Effective ocean management and conservation of highly migratory species depends onresolving overlap between animal movements and distributions, and fishing effort.However, this information is lacking at a global scale. Here we show, using a big-data approach that combines satellite-tracked movements of pelagic sharks and global fishing fleets, that 24% of the mean monthly space used by sharks falls under the footprint of pelagic longline fisheries. Space-use hotspots of commercially valuable sharks and of internationally protected species had the highest overlap with longlines (up to 76% and 64%, respectively), and were also associated with significant increases in fishing effort.We conclude that pelagic sharks have limited spatial refuge from current levels of fishing effort in marine areas beyond national jurisdictions (the high seas). Our results demonstrate an urgent need for conservation and management measures at high-seas hotspots of shark space use, and highlight the potential of simultaneous satellite surveillance of megafauna and fishers as a tool for near-real-time, dynamic management.Industrialised fishing is a major source of mortality for large marine animals (marine megafauna) 1-6 . Humans have hunted megafauna in the open ocean for at least 42,000 years 7 , but international fishing fleets targeting large, epipelagic fishes did not spread into the high seas (areas beyond national jurisdiction) until the 1950s 8 . Prior to this, the high seas constituted a spatial refuge largely free from exploitation as fishing pressure was concentrated on continental shelves 3,8 . Pelagic sharks are among the widest ranging vertebrates, with some species exhibiting annual ocean-basin-scale migrations 9 , long term trans-ocean movements 10 , and/or fine-scale site fidelity to preferred shelf and open ocean areas 5,9,11 . These behaviours could cause extensive spatial overlap with different fisheries from coastal areas to the deep ocean. On average, large pelagic sharks account for 52% of all identified shark catch worldwide in target fisheries or as bycatch 12 . Regional declines in abundance of pelagic sharks have been reported 13,14 , but it is unclear whether exposure to high fishing effort extends across ocean-wide population ranges and overlaps areas in the high seas where sharks are most abundant 5,13 .Conservation of pelagic sharkswhich currently have limited high seas management 12,15,16would benefit greatly from a clearer understanding of the spatial relationships between sharks' habitats and active fishing zones. However, obtaining unbiased estimates of shark and fisher distributions is complicated by the fact that most data on pelagic sharks come from catch records and other fishery-dependent sources 4,15,16 .Here, we provide the first global estimate of the extent of space use overlap of sharks with industrial fisheries. This is based on the analysis of the movements of pelagic sharks tagged with satellite transmitters in the Atlantic, Indian and Pacific oceans, together with fishing vessel movements m...

International audienceMarine resources are under increasing pressure from a wide variety of threats such as overfishing, offshore energy development, and climate change. As marine ecosystems degrade, so do the well-being and livelihoods of humans that depend directly on the ecosystem goods and services they provide. Marine protected areas have been proposed to protect biodiversity, restore damaged ecosystems, sustain fisheries, and rebuild overexploited stocks. The effectiveness of marine protected areas depends in part on their effectiveness as connected networks, linked over large areas by ecological processes such as larval dispersal. Here, we applied a biophysical model driven by ocean currents derived from satellite altimetry to evaluate connectivity between Western Indian Ocean reefs. We applied graph-theoretic analysis, including clustering and a betweenness centrality metric. Our results show high interconnectivity within several regions (Mozambique Channel, Mascarene archipelago) and lower connectivity across the WIO region. We compared the results with the current MPA network, and proposed sites/reefs that should be considered priority sites for MPA implementation: Pebane, Cosmoledo, Majunga, Masoarivo, Platte Island, Farquhar, Agalega and Geyser bank. Our results are timely, considering the oil and gas exploration that is ongoing in the region. We discuss implications for transboundary marine policies and regional cooperation in the Western Indian Ocean, and advocate the creation of a regional-scale organization to structure interactions among the different actors

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Mauritius and Réunion are part of the Mascarene Islands situated in the southwestern Indian Ocean, c. 800 km east of Madagascar. A total of 44 different Sargassum species and subspecific taxa was listed in the literature for these islands. This was a remarkable number for such isolated and small islands, and was more than have been recorded from Madagascar or other East African countries. The first aim of our study was to revise these species lists using newly collected specimens to provide a reliable and illustrated tool for the identification of the Mauritian and Reunion Sargassum On the basis of morphological and molecular analyses, a total of six taxa was identified as S. cymosum f. borbonicum S obovatum, S. pfeifferae (reinstated), S. polycystum, S. portierianum and S. robillardii (stat. nov.). A seventh taxon S scopula, was identified from a herbarium collection but not re-collected. As a result of our taxonomic revision, we concluded that most of the species listed in the literature for both Mauritius and Réunion were misidentifications or synonyms, and we proposed three new taxonomic synonyms. The biogeography of the six Sargassum taxa was further investigated using local and regional hydrodynamic dispersal models. Results underlined the isolated position of the Mascarene Islands with (1) unlikely import o[ Sargassum in present-day conditions, (2) likely dispersal/exchanges withm the archipelago and (3) a two-pronged export of Sarga.ssum to the east coast of Madagascar and the Seychelles with a major stream northwestward and a weaker stream south westward. These oceanic conditions had probably shaped the Sargassum diversity of the Mascarene Islands and in particular explained the endemicity of 5. cymosum f borbonicum and restricted distribution of S. obovatum, S. pfeifferae, S. robillardii and S. scopula.