Visual and Acoustic Line Transect Surveys

An observer uses
An observer uses "big-eye" binoculars to scan for cetaceans from the flying bridge of the Oscar Elton Sette.
A group of melon-headed whales encountered off of Hawaii Island. Photo: Marie Hill; NMFS Permit #774-1714.
A group of melon-headed whales encountered off of Hawaii Island. Photo: Marie Hill; NMFS Permit #774-1714.

Visual surveys to assess cetacean abundance and distribution are conducted from the flying bridge of a large (> 200 ft.) research vessel using 25 × 150 "big eye" binoculars. Observers scan the water for cetaceans while the vessel steams along predetermined transect lines. Often cetaceans are encountered in groups. When cetaceans are seen, the observers record data such as the species, location (latitude and longitude) of the encounter, general behavior of the animals, and estimates of the number of cetaceans in the group. The sighting data are later analyzed using distance sampling statistical models and imported into a Geographical Information System (GIS) for further spatial analysis. Whenever possible, digital photographs of encountered cetaceans are obtained to help confirm the species identification and document unique characteristics that enable identification of individual cetaceans.

Simultaneously, observations of the underwater sound field along the survey track are made using a towed hydrophone array. A typical towed array consists of 4 hydrophone elements with linear sensitivity to frequencies between 1 kHz and 100 kHz. As the array is towed on a cable behind the ship, acoustic signals are transmitted up the cable to on-deck computers outfitted with software that allows real-time visualization, recording, and spatial localization of cetacean sounds. Acoustic detections of cetaceans are noted. Data on the location of detections can be used in line transect models and incorporated into GIS databases for comparison with corresponding visual detections and habitat data.