Senadeera, S.K.2021-10-152021-10-152014UWU/AQT/14/0031http://www.erepo.lib.uwu.ac.lk/bitstream/handle/123456789/7298/AQT%2014%20031-19022021101344.pdf?sequence=1&isAllowed=ySri Lanka is rich with marine cetaceans (whales, dolphins and porpoises). Mainly whale watching industry has immensely developed in Mirissa. But up to date survey data on cetacean populations in Southern marine water is not well documented. During 48 survey days from August - December 2018, along Southern coast off Mirissa on sighting a cetacean, time of sighting, GPS location of boat, number of individuals in the group and observed behaviors were recorded. 5 baleen whales (suborder Mysticeti) and 8 toothed whales (suborder Odontoceti) were recorded during the survey period (13 total species). Blue whale (Balaenoptera musculus) was the most common type of baleen whale with 443 sightings in 80°10'- 80°46' E ,05°55'- 05°17'N and 1 - 3 range was their average pod size while spinner dolphin (Stenella longirostris) was the highest abundant toothed whale with 2765 sightings. 60,4 and 3 sightings of Bryde's Whales (Balaenoptera edeni), Fin whales (Balaenoptera physalus) and Ormura Whales (Balcienoplera mural) were recorded respectively. Under the suborder Odontoceti, 2765 of Spinner Dolphin (Stenella longirostris), 571 of Bottlenose Dolphin (Tursiops truncatus), 125 of Stripped Dolphin (Stenella coeruleoalba), 75 of False Killer Whale (Pseudorca crassidens), 54 of Rissos Dolphin, (Grampus griseus), 30 of Pilot whale (Globicephala macrorhyncus). 7 of Killer Whale (Orcinus orca), 6 of Sperm whale (Physeter macrocephalus) and 3 of Humpback Whale (Megaptera novaeangliae) were sighted. As per whale watcher's data, Humpback Whales were recorded in. Southern coast afttx 2015. Since the whale watching industry in Southern coast is mainly rely on Blue whale population; they are highly vulnerable group from whale watchinL, shipping and fishing activities. Results provide important iniormation on the conservation status °lc:eta:cans. Therefote, a powerful management measure is highly needed tier the conservation of cetacean populations and the sustainability of whale watching industry. Survey was conducted with the support of commercia44hale watching cruise "Raja and the Whales". Keywords: Cetaceans, Mirissa, Abundance, SightingsenAquatic Resources Technology Degree Programme ( AQT)Thesis