A robust line transect method

Standard line transect estimation is not robust to departures from the assumptions that animals do not move in response to the observer before detection and that all animals on or near the trackline are detected. We propose field methodology and analysis methods for which estimates are robust to the first assumption and do not require the second. The approach is applicable only to surveys in which two observation platforms are available, such that conventional line transect field methods, with effort concentrated close to the observer, are carried out from one, and a wider area is scanned from the second, possibly with low probability of detection. An example is a ship or terrestrial line transect survey together with a helicopter searching a wider area ahead of the observer. In the case of ship surveys, both observation platforms might be on the same vessel; observers might scan with hand-held binoculars or the naked eye on the first platform, and an observer with powerful tripod-mounted binoculars, screened from the view of the other observers, might search from the second.