuss the main knowledge gaps and uncertainties concerning their stock structure, and develop biological sampling approaches to support the provision of this information. Here, we synthesise the discussions of this latter component. For each tuna species, we identify several general sampling considerations needed to reduce uncertainty, including i) the need for broadscale sampling in space, ideally covering each species' distribution, targeting adults in spawning condition and adopting a phased approach; ii) the need for temporally-repeated sampling of the same geographical areas to assess stability in observed patterns over time; iii) the need to resolve patterns in spatial dynamics, such as those resulting from movements associated with the seasonal extensions of poleward flowing currents, from underlying stock structure, iv) the importance of adopting a multidisciplinary approach to stock identification, and v) the need for careful planning of logistics and coordination of sampling efforts across agencies. Finally, we present potential sampling designs that could be adopted to help overcome uncertainties around the initial identification of stocks and the provenance, mixing and proportional contributions of individuals in harvested assemblages, as well as how these uncertainties could be accounted for in fisheries management via the use of management strategy evaluation.

Mesopelagic resources are central to the ecosystem but remain poorly studied mainly due to the lack of observations. This paper investigates the assemblages of micronekton organisms and their habitat in the Natural Park of the Coral Sea around New Caledonia (southwest Pacific) using data from 141 pelagic trawls. A total of 67,130 micronekton individuals (fish, crustaceans, and mollusks) were collected with 252 species identified among 152 genus and 76 families. In the analyses, we focused on 22 species; each were present in more than 33 trawls (i.e., in more than 25% of the total number of trawls) and studied their spatial distribution and vertical dynamic behavior. Community structure was investigated through region of common profile (RCP), an innovative statistical multivariate method allowing the study of both species assemblages and environmental conditions’ influence on species occurrence probability. Nine major assemblages were identified, mainly driven by time of the day and sampling depth. Environmental variables, such as mean oxygen concentration, mean temperature, and bathymetry, also influenced micronekton assemblages, inducing a north/south distribution pattern. Three major day-assemblages were identified, distributed over the whole EEZ but segregated by depth: one assemblage in waters shallower than 200 m and the other two in deeper waters, respectively, in the north and the south. The night-assemblages were mostly segregated by depth with two community changes at approximately 80 and 200 m and spatially with a north–south gradient. The predominant northern night assemblages were dominated by crustacean, whereas the southern assemblage mostly by cephalopods and fish species. Generally, the southwest part of the EEZ was the most diverse part. Statistical analyses allowed the prediction of the spatial distribution of each species, and its vertical migration behavior was determined. Based on results, three important areas were identified to be considered for special management measures as part of the Natural Park of the Coral Sea.

Lunar phase and illumination are known to affect nocturnal behavior of many organisms, particularly through predator-prey interactions. Visual predators can benefit from higher light levels to increase their activity, while prey may decrease their activity to avoid predation. The lower number of nocturnal seabirds observed on colonies during full moon nights has been mostly interpreted as a predation avoidance strategy. However, it is also possible that shearwaters take advantage of the moon's illumination to feed also at night, and stay at sea to forage during full moon nights. We used miniaturized GPS-loggers to obtain 179 tracks from 99 wedge-tailed shearwaters breeding in New Caledonia, to investigate moonlight effects on individual behavior. Lunar phase significantly predicted self-provisioning trip duration, with individuals performing longer trips around the full moon. However, this relationship was not significant during chick-provisioning trips when adults have to frequently return to the colony. Adults mostly returned to the colony during moonlit periods, refuting the predation avoidance theory. Tracked individuals showed an unexpectedly high amount of nocturnal foraging activity (28% of total activity), positively influenced by the presence of the moon. δ15N stable isotope values were significantly related to the percentage of nocturnal foraging, but with a weak relationship, impeding our ability to confirm that wedge-tailed shearwaters fed on different prey when foraging at night. This study suggests that reduced colony attendance around the full moon may be linked to greater at-sea foraging opportunities in distant oceanic areas than to increased predation risk on land.