Abstract:
Knowledge of cetacean occurrence and behaviour in southern African waters is limited, and passive acoustic monitoring has the potential to address this gap efficiently. Seasonal
acoustic occurrence and diel-vocalizing patterns of sperm whales in relation to environmental
conditions are described here using passive acoustic monitoring data collected off the west coast
of South Africa. Four autonomous acoustic recorders (AARs) were deployed on 3 oceanographic
moorings from July 2014 to January 2017. Sperm whale clicks were detected year round in most
recording sites, with peaks in acoustic occurrence in summer and late winter through spring. Dielvocalizing patterns were detected in winter, spring and summer. Higher percentages of sperm
whale clicks were recorded by AARs deployed at 1100 m water depth compared to those concurrently deployed at 850 and 4500 m, likely inferring that the whales exhibited some preference to
water depths around 1100 m. Acoustic propagation modelling suggested a maximum detection