Sighting of cetaceans off Kochi: A spatial and temporal analysis

Abstract

The distribution of marine mammals in the Indian seas is poorly understood. The aim of this study was to use visual sighting survey to examine the distribution of cetacean species off Kochi, south-west coast of India. The relation between cetacean distribution and physiographic variables (depth and distance from the shore), oceanographic variables [sea surface temperature (SST) and sea surface salinity (SSS)] are also discussed. A total of 577 pods of dolphins were sighted from 625 fishing operations off Kochi coast during October 2018 to September 2019. In the encounters with 577 pods of dolphins, a total of 2599 individuals, represented by four species viz, Sousa chinensis (Indo-pacific humpback dolphin), Stenella longirostris (spinner dolphin), Tursiops aduncus (Indo-Pacific bottlenose dolphin) and Delphinus capensis (long beaked common dolphin) were recorded. A total area of 20,456.011 km2 was covered in this survey and the distribution of cetaceans was plotted in grid and Shannon-Weiner diversity index was calculated. There was no significant difference in seasonal distribution of species in the study area, however sightings were less in the monsoon season compared to other seasons. Dolphins were observed in water depths ranging from 7 to 127 m, with majority of the sightings occurring in less than 50 m and between 3 and 65 km from the nearest shore. All the species were recorded at mean SST of 28°C and mean SSS of 34 ppt. The study shows the need for further research covering other locations along the Indian coast.

Keywords: Arabian sea, Distance from shore, India, Seasonality, Sightings, Water depth