Cetacean Distribution, Abundance and Habitat Use in the Central Indian Ocean

Abstract

Cetaceans face unprecedented anthropogenic threats globally, the biggest being fisheries related mortality, along with climate change, and habitat degradation. Establishing baseline abundance and distribution of cetaceans, along with continued long-term monitoring is important for identifying declining trends early enough to implement effective mitigation measures. However, absolute abundance and habitat use of cetaceans in the central Indian Ocean, particularly the Maldives and the Chagos Archipelago remain a significant data gap. This thesis aims to address these critical data deficiencies by presenting the first abundance estimates of cetaceans and investigating the distribution and habitat use of cetaceans across the Maldives and the Chagos Archipelago. Density Surface Models (DSMs) and Bayesian Integrated Species Distribution Models (iSDMs) were applied to multi-decadal sightings collected from scientific surveys, fisheries patrols, and whale-watching platforms to model the abundance and distribution of cetaceans. In the Northern Maldives, the first abundance estimates of cetaceans were produced from data across 1998 to 2022. Spinner dolphins were the most abundant species in the region, with a notable abundance of elusive and globally important beaked whales. In the Chagos Archipelago, a total of 18 species were documented, ranging from commonly encountered spinner dolphins to rarer, cryptic beaked whales and pygmy killer whales. Models revealed that coastal areas and reef banks were important habitats particularly sheltered lagoons and the steep atoll slopes of the Great Chagos Bank. Lastly, DSMs were used to produce the first abundance estimates of cetaceans in the Chagos Archipelago. This thesis demonstrates that the bathymetric features of the Maldives and Chagos Archipelago are important habitats for cetaceans. The results presented here help fill a major biogeographic gap and complements emerging work on insular cetacean communities elsewhere in the Indian Ocean. These results, therefore, contribute to the development of a more complete regional understanding of cetacean ecology and provide a vital benchmark for monitoring population health and resilience. Ultimately, the long-term survival of these populations depends on efficient conservation management – needing continued application of such multi-modal research strategies, ensuring that the central Indian Ocean remains a functional sanctuary for these ecologically vital marine megafauna.

Awarding Institution(s)

University of Plymouth

Award Sponsors

Bertarelli Foundation

Supervisor

Clare Embling, Simon Ingram, Danielle V Harris, Tom LETESSIER

Keywords

Cetaceans, Chagos Archipelago, Abundance Estimation, Density Surface Modelling, Integrated Species Distribution Modelling, Central Indian Ocean

Document Type

Thesis

Publication Date

2026

Embargo Period

2027-04-01

Deposit Date

September 2026

This document is currently not available here.

This item is under embargo until 01 April 2027

Share

COinS