Tracking and source level estimation of multiple sperm whales in the Gulf of Alaska using a two-element vertical array

Abstract

Between 15-17 August 2010 a two-element vertical array (VA) was deployed in 1200 m deep water off the continental slope of Southeast Alaska. The array was attached to a longline fishing buoyline at 300 m depth, close to the sound-speed minimum of the deep-water profile. The line also attracted seven depredating sperm whales to the area, each generating impulsive `clicks' that arrived on the VA via multiple ray paths. The propagation model BELLHOP was used to model relative arrival times and vertical elevation angles of click ray paths as a function of depth and range from the VA. The resulting tracking system yielded range-depth tracks of multiple animals out to at least 35 km range. These locations, along with the transmission loss estimates of the model, permitted the sound source levels to be recovered. Here we present the consistency of source levels from individuals over time, the degree of source level variation between individuals, and possible correlations between inter-click interval and source level. This analysis suggests how a relatively simple ocean observing acoustic system could localize bioacoustic signals over large ranges, given the appropriate deployment configuration.