Passive acoustic monitoring of Antarctic blue and fin whales off the South African west coast

Abstract

Antarctic blue and fin whales were assumed to be common migratory whale species prior to being drastically reduced to almost extinction in the Southern Hemisphere by industrial whaling between 1904, and the mid-1970s. Following the end of commercial whaling some four decades ago, still little is known of their distribution, migrations, current population levels and post-whaling population recoveries. Both of these whale species emit low-frequency, highintensity calls which are thought to be used for feeding and communication during mating. The development of Passive Acoustic Monitoring (PAM) systems has assisted researchers across the globe to understand the distribution and seasonal abundance of different whale species through the detection of calls from these whales. Understanding the acoustic ecology of Antarctic blue and fin whales might improve the conservation and management strategies of these highly depleted species. Seasonal acoustic occurrence and behaviour of Antarctic blue and fin whales off the South African west coast were determined using bioacoustic data collected through two autonomous acoustic recorders (AARs) between December 2015 and January 2017. Blue whale Z-calls were detected yearround with a peak in July, while fin whale 20 Hz pulses were detected seasonally with a peak in June by AAR1 deployed at 1118 m water depth. Blue and fin whale calls were detected seasonally with a similar peak in May by AAR2 deployed at 4481 m water depth. The blue whale 27 Hz chorus, and blue and fin whale 28-18 Hz chorus followed a similar trend as the seasonal acoustic occurrence of individual Z-calls and 20 Hz pulses of the two species respectively. Around AAR1, blue whale Z-call rates peaked during the day in autumn and winter, day and night in spring, and night in summer. Fin whale diel 20 Hz call rates peaked during the day in autumn, and peaked slightly during dawn in winter, with no calls for spring and summer. Around AAR2, blue whale diel Z-call rates peaked during the day in autumn, winter and summer, with no calls recorded in spring. Fin whale diel 20 Hz call rates peaked during the day in autumn, day and night in spring, winter showed no peak differences and no calls were recorded for summer. Results from the ANOVA one-way analysis of variance showed significant differences in both Antarctic blue and fin whale call presence and call rates by month. Diel call occurrence by season for both Z-calls and 20 Hz pulses showed no significant difference in both stations. Diel call rates by season for Z-calls were only significant in winter and spring in AAR1, and only in autumn in the deep station. Diel call rates by season for the 20 Hz pulses showed no significant difference in both stations. In addition to the detection of the key species of this study, other known biological calls produced by Antarctic minke whales, humpback whales and southern right whales were acoustically detected seasonally in both the shallow and deep stations. Other unknown sounds and anthropogenic sounds were also acoustically detected seasonally in both stations. This study highlights the South African west coast as an important year-round habitat and seasonal breeding or overwintering habitat of these whales. Additionally, the year-round acoustic occurrence in this region supports the notion that blue whale migration patterns are more dynamic than previously perceived. Bioacoustic data allowed us to continue to monitor the seasonal acoustic occurrence and behaviour of blue and fin whales through passive acoustics.