The relationship between blue whale (Balaenoptera musculus) visual and acoustic encounter rates was quantitatively evaluated using hourly counts of detected whales during shipboard surveys off southern California. Encounter rates were estimated using temporal, geographic, and weather variables within a generalized additive model framework. Visual encounters (2.06 animals/h, CV = 0.10) varied with subregion, Julian day, time of day, and year. Acoustic encounters of whales producing pulsed A and tonal B call sequences (song; 0.65 animals/h, CV = 0.06) varied by Julian day, survey mode (transit or stationary), and subregion, and encounters of whales producing downswept (D) calls (0.41 animals/h, CV=0.09) varied by Julian day and the number of animals seen. Inclusion of Julian day in all models reflects the seasonal occurrence of blue whales off southern California; however, the seasonal peak in visual encounters and acoustic encounters of D calling whales (July–August) was offset from the peak in acoustic encounters of singing whales (August–September). The relationship between visual and acoustic encounter rates varied regionally, with significant differences in several northern regions. The number of whales heard D calling was positively related to the number of animals seen, whereas the number of singing whales was not related to visual encounter rate.
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