Spatial and temporal patterns in visual and acoustic detection rates of blue whales in the southern California Bight

Abstract

We have been acoustically monitoring blue whale calling within the Southern California Bight, and compared detection rates with sighting rates from visual surveys. Northeast Pacific blue whales produce 3 different call types, a long duration, pulsed 'A' call, a long duration, tonal 'B' call, and down swept 'D' call. The function of each of these call types remains unknown; however, it is well documented that blue whales routinely produce type A and B calls in an alternating sequence for several hours at a time. Acoustic Recording Packages (ARPs) at Cortez and Tanner Banks have been acoustically monitoring the region since August, 2000 providing a detailed continuous record of calling presence for over 2.5 years. There are significant interannual differences in the number of calls detected. In addition to acoustic monitoring at Cortes and Tanner Banks, opportunistic visual and acoustic ship-based surveys have been conducted throughout the southern California Bight. Ship-based acoustic surveys use expendable sonobuoys which allow for the localization of individual calls. By localizing individual calls, the number of animals calling rather than the number of call counts could be determined. These findings suggest seasonal variation in the proportion of animals producing both call types, as well as variation in call rate. Interannual, seasonal, and spatial differences in acoustic and visual detection rates suggest that the relationship between the two detection methods is complex, and cannot be derived from a single "calibration" season. The differing temporal and spatial patterns of the various call types likely indicate different functions for each. The results of this work will help us to determine if there are habitat preferences for blue whales in the Southern California Bight and also if certain habitats lead to the production of different call types, indicating call type, behavior, and habitat relationships.