Measuring Sperm whales via Acoustics and Photogrammetry

Abstract

The most common vocalisations heard from sperm whales are short, broadband clicks which often display a decaying, evenly spaced, multi-pulse structure. The time between these pulses (inter-pulse interval: IPI) represents the two-way time for a pulse to travel between the air sacs located at either end of the sperm whale‟s head. The IPI therefore, is measure of head length and via an allometric relationship, total length. In order to compare IPIs of known individuals to an independent measure of length, a new, boat-based fully digital stereo photogrammetric system was developed and its measurement accuracy assessed. A field test was conducted measuring objects of known length throughout a distance/angle network. Mean measurement error over the entire network was 0.82% and it was found that the largest contributor to measurement error was the repeated manual selection of conjugate points in stereo images. IPIs were measured using a newly developed software plugin1 for Pamguard, an open-source software package for passive acoustic monitoring. The plug-in was based on the recently developed “bent horn” theory of sound production in sperm whales. Previous studies relating IPI to an independent measure of length have suffered from very small sample sizes. Therefore, this study measured 21 sperm whales off Kaikoura, New Zealand using the new photogrammetric and acoustic method. Both measurement methods produced results which were repeatable and more accurate than previously published studies (mean C.V = 1.57% and 0.63%, respectively). A new equation describing the relationship between IPI and total length was proposed. To further boost sample size, the IPI plugin was also used to measure IPIs from archived recordings of whales that had been measured with a previous photogrammetric system. This brought the total sample size of different whales with both IPI measurements and stereo photogrammetric measurements to 33. Finally the IPI plugin was used to estimate acoustically the growth rates of 29 seasonal resident sperm whales which had been repeatedly recorded between 1991 and 2009. Most whales showed an increase in IPI over time representing growth. For individuals that were recorded many times over several years von Bertalanffy growth curves fitted the data well (mean r2 = 0.83; range = 0.13- 0.99). This is the first time growth has been estimated using an acoustic method and because it is non-lethal it has the potential to allow different types of questions related to growth to be investigated.