Passive acoustic and visual monitoring of humpback whales (Megaptera novaeangliae) in the Olympic Coast National Marine Sanctuary: Importance of quantifying call type

Abstract

Humpback whales (Megaptera novaeangliae) produce a variety of vocalizations such as social and feeding calls as well as patterned calls that comprise song. Typically, social and feeding vocalizations do not follow the highly structural format of song. High-frequency acoustic recording packages were deployed in the Olympic Coast National Marine Sanctuary July 2004 through June 2009 while visual surveys were conducted throughout the year at approximately monthly intervals. Humpback whales were detected visually and acoustically; however, there was a mismatch in the peak seasonality of these detections. Visual detections occurred in all seasons but peaked in summer and early fall. Acoustic detections were documented primarily in late summer to early winter. Male humpback whales are known to produce long songs primarily during their winter breeding season. To test whether the detection differences between visual and acoustic surveys could be explained by changes in the whales' vocal behavior, we quantified the relative occurrence of song and non-song calling using a variety of metrics and related the occurrence to the visual survey sightings. We show how identification of the type of acoustic detection is an important consideration and can help address biases introduced by seasonal differences in the production rate of reproduction-related calls.