HAL Collection UNC (Univ. de la Nouvelle Calédonie)
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Cyclone Chido à Mayotte : une méthode rapide et reproductible par télédétection pour évaluer l’impact sur les mangroves
No full textInternational audienceLes mangroves jouent un rôle écologique et socio-économique essentiel dans les territoires insulaires tropicaux : elles participent à la dynamique sédimentaire du littoral, atténuent l’énergie des vagues, stockent du carbone et servent d’habitat pour de nombreuses espèces. À Mayotte, ces écosystèmes sont soumis à une pression croissante, liée à l’anthropisation, à l’érosion et aux effets du changement climatique.Le 14 décembre 2024, le cyclone Chido (catégorie 4) a traversé le nord de Mayotte, causant des dégâts importants aux infrastructures et aux milieux naturels. Dans un tel contexte, l’évaluation rapide de l’impact sur les mangroves est un enjeu majeur pour orienter les efforts de gestion, notamment lorsque l’accès au terrain est limité.Cette étude propose une approche reproductible basée sur l’analyse de séries temporelles d’images satellites Sentinel-2 (programme Copernicus), disponibles en open data. Grâce à leur résolution spatiale (10 m), leur fréquence d’acquisition (5 jours) et leur compatibilité avec des outils open source, ces données permettent de suivre l’évolution de la végétation via l’indice NDVI avant et après le passage du cyclone.L’approche développée permet : (i) de quantifier les pertes de végétation et les signes précoces de reprise ; (ii) d’identifier des gradients spatiaux de perturbation ; et (iii) de localiser les zones prioritaires pour la restauration. Elle a révélé une baisse moyenne du NDVI de 55 % dans les mangroves du Nord, contre 20% dans le Sud, reflétant fidèlement la trajectoire du cyclone.Les résultats ont été validés a posteriori par des observations de terrain selon la méthode DARAMA. Cette démarche démontre l’intérêt d’une télédétection opérationnelle pour renforcer le suivi des écosystèmes côtiers et mieux anticiper les réponses post-cycloniques, en particulier dans les contextes insulaires à données limitées
Marine predator super-habitats: Channels used for foraging, resting, mating, and parturition in sharks
No full textConservation measures for marine animals include protecting habitats that are important for foraging, reproduction, and refuging. This can be particularly challenging for mobile marine animals such as sharks that may use a wide range of habitats across life stages and for important ecological events. Channels connecting coastal ocean habitats with lagoons or estuaries are well known for their high abundance of predators and other fishes. Using a combination of biologging and diving observations, we show that a single channel in Fakarava, French Polynesia, is used for foraging, refuging, mating, and parturition by the grey reef shark, Carcharhinus amblyrhinchos. Despite over 500 sharks using and being residential to this channel, grey reef sharks can complete most of their life stages in a single restricted habitat. We classify this channel as a ‘super-habitat’ and propose that channels may be even more important to marine predators than previously thought and should be incorporated into conservation planning
ROBIN: Reference observatory of basins for international hydrological climate change detection
No full textInternational audienceHuman-induced warming is modifying the water cycle. adaptation to posed threats requires an understanding of hydrological responses to climate variability. Whilst these can be computationally modelled, observed streamflow data is essential for constraining models, and understanding and quantifying emerging trends in the water cycle. to date, the identification of such trends at the global scale has been hindered by data limitations -in particular, the prevalence of direct human influences on streamflow which can obscure climate-driven variability. By removing these influences, trends in streamflow data can be more confidently attributed to climate variability. Here we describe the Reference Observatory of Basins for INternational hydrological climate change detection (ROBIN) -the first iteration of a global network of streamflow data from national reference hydrological networks (RHNs) -comprised of catchments which are near-natural or have limited human influences. This collaboration has established a freely available global RHN dataset of over 3,000 catchments and code libraries, which can be used to underpin new science endeavours and advance change detection studies to support international climate policy and adaptation
Perception and discrimination of chemical and visual stimuli in Antillean manatees under human care
No full textInternational audienceAbstract Chemoreception represents a primitive sense in many mammals. Although evidence of functional olfaction is described in some cetaceans and pinniped species, findings considering Sirenians are scarce. We investigated the reactions of three Antillean manatees under human care to chemical stimuli: two food-related stimuli, one social-related stimulus and frozen water (control). A fish aggregation around the diffuser on manatees’ behavior was also monitored to test its influence on manatees’ behavior. Behaviors related to interest, social groupings and acoustic activity were recorded. The results showed that the density of fish around the diffuser influenced the manatees’ visual orientation and vocal rate. The manatees were also more present in the test area and decreased the duration and peak frequency of their calls after the diffusion of the feces odor compared to the control condition. Finally, they spent less time close to the device during fennel (plant) diffusion compared to the control condition. This work provides new evidence to the functionality of chemical perception in manatees. It also discusses the implication of other sensory cues and learning mechanisms in their foraging behaviors. We suggest that investigation of manatees’ reactions to environmental changes should include behavioral and acoustical measure
Deterministic and stochastic processes drive sessile cryptobenthic community composition on coral reefs at different spatial scales
