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Enhanced pelagic biomass around coral atolls

Tom Letessier, Jessica Meeuwig | Mar 22, 2016

Tom Letessier, Jessica Meeuwig

Mar 22 2016

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Productive waters surround the remote Palmyra atoll in the Pacific.

Photo: Zafer Kizilkaya.

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Dr. Tom Letessier
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Letessier TB, Cox MJ, Meeuwig JJ, Boersch-Supan PH, Brierley AS. 2016. Enhanced pelagic biomass around coral atolls. Marine Ecology Progress Series, 546: 271-276.


  • Abrupt topographical features in the open ocean are known to attract threatened species like tuna and sharks.
  • We explored the spatial distribution of mid-water organisms residing in scattering layers using scientific echosounders in the Chagos Archipelago.
  • We found that coral atolls and seamounts enhance surrounding mid-water biomass by approx 100 times.
  • The spatial range of influence of bathymetry was estimated to be approx 1.8 km.
  • This information is useful, particularly in the context of marine protected area design, and will allow us to adequately protect important ecological processes in the mid-water.


Understanding the processes driving the distribution of mid-water prey such as euphausiids and lanternfish is important for effective management and conservation. In the vicinity of abrupt topographic features such as banks, seamounts and shelf-breaks, mid-water faunal biomass is often elevated, making these sites candidates for special protection. We investigated the spatial distribution of water column acoustic backscatter—a proxy for macrozooplankton and fish biomass—in the 9 km transition zone between the pelagos and coral atolls in the Chagos Archipelago (6° N, 72° E). The purpose was to determine the magnitude and distance over which bathymetry may enhance biomass in the mid-water, and thereby identify the scale over which static topographic features could influence the open ocean. Two distinct sound scattering layers were identified, from the surface to 180 m and from 300 to 600 m, during daytime. Both layers exhibited significant increases in backscatter near features. Close to features, the shallow layer backscatter was ca. 100 times higher and was driven partly by increasing numbers of larger individuals, evident as single target echoes. We determine the regional scale of influence of features on pelagic biomass enhancement to be ca. 1.8 km in the Chagos Archipelago, and suggest possible ecological explanations that may support it. Our approach determining the scale of influence of bathymetry should be applied during the process of marine reserve design, in order to improve protection of mid-water fauna associated with topographical features, such as seamounts and coral reefs.



Tom Letessier was supported by the Marine Biodiversity Hub through the Australian Government’s National Environmental Research Program (NERP). Phillip Boersch-Supan was supported by a Cusanuswerk doctoral fellowship, a Lesley & Charles Hilton-Brown Scholarship, University of St. Andrews, and a grant from the Fisheries Society of the British Isles. Martin Cox was supported by Australian Research Council grant FS110200057. Millennium Coral Reef Mapping Project validated maps were provided by the Institute for Marine Remote Sensing, University of South Florida and Institut de Recherche pour le Développement, Centre de Nouméa, with support from NASA. Ship time was provided by the UK Foreign & Commonwealth Office and BIOT Administration. We are indebted to the Master, Chief Engineer and crew of FPV ‘Pacific Marlin’ for their outstanding support.



Two distinct layers of acoustic backscatter were identified in the vicinity of the Peros Banhos atoll in the Chagos archipelago. These are indicative of aggregations of pelagic fauna within  ca. 1.8 km of the topographic features. Figure extracted from Letessier et al. 2016.