Medium-term stereophonic recording of humpback whales in Sainte Marie channel, Madagascar: daily variation in whale density

Abstract

Humpback whales (Megaptera novaeangliae) are well known to be particularly active acoustically. In 2007 the research team started to investigate humpback whales in the breeding area in the Sainte Marie channel (Madagascar). An array of 2 synchronous hydrophones was deployed in 2012 as part of a feasibility study for the deployment of a permanent acoustic array in the channel for the research programme, BAOBAB. Eight continuous sets (mean duration: 27 h 15 min) were recorded between 3 August and 11 September. Stereophonic recording allows the discrimination of acoustic sources that are not spatially overlapping, as the bearing to the emitting whale can be estimated from the Time Difference Of Arrival between the sensors. Based on cross-correlation functions analyses, this paper proposes an assessment of the number of emitting whales in a given underwater soundscape, and changes in their numbers over the covered time period. The first results showed that this value changed between 0 and 10 singers over the survey period of more than a month, and a peak in density was observed in the third week of August. Daily trends indicated highest density at night and lowest density just before nightfall. The study attempts to link acoustic activity and spatial occupation to reveal interactions between the detected emitting whales.