Mark recapture distance sampling: using acoustics to estimate the fraction of dolphins missed by observers during shipboard line-transect surveys

Cetacean abundance estimation often relies on distance sampling methods using shipboard visual line-transect surveys, which assumes that all animals on the trackline are detected and that the detection of animals decreases with increasing distance from the trackline. Mark-Recapture Distance Sampling (MRDS) typically employs a secondary visual observation team and may be used to identify the fraction of animals detected on the trackline when it is suspected that animals may have been missed. For species that are difficult to detect using visual observation methods, such as deep-diving species or those with cryptic surfacing behavior, this secondary team may be prone to the same limitations in detection as the primary observation team and alternative modes of detection may improve estimates. Here we examine the potential use of passive acoustic detection as a secondary platform for MRDS of rough-toothed dolphins (Steno bredanensis) during a combined visual and acoustic shipboard line-transect survey. The average trackline detection probability for rough-toothed dolphins was less than one for both the trial configuration (for the visual team) and independent observer configuration (average p(0) 0.45 for the visual team) and independent observer configuration (average p(0) 0.37 for the visual, p (0) 0.77 for the acoustic and p(0) 0.84 for both teams combined). This study, while limited in scope, strongly suggests that passive acoustic methods may be an effective alternative for estimating p(0) for some cetaceans species.