Instantaneous acoustical response of marine mammals to abrupt changes in ambient noise

Abstract

Four months of passive acoustic data recorded at Thirtymile Bank in offshore southern California have been analyzed to describe instantaneous vocal response of marine mammals to abrupt changes in ambient noise. Main contributors to the distinctive regional soundscape are heavy commercial shipping, military activities in the naval training range, diverse marine life and natural sources including wind and tectonic activity. Many of these sources produce intense, irregular and short-term events shaped by local oceanographic conditions, bathymetry and bottom structure (Thirtymile Bank blind thrust). We seek to attribute detected changes in cetacean vocal behavior (loudness, calling rate, and pattern) to these events and differentiate the reaction by noise source, its intensity, frequency and/or duration. Main target species are blue and fin whales. Initial hypotheses formulated after data scanning are tested statistically (2D histograms and PCA). To quantify the vocal behavior variations, an innovative detection approach based on pattern recognition is applied, which allows for extraction of individual calls with low false alarm and high detection success comparable to those of a human analyst. Obtained results relate cetacean acoustic behavior to ambient noise variability and thus help refine existing cue-based formulae for estimation of whale population density from PAM data.