No full textInternational audienceCoral reefs host immense biodiversity. The species that live concealed within the reef matrix, the cryptobiome, represent a major component of this diversity and are essential to reef functioning. However, the drivers of their composition and distribution remain incompletely understood. We deployed 27 autonomous reef monitoring structures (ARMS) for 2 yr at 9 outer slope sites on the fringing reefs along the west and southwest coasts of Reunion Island (south-west Indian Ocean). Photo-analysis of the sessile cryptobenthic communities (SCCs) on ARMS plate faces revealed that Rhodophyta, Annelida, Bryozoa, Porifera, Foraminifera, and Ascidiacea were the most abundant taxa. Oceanic distance, habitat characteristics, and environmental parameters were analysed to assess factors controlling the SCCs at various spatial scales. Presence-absence and abundance-based dissimilarity metrics revealed the relative contributions of deterministic and stochastic processes driving SCC beta diversity. At the ARMS unit scale, SCC composition was strongly linked to plate face orientation and exposure to the environment. Along the Reunion coral reef coast, community dissimilarity was associated with discontinuities of reef habitat, grouping SCCs into 4 geographical zones based on environmental factors (sea surface temperature, wave height, water flow velocity). Our results suggest that the SCC membership of taxa may be primarily driven by stochastic processes such as random larval dispersal and settlement on each reef, while their relative abundance within the community may be influenced by deterministic niche-based processes. These findings improve our understanding of the drivers of coral reef biodiversity in support of the conservation of these ecosystems
Evidence of vertical stratification in marine environments: insights from passive acoustic monitoring in French Polynesia
No full textInternational audienceThe stratification of species distribution in marine environments is well-documented, reflecting the influence of depth-related factors such as light availability, temperature, pressure, and habitat structure on the organization of marine communities. This study aimed to determine whether this stratification can also be highlighted using Passive Acoustic Monitoring (PAM), a technique proven effective in the study of various marine habitats. Sounds were recorded at 300 m in July 2022 off Moorea Island (French Polynesia) using a SNAP acoustic recorder. These data were compared to both those collected at shallower depths at Moorea Island (20, 60, and 120 m) and to data from other Polynesian islands obtained during a previous, non-concurrent study. Fish sounds in the recordings were manually identified. Richness and abundance of sounds from 20 m to 300 m depth were compared to assess whether fish communities in the altiphotic reef, upper mesophotic reef, lower mesophotic reef, and rariphotic zone could be distinguished based on the sounds they produce. Comparisons between zones over a 24-h period revealed for the first time the existence of acoustic stratification, with a fourfold decrease in species richness, from the altiphotic reef to the rariphotic zone, along with a general decline in sound abundance. However, this general pattern was not uniform in terms of abundance throughout the day, as sound abundance increased with depth at dusk from 950 sounds per hour at 20 m to 3600 at 300 m. It further confirms that diel activity patterns were also evident in light-deprived zones. This first exploration of vertical acoustic stratification based on fish sounds highlights the undeniable contribution of PAM to studies on the ecology of deep-sea communities
Competitive cleaning: behavioural variation supports coexistence of two juvenile sympatric cleanerfishes
No full textInternational audienceCleanerfish support coral reef ecosystems by providing important services to client fishes. Cleanerfish species often coexist, but how interspecies dynamics influence cleaning networks across life-history stages remains largely unknown. Here, we investigated the effects of interspecific competition on the behaviour of two juvenile cleaner wrasses, Labroides dimidiatus and Labroides bicolor . Under competitive scenarios, L. dimidiatus exhibited higher levels of client association compared to L. bicolor , while no differences were observed under non-competitive conditions. These behavioural differences likely facilitate resource partitioning and coexistence, supporting the stability of coexisting mutualistic networks in coral reef ecosystems
Climate change impacts on floods in West Africa: new insight from two large-scale hydrological models
No full textInternational audienceAbstract. West Africa is expected to face unprecedented shifts in temperature and extreme precipitation patterns as a result of climate change. The devastating impacts of river flooding are already being felt in most West African countries, emphasizing the urgent need for comprehensive insights into the frequency and magnitude of floods to guide the design of hydraulic infrastructure for effective flood risk mitigation and water resource management. Despite their significant socioeconomic and environmental impacts, flood hazards remain poorly documented in West Africa due to the data-related challenges. This study aims to fill this knowledge gap by providing a large-scale analysis of flood frequency and magnitudes across West Africa, focusing on how climate change may influence future flood trends. To achieve this, we have used two large-scale hydrological models driven by five bias-corrected sixth Coupled Model Intercomparison Project (CMIP6) climate models under two Shared Socioeconomic Pathways (SSPs). The generalized extreme value (GEV) distribution was utilized to analyze trends and detect change points by comparing multiple non-stationary GEV models across historical and future periods for a set of 58 catchments. Both hydrological models consistently projected increases in flood frequency and magnitude across West Africa despite their differences in hydrological process representations and calibration schemes. Flood magnitudes are projected to increase at 94 % (96 %) of stations for the 2-year (20-year) event in the near-term future and at 88 % (93 %) of stations for the 2-year (20-year) event in the long-term future, with some locations expected to experience increases exceeding 45 %. The findings from this study provide regional-scale insights into the evolving flood risks across West Africa and highlight the urgent need for climate-resilient strategies to safeguard populations and infrastructure against the increasing threat of flood hazards
Human Habitability and Economic Growth in Pacific Island Countries
No full textInternational audienceClimate change is a major and growing challenge for Pacific Island countries. Beyond the dramatic consequences of more frequent disasters and sea level rise, climate change is also reducing the human habitability of these zones. Building on the work of Gajewski, Klein and Tahri (2025), we employ the concept of climate niche and human habitability for oceanic countries. We first document a decline in habitability over the last century. We then show that this has a negative impact on economic output. We show that Australia and New Zealand tend to be more affected than the rest of the world, but the Fiji Islands seem to be better able to cope with the decline in habitability. This suggests that some adaptation policies, such as those adopted in Fiji, can increase resilience in the context of oceanic islands, which suffer disproportionately from the effects of climate change