Whale shark habitat assessments in the northeastern Arabian Sea using satellite remote sensing

One of the major requirements for the growing whale shark tourism industry is to identify potential areas of their aggregation for sighting. This would require baseline information on the occurrence of whale shark and the associated environment. In this context, the relationship between whale shark landings, phytoplankton concentration and sea surface temperature (SST) in the continental shelf and offshore regions of Gujarat coast were examined using satellite data from 1998 to 2000. Monthly images of chlorophyll-a (chl-a) concentration, an index of phytoplankton biomass and SST were derived for the eastern Arabian Sea from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and National Oceanographic and Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR), respectively. Whale sharks (Rhincodon typus) landing data were obtained from a survey conducted by Trade Records Analysis of Flora and Fauna In Commerce (Traffic)-India of the World Wide Fund (WWF)-India and the Central Institute of Fisheries Technology (CIFT), India. Mean chl-a concentration in the study area (between 20-22° N and 69-70° E) covering the continental shelf and adjoining offshore region of coast (depth > 25 m) was observed to be significantly higher (4.23 mg m-3 in February and 3.88 mg m-3 in March) compared to regions seaward of the study area (mean of 1.51 mg m-3 for February and 1.16 mg m-3 for March) and in southern latitudes of the eastern Arabian Sea (mean of 0.27 mg m-3 for February and 0.23 mg m-3 for March). The SST in the study area ranged from 23-26°C for February and March, whereas in the southern latitudes, it ranged from 27-29°C. The SST in regions outside the study area was marginally warmer by 0.5°C. A significant relationship between whale shark landings off Gujarat, chl-a concentration and SST was observed. Results presented in this study contribute to the idea that the combined use of ocean colour and SST images are an appropriate tool to identify potential areas of whale shark aggregation for sightings.