Automatic detection of sperm whale sounds from the Gulf of Alaska

Abstract

Records from whaling ships in the Gulf of Alaska indicate that sperm whales were present in summer but were much less common in winter. These records could have been biased, perhaps strongly, by seasonal conditions: ships were less likely to be present in winter than summer, and sighting conditions may have been worse in winter. To obtain an independent and newer estimate of sperm whale seasonality, six autonomous hydrophone recorders were deployed for one year. After retrieval, recordings were analyzed automatically to detect sperm whale sounds. The detection technique comprised several successive stages: spectral transform, noise cancellation, band-limited energy summation, periodic autocorrelation, and thresholding. The autocorrelation step was highly effective for distinguishing between sounds of sperm whales and noise sources. The detection threshold was set so that, in test data, no noise sources were detected and approximately 2/3 of known sperm whale sounds were detected. Use of such a high threshold allows for an accurate index of sperm whale seasonal and geographic occurrence. Results indicate that sperm whales are indeed present in winter in the Gulf of Alaska. Use of automatic acoustic detection allows results to be replicated in the future.