Kathleen M. Stafford
Hatfield Marine Sci. Ctr., Oregon State Univ., Newport, OR 97365
Christopher G. Fox
Natl. Ocean. and Atmos. Admin., Newport, OR 97365
Bruce R. Mate
Oregon State Univ., Newport, OR 97365
Blue whale calls were recorded off central California in the Fall of 1993. These calls were characterized as to duration, frequency downsweep, intercall interval, and sound-pressure level. Average values were determined from 303 calls, including up to three harmonics (fundamental downsweep from 18.9 to 17.2 Hz over 16 s). These frequency-domain characterizations were then used to develop numerical time series (kernels) that, when convolved with the original time series, produce correlation peaks indicating the presence of blue whale calls (matched filter). When harmonics were present in the data, a combined kernel, including the fundamental frequency and first harmonic, improved the signal to noise ratio over use of the fundamental kernel alone. These matched filters were able to detect blue whale calls even in very ``noisy'' time series. When applied to hydrophone recordings from three U.S. Navy SOSUS (SOund SUrveillance System) arrays, it is possible to produce locations for blue whale calls by timing the arrival of individual calls and applying least-squares techniques. This information can be used to increase our knowledge of blue whale distribution in the northeast Pacific. The methods described here may also be extended to other species that employ low-frequency vocalizations or to other ocean areas.