Diversity and distribution of cetaceans off Mirissa in the southern coast of Sri Lanka I: Relationship with depth
E.P.D.N. Thilakarathne , P.B.T. Pradeep Kumara, R.M.G.N. Thilakarathna
Dolphin and whale watching has become a blooming activity in the tourism industry in Sri Lanka. Proper scientific data is lacking for the understanding and management of dolphin and whale watching activities particularly in the Southern coast. Based from Mirissa, a shipboard survey was conducted for 43 days from January to May 2012 covering 940 km2 .The diversity and distribution of cetaceans inhabits in the continental shelf and slope were recorded from shipboard sighting data. Their species composition, pod size, behavioral pattern were observed and the distribution pattern was mapped.
Total of 8 species of cetaceans were recorded during the study period. They were spinner dolphin (Stenella longirostris), bottlenose dolphin (Tursiops truncates), melon headed whale (Peponocephala electra), fin whale (Balaenoptera physalus), killer whale (Orcinus orca), short finned pilot whale (Globicephala melas), sperm whale (Physeter macrocephalus) and blue whale (Balaenoptera musculus). Distribution pattern and pod size varied with the species. Out of 6675total sightings, 5382 were spinner dolphins and 843 were bottlenose dolphins. Depth levels showed a significant relationship with different cetacean species (p < 0.05). Spinner dolphins, bottlenose dolphins and melon headed whales were always observed near to the shore at relatively shallow areas around 100m depth contours. Fin whales, killer whales and short finned pilot whales were observed around 500 m to 650 m depth while sperm whales and blue whales were observed around 1000 m depth level located further away from the shoreline. Most of the dolphin groups were consisted with relatively large number of individuals about 160 while, most baleen whales were found solitary or small pods consisted with few individuals. Large toothed whales such as sperm whale pods were observed with intermediate number groups about 20 individuals. Those findings can be directly used to conserve and protect cetaceans and prepare a proper management plan to regulate whale watching industry.
Keywords: marine mammals, whale watching, dolphins, porpoises, southern Sri Lanka