Seasonal and Diel Vocalization Patterns of Antarctic Blue Whale (Balaenoptera musculus intermedia) in the Southern Indian Ocean: A Multi-Year and Multi-Site Study

Abstract

Passive acoustic monitoring is an efficient way to provide insights on the ecology of large whales. This approach allows for long-term and species-specific monitoring over large areas. In this study, we examined six years (2010 to 2015) of continuous acoustic recordings at up to seven different locations in the Central and Southern Indian Basin to assess the peak periods of presence, seasonality and migration movements of Antarctic blue whales (Balaenoptera musculus intermedia). An automated method is used to detect the Antarctic blue whale stereotyped call, known as Z-call. Detection results are analyzed in terms of distribution, seasonal presence and diel pattern of emission at each site. Z-calls are detected year-round at each site, except for one located in the equatorial Indian Ocean, and display highly seasonal distribution. This seasonality is stable across years for every site, but varies between sites. Z-calls are mainly detected during autumn and spring at the subantarctic locations, suggesting that these sites are on the Antarctic blue whale migration routes, and mostly during winter at the subtropical sites. In addition to these seasonal trends, there is a significant diel pattern in Z-call emission, with more Z-calls in daytime than in nighttime. This diel pattern may be related to the blue whale feeding ecology.

Long‐Term and Seasonal Changes of Large Whale Call Frequency in the Southern Indian Ocean

“…Our dense set of observations also shows intra‐annual shifts in call frequency with regular and yearly patterns, particularly clear for the Antarctic blue and fin whales (Figures e and b). They are best documented for the Antarctic blue whale, based on precise measurements of more than 1,000,000 individual calls detected year‐round over the network (Leroy et al, ). These short‐term changes in the Antarctic blue whale call frequency were first documented off western Australia by Gavrilov et al (), based on PSD measurements.…”

“…The data are continuously sampled at a rate of 240 Hz (see D'Eu et al, , for instrument details) and are collected every year during the annual voyages of the R/V Marion Dufresne to the French Southern and Antarctic Territories. The records are almost continuous for the past 6 years (2010–2015), except for a few months or years depending on the site, due to battery failures or instrument losses (see Leroy et al, , for details). The whole 6‐year‐long data set was analyzed in this study, except the NCRO site in 2010 and 2013, hindered by high noise levels likely due to strumming of the mooring line.…”

“…Frequencies were averaged per day and then per week, and over all sites. Note that since the Antarctic blue whale Z-calls are present year-round in our study area; although in smaller number during the austral summer (Leroy et al, 2016), the frequency measurements are available year-round for this call type.…”

Antarctic Blue Whale calls detection based on an improved version of the stochastic matched filter

“…As for other blue whales [1], [2], [3], Passive Acoustic Monitoring (PAM) seems to be a very efficient tool to develop automatic Antarctic Blue Whale calls (Z-calls) detection algorithms and then facilitate large data analysis to monitor the specie in vast areas. They are mostly based on signal cross-correlation theory: matched filters have been applied [4], as well as spectrogram-based template matching correlation [3], or more recently subspace-detection algorithm [5], [6]. However, those methods do not perform well at low Signal to Noise Ratio (SNR), whether it is due to high background noise environment (e.g.…”

“…Z-call monitoring is usually performed using acoustic measurements in the SOund Fixing And Ranging (SOFAR) channel [2], [3], [6]. However, data recorded by threecomponent Ocean Bottom Seismometers (OBS) and hydrophone (OBSh) landed on the sea-floor, cover the desired frequency range.…”