Species rich but data poor: leveraging distribution modelling techniques to map cetacean occurrence in South Asian waters

Abstract

Cetaceans play important ecosystem roles but are challenging to study since they are highly mobile and spend their entire life at sea. More than a third of all cetaceans occur in South Asian waters where they overlap with several key threats including fisheries, pollution, and high vessel traffic. However, baseline information on their occurrence at the sub-continental scale is lacking even for relatively well studied species. To address this gap, we collated sighting information on cetaceans from the North Indian Ocean region (5-25{degrees}N, 65-95{degrees}E) and used species-tuned distribution models and ensemble modelling approaches with physiographic and oceanographic predictors to map habitat suitability and species richness. We used Multivariate Environmental Similarity Surfaces (MESS) indices and expert reviews to validate our predictions and better infer species distribution. We collated 2329 sighting records for 31 species and modelled the distribution of 18 species. Areas of high species richness, defined by high slope and bathymetric complexity, occurred along the east coast of India, in south Sri Lanka, and around the Lakshadweep and Andaman and Nicobar archipelagos. Species richness patterns were generally consistent with the six identified ecoregions in the area. Most of our predictions were given high scores by the expert reviewers. Our maps underpredicted the occurrence of a few oceanic species, highlighting the need for focused offshore surveys, and overpredicted areas with high MESS indices for all species. We discuss the occurrence patterns and their drivers for these cetacean species while highlighting knowledge gaps and the importance of fine-scale distribution data for conservation planning, especially during the development of regional management plans for cetaceans.