Abstract
At-sea observations of seabirds and cetaceans provide essential baseline information about their biogeography and behaviour, facilitating marine spatial planning and management. Much of the world's oceans have been surveyed, yet some regions remain particularly data-poor for seabirds and cetaceans, including the Bay of Bengal. We performed 39 d of vessel-based observations within the Bay of Bengal from 2012 to 2014, surveying an overall linear distance of 4722.3 km. We observed 2697 seabirds of 17 species and 1441 cetaceans of at least eight species. Among the seabirds, Sooty Terns Onychoprion fuscatus (n = 2282, 85% of all birds) and Wedge-tailed Shearwaters Ardenna pacifica (n = 327, 12%) predominated, whereas cetacean numbers were dominated by Spinner Dolphins Stenella longirostris (n = 772, 54% of all cetaceans) and Indo-Pacific Bottlenose Dolphins Tursiops aduncus (n = 533, 37%). Other seabirds and cetaceans accounted for only 4% and 7%, respectively, of all sightings. The abundance and diversity of both groups was low compared to other tropical areas. We propose that low seabird and cetacean abundance results from low productivity due to stratification in the Bay of Bengal, as well as long-lasting disturbance, overexploitation of marine resources, possible impacts of longline fisheries, and the near absence of seabird breeding sites.