Acoustic surveys of humpback whales: Calibration experiments off the east coast of Australia

Abstract

Male humpback whales are frequently heard singing during migration and on their breeding grounds, often continuously for many hours, and the sounds are audible for tens of kilometers. Acoustic surveying may be useful in quantifying whale movements in oceanic waters beyond the range of land surveying, and it might also be a useful addition to visual monitoring in areas where this is possible. For acoustic surveys to be of use, the acoustic cues of the whales must be quantified and calibrated against the numbers of whales in an area. A combined visual and acoustic survey of whales migrating close to shore was conducted off the coast of southeast Queensland. Song activity, measured using two indices, correlated significantly with the number of passing whales determined visually. Linear regressions of daily measures of song activity with numbers of whales seen lead to estimates of ratios of singers with whales seen at 0.127±0.027 (95% confidence interval) and singer hours with whales seen at 0.288±0.065. The possible use of these indices for conducting stand-alone acoustic surveys will be discussed.