Long-range tracking of bowhead whale calls using directional autonomous seafloor acoustic recorders

Abstract

Since 2007 “Directional Autonomous Seafloor Acoustic Recorders” (DASARs) have been deployed at five sites across a 280 km swath of the Beaufort Sea continental shelf to record bowhead whale (Balaena mysticetus) calls during their autumn migration. Composed of an omnidirectional pressure sensor and two horizontal directional sensors measuring particle motion, DASARs provide information for determining the bearing to a sound source. In previous analyses, bearings obtained from multiple DASARs within a single site have been used to localize calls, with a maximum baseline separation of 21 km between instruments. Here, we use data collected from two different sites to exploit a 45 km instrument separation for tracking bowhead whale calls detected under low ambient-noise situations in 2009 and 2011. These data sets have been manually analyzed to extract a series of calls from individual whales swimming from one site toward another. The fact that the tracking method does not require relative arrival time information makes matching calls between these widely separated shallow-water recordings practical. Both empirical and numerical transmission loss models are used to investigate the relationship between animal orientation, source level, and the presence of harmonics.