Distribution and Abundance Estimates for Cetaceans in the Waters off Guam and the Commonwealth of the Northern Mariana Islands

Abstract

Cetacean distribution and abundance are reported from the first systematic line-transect visual survey in the waters of Guam and the Commonwealth of the Northern Mariana Islands (CNMI). The survey was conducted during January – April 2007 following standard line-transect protocols. Trackline coverage (11,033 km) was dominated by high sea states (88.2%); however, 13 cetacean species were recorded. The sperm whale (Physeter macrocephalus) was the most frequently encountered whale, followed by Bryde’s and sei whales (Balaenoptera edeni and B. borealis, respectively). Occurrence of the sei whale is unique, because the species had not been confirmed to occur south of 20° N. The pantropical spotted dolphin (Stenella attenuata) was the most frequently sighted delphinid, followed by the striped dolphin (Stenella coeruleoalba) and false killer whale (Pseudorca crassidens). Humpback whales (Megaptera novaeangliae)were acoustically detected and later seen off Saipan. Numerous cetacean sightings were associated with steep bathymetric features including the West Mariana Ridge, the Mariana Ridge, and the Mariana Trench. Abundance estimates were based on 80 on-effort sightings for 12 species. Species were pooled into three separate groups for estimating detection probabilities: Balaenoptera spp., blackfish (medium-size odontocetes), and small dolphins. A separate detection function was generated for the sperm whale. Precision of abundance estimates are very low for all species due to low sighting rates and high sea states; however, these abundance estimates serve as the best scientific data available for the area and establish vital baseline information for future research efforts.