A combined acoustic and visual survey of humpback whales off southeast Queensland

During their migrations between low latitude breeding areas and high latitude feeding areas, male humpback whales, Megaptera novaeangliae, are frequently heard singing, often continuously for many hours, and the sounds are audible for tens of kilometres. The stock that passes close to the coast of southeast Queensland has been extensively surveyed visually, but little is known of movements of whales that pass out of sight of land here, or in other areas of the world, where the migratory paths of humpback whales are often across open ocean. Acoustic surveying may be useful in quantifying whale movements in oceanic waters beyond the range of land surveying and an addition to visual monitoring. 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. In 1997 we performed a combined visual and acoustic survey of whales migrating close to shore on the coast of southeast Queensland. Song activity was measured using two indices: number of passing singers and number of singer-hours observed within a 10km sector, and correlated with the number of whales passing through the area determined visually. Both were significantly correlated with r = 0.68 and 0.64 for singers and singer-hours respectively on a daily basis, and 0.79 and 0.89 respectively on a weekly basis. Linear regressions of daily measures of song activity with numbers of whales seen lead to estimates of ratios of singers with whales seen of 0.127 ± 0.027 (95% confidence interval) and singer-hours with whales seen of 0.288 ± 0.065. We discuss the possible use of these indices for conducting stand-alone acoustic surveys