Population density of sperm whales in the Bahamas estimated using propagation modeling.

Abstract

The population density of sperm whales is estimated using an acoustic model to calculate the probability of receiving their clicks. The model uses estimates of (1) the source level of clicks, (2) the beampattern of the whales’ emitted clicks, (3) the distribution of whale orientations, (4) the loss between source and receiver (derived from acoustic propagation modeling), (5) noise levels at the receiver, (6) the detector’s rates of missed calls and false detections, and (7) sperm whales’ average click rate. These data are combined in a model that propagates simulated clicks from whales at various simulated positions to the receiving hydrophone to estimate the detection function—the probability of receiving a click as a function of distance. This function is then combined with information on whale click rates (performed at a chosen time of day when sperm whales appear to click at a measurably predictable rate) to estimate the population density corresponding to the number of received clicks in a given period of time. Data from the U.S. Navy’s Atlantic Undersea Test and Evaluation Center (AUTEC) in the Bahamas are used to estimate sperm whale population density there. [Thanks to ONR, NOAA, and the Navy’s Environmental Readiness Division for funding.]