Distribution and abundance estimates for cetaceans in the waters off Guam and the Commonwealth of the Northern Mariana Islands (CNMI)

The Mariana Islands Sea Turtle Cetacean Survey (MISTCS) was the first systematic line-transect visual survey of Guam and the Mariana Islands waters designed to estimate the diversity, distribution and abundance of cetaceans and sea turtles. The study area was 573,278 km² defined as 18-10°N and 142-148°E and the survey was conducted from 16 January – 13 April 2007. Standard line-transect protocols were followed with six observers trained in tropical cetacean species identification searching the trackline using 25x “bigeye” binoculars. There were 123/146 on-effort sightings that  included  13 cetacean species (Balaenoptera borealis, B. edeni, Feresa attenuata, Globicephala macrorhynchus, Peponocephala electra, Physeter macrocephalus, Pseudorca crassidens, Stenella attenuata, S. coeruleoalba, S. longirostris, Steno bredanensis, Tursiops truncatus, and Mesoplodon sp.). There were numerous sightings of the sei whales and were the first confirmed sighting of this species in the northern hemisphere south of 20° latitude. Due to extremely difficult viewing conditions we analyzed all on-effort sightings collected in BSS < 7. Because sighting rates were so low, detection functions to estimate abundance were generated by pooling species (similar in sighting characteristics) into for four separate groups (Balanopterids, sperm whales, blackfish [medium size whales] and dolphins) thereby allowing for  individual abundance estimates to be determined.