Measuring the Lengths of Sperm Whales of the Northern Gulf of Mexico by Wavelet Analysis of their Usual Clicks

Abstract Acoustic recordings of underwater sounds produced by marine mammals present an attractive alternative to costly and logistically complex ship based visual surveys for collecting population data for various species. The first reported use of underwater acoustic recordings in the long-term monitoring of sperm whale populations was by Ackleh et al. (Ackleh et al., 2012). The paper describes counting sperm whale clicks at different locations to track population changes over time. Analysis of sperm whale clicks offers additional insight into sperm whale populations. The echo location clicks (usual clicks) of sperm whales can be used to give an estimate of the whale’s length. The acoustic axis of the whale must be aligned with the hydrophone for accurate length estimates to be made. Most previously reported work using acoustics to measure whale lengths has been done with surface hydrophones recording clicks from a diving whale. With that geometry the whale’s acoustic axis will be approximately aligned with the hydrophone. In the case of monitoring a diving whale it is usual that most of the recorded clicks are from a single whale reducing interfering clicks from other whales. When recording clicks with bottom mounted hydrophones no whale/hydrophone orientation can be assumed. The vast majority of clicks recorded are from off axis whales and therefore produce inaccurate length measurements. It is also not unusual for several clicking whales to be recorded at the same time, producing incorrect whale length measurements due to interference between the clicks. Cepstrum Analysis is the favored technique for making acoustic whale length measurements from bottom moored hydrophones. If enough clicks are recorded from the same whale in enough different orientations, the spurious signals in off axis clicks are “averaged out”. It has been observed, however, that multiple whales clicking at the same time produced errors in the results from Cepstrum Analysis. In this paper a new method for selecting on axis sperm whale clicks from which to determine repeatable length estimates will be presented. It uses wavelet decomposition and a unique quality control test to make better whale length estimates.