Temporal separation of blue whale call types on a southern California feeding ground

Abstract

Northeast Pacific blue whales, Balaenoptera musculus, migrate annually between productive summer feeding grounds off North America and tropical winter breeding grounds off Central America. These migratory movements have been confirmed through acoustic monitoring of the long-duration, low-frequency sounds produced by males (type B calls). However, other calls in the species' repertoire might prove a better proxy for the migratory and foraging behaviour of the population as a whole. To explore the seasonal and daily calling behaviour of this population, we evaluated the occurrence of three blue whale call types (song B, singular B and D) recorded between 2000 and 2004 at Cortez and Tanner Banks, a summer feeding area offshore of southern California. We recorded a significant temporal separation among the type B and D calls, both seasonally and daily, suggesting preferred use of certain call types during different behavioural states. A consistent seasonal pattern was evident, with D calling from April to November and song and singular B calling from June to January. In addition, D calls were heard primarily from dawn through to dusk, in contrast to the crepuscular pattern of song, suggesting that the production of D calls is related to feeding behaviour, which occurs primarily during the day on aggregated krill at depth. An increase in the length of the overall calling season was also observed from 2000 to 2004 and may be related to increased prey availability in the Southern California Bight relative to more southerly feeding areas.