Abstract:

Preliminary results are presented from a combined acoustic and visual study of blue whales in the Channel Islands National Marine Sanctuary off the California coast, conducted during four 3-day expeditions in July 1996. Acoustic recordings were collected using GPS positioned sonobouys arrays, with up to five sonobuoys deployed at approximately 1-km spacing. Visual observations were made to estimate the number of blue whales in the vicinity of the arrays, and to record whale behaviors. Significant numbers of blue whales (5--15) were sighted each day, and as many as 100 blue whale calls per hour were recorded. About one-third of the time, large ships transited through this area, providing loud low-frequency sound sources to examine blue whale sensitivity to noise. Low-frequency blue whale calls, similar to those previously designated ``A'' and ``B'' in the literature, were recorded. Additional blue whale calls include an approximately 4-s duration frequency downswept tone between 30 and 80 Hz, often heard at about the same intensity as the more typical ``A'' and ``B'' calls. Cross correlation of signals received by sonobuoys provides information to localize calling whales. The location accuracy relative to the different call types will be discussed.