We present a passive acoustic method for estimating the density of echolocating cetaceans that dive synchronously, based on isolation of dive starts using a field of distributed bottom-mounted hydrophones. The method assumes that all dive starts of the target species within a defined area are detected, and that independent estimates of dive rate and group size are available. We apply the method to estimate the density of Blainville’s beaked whales (Mesoplodon densirostris) at the Atlantic Undersea Test and Evaluation Center (AUTEC) in the Bahamas during the time of a multi-ship active sonar exercise. Estimated densities for the 65 h before the exercise, 68 h during, 65 h after, and the final 43 h monitored were 16.99 (95% CI 13.47–21.43), 4.76 (3.78–6.00), 8.67 (6.87–10.94), and 24.75 (19.62–31.23) respectively, illustrating a possible avoidance reaction. Results for the 65 h before were compared with those from the click count density estimation algorithm developed by Marques et al. [Marques T, Thomas L, Ward J, DiMarzio N, Tyack P. Estimating cetacean population density using fixed passive acoustic sensors. An example with Blainville’s beaked whales. J Acoust Soc Am 2009;125(4):1982–1994]. The click count-based estimate was 19.23 animals/1000 km2 (95% CI 12.69–29.13)—similar (13% higher), but with higher variance (CV 21% for click count method versus 12% for the dive count method). We discuss potential reasons for the differences, and compare the utility of the two methods. For both, obtaining reliable estimates of the factors that scale the measured quantity (dive starts or detected clicks) to density is the key hurdle. Defining the area monitored in the dive count method can also be problematic, particularly if the array is small.
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