Identifying and quantifying pelagic organisms around seamounts, atolls, and islands in the Chagos Archipelago in relation to local oceanographic processes

Abstract

Topographic features, such as seamounts and atolls, are commonly identified as biodiversity hotspots that are key areas of productivity in an otherwise oligotrophic Indian Ocean. Whilst these sites of increased biomass are frequently identified, the oceanographic drivers that underpin the spatiotemporal distributions of biomass are not. If effective conservation measures are to be continued and implemented in regions such as the Chagos Archipelago Marine Protected Area (MPA) then the spatial variability of biomass and the physical drivers must be elucidated. This study presents case studies at Sandes seamount and Egmont atoll in the Chagos Archipelago identifying the distributions and behavioural responses of pelagic biota to oceanographic processes. Multifrequency fisheries acoustics data was combined with synoptically collected oceanographic sensor data to determine the oceanographic drivers of pelagic biota. In-situ sampling was also conducted to determine the composition of species over topographic features. Both regional and fine-scale physical processes were identified driving the distribution of biological aggregations over the different topographic features in the Chagos Archipelago. Around Sandes seamount, a joint climatic-oceanographic event, the Indian Ocean Dipole (IOD), affected sea surface temperatures and current velocities causing the deepening of the thermocline. This dramatically reduced the number of schooling fish over the summit of Sandes seamount re-distributing schools into deeper water around the flanks of the seamount. Analysis of the plankton distributions at Sandes seamount also highlighted the role of the thermocline in aggregating biota. At Egmont atoll, flow-topography interactions dictated the spatial distribution of schooling fish, with canyons and lagoons influencing fish behaviour. This research emphasises the biological importance of seamounts and atolls in the Chagos Archipelago. Understanding the spatial variability of biota around topographic features is vital to improve the efficiency and efficacy of marine spatial plans. By recognizing the behavioural responses of pelagic fish and plankton to oceanographic processes in the Chagos Archipelago this work can contribute to a wider understanding of biophysical interactions around topographic features globally.