Sound production of gray whales, Eschrichtius robustus, along their migration route: a new approach to signal analysis.

Sound production of gray whales was investigated to determine their acoustic repertoire along the migration route, and to compare sound production in deep and shallow water. Recording was conducted off Monterey Bay and Carmel Bay, California, during the annual migrations of 1988 through 1991. Sounds were analyzed through digital signal processing. Six acoustic variables were measured. Three variables were used for final classification, chosen for their clarity in the presence of high levels of ambient noise (poor signal-to-noise ratio). These variables were 3-dB bandwidth, center frequency, and harmonic/sideband interval. Q ratio was used as an indicator of how broadband or narrow band a signal was relative to other gray whale signals. Four categories of signals were determined: M1, M3, M4, and M5. All signal types were concentrated below 1500 Hz. M3 signals had the lowest center frequency, averaging below 100 Hz. M1 were pulses and bonging signals, M3 were low-frequency moans, M4 were grunts, and M5 were subsurface exhalations. The rate of sound production was lower when whales traveled over deep water than over shallow water. Rate of sound production was less along the migration route compared to the lagoons. M3 signals were the most common, comprising 46.6% of the repertoire along the migration route, with M1 signals comprising 37.4%.