Acoustic censusing of sperm whales at Kaikoura, New Zealand : an inexpensive method to count clicks and whales automatically
Commercial hunting of sperm whales ended in 1988. Today, populations appear to suffer from a shortage of large males, depressing fertilisation rates. Sperm whales spend most of their time underwater, feeding at depths to which little or no sunlight penetrates. They produce a variety of sounds, most of which are echolocation signals called clicks. To gather data on distribution and abundance of bachelor males off Kaikoura, New Zealand, clicks were recorded along stratified acoustic transects between 1991 and 1994. Transect recordings were made using a single omnidirectional hydrophone. In this study, dedicated DSP-software was developed to count clicks automatically, and to facilitate the calibration of these counts through automated and interactive analyses of dive cycle and range experiments. Calibration parameters needed to estimate whale abundance comprise the proportion of time sperm whales spend clicking regularly, the rate at which they typically produce regular clicks, and the proportion of time we are able to detect regular clicks when recording at the dive location and at range. Interactive analysis of an example dive of a single whale recorded under ideal conditions gave the following results: The mean of all inter-click intervals measured within bouts of regular clicks was 0.801 seconds (n=2203, cv 27.5%, min 0.076 s, max 1.735 s), which corresponds to an average click rate of 1.25 s-1 . Derived from instantaneous click rates [Footnote: The mean of a set of intervals Xi is not equal to the reciprocal mean of the corresponding set of instantaneous rates Xi-1], however, the mean click rate equals 1.382 s-1 (n=2203, cv 48.3%, min 0.576 s-1 , max 13.197 s-1). Including silences between bouts, the 2244 clicks analysed account for an average rate of 0.97 s-1 over a period of 38:36 minutes between the first and the last regular click detected. Considering ten seconds between fluke-up and the first click recorded, and the silent ascend to the surface between the last regular click and two surface clicks found 3:37 minutes later, the average click rate over a dive time of 42:23 minutes is 0.88 s-1. Using an optimised set of detection parameters during automatic click counts of this dive, about two thirds of all energy peaks registered by the software were real sperm whale clicks. The other third consisted mainly of ambient sea noise and click echoes. For recordings with a lower proportion of echoes, and more than one whale clicking, the percentage of real clicks in automatic counts is likely to reach about 80%. Off Kaikoura, two to three whales are expected within hydrophone range most of the time. Interactive analyses of dive cycles also yield valuable information on the acoustic behaviour of sperm whales. In several dive cycles analysed, inter-click intervals were found to oscillate periodically during bouts of regular clicks. These oscillations correlated with click amplitude and may indicate echolocation sweeps of a whale scanning its surroundings. To quantify absolute abundance of whales, the proportion of time we are able to detect clicks at range must be known. When recording an individual from the distance, its identity could be verified objectively via a sonobuoy deployed at the dive location.
Advisor: Dawson, Steve; Slooten, Liz
Degree Name: Master of Science
Degree Discipline: Marine Science
Publisher: University of Otago
Research Type: Thesis