Acoustic techniques have the potential to increase the reliability of cetacean species identification during shipboard surveys. The whistles of nine odontocete species were compared using data collected from a towed array and sonobuoys deployed during dolphin abundance surveys in the eastern tropical Pacific. Twelve variables were measured manually from spectrographic displays of each whistle (n=912). Multivariate discriminant function analysis (DFA) resulted in 49.9% of whistles being classified to the correct species. It was hypothesized that some whistles carry less species-specific information than others, therefore, groups of five whistles were averaged to reduce the effect of these ambiguous whistles. Correct classification increased to 65.4% when DFA was run on the averaged data set. A species identification decision tree that used 7 of the 12 whistle variables was constructed using nonparametric techniques (classification and regression trees) and resulted in 53.1% correct classification when applied to the original data set. Prior probabilities were added to the decision tree based on sighting rates for each species in the study area, resulting in 56.7% correct classification. The species identification decision tree provides a relatively simple acoustic method that can be used to augment conventional visual techniques.

Minke whales, Balaenoptera acutorostrata (Lacépède), have been considered a rare species in Hawaiian waters due to limited sightings during visual and aerial surveys. However, our research suggests that they are more common than previously considered. In spring 2005, a combined visual-acoustic survey of cetaceans in Hawaiian waters resulted in the sighting of a minke whale within 22 km of Kaua'i. Minke whale vocalizations were also detected at several other locations near Kaua'i and O'ahu. These 2005 reports are the first from nearshore (<50 km) Hawaiian waters despite years of previous shipboard and aerial surveys. The lack of historical sightings is likely due to misidentification or the inability to detect these animals during poor sighting conditions. We recommend that future cetacean surveys in Hawaiian waters include a passive acoustic component to increase the likelihood of detecting minke whales.

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Recordings of low-frequency sounds were made in the presence of blue whales (Balaenoptera musculus) in 1998 and 1999 in waters off northern Peru during a marine mammal research cruise. Recordings were made on DAT from sonobuoys deployed near vocalizing whales. Photographic identifications and biopsy samples were collected from several animals that were recorded. Over 20 signals were extracted and analyzed from DAT segments that contained the best signal quality. All signals from the 1998 encounter consisted of rapid, frequency modulated (FM) downsweeps (begin frequency x̄=94, S.D.=13.6 Hz; end frequency x̄=59, S.D.=7.8 Hz; frequency range x̄=41.7, S.D.=14.9) of relatively short duration (x̄=1.2, S.D.=0.3 s). Approximately half of the signals exhibited a characteristic s-shape pattern when examined spectrographically. Long duration sweeps and pulses, typical of blue whales sounds from the eastern North and Central Pacific, were not detected. The vocalizations recorded in this study are compared to recordings of short duration blue whale sounds made elsewhere, including off California, Baja California, and southwestern Australia. In general, short duration blue whale sounds from these other areas are more variable and/or have different frequency characteristics. The function of short duration, FM blue whale sounds is unknown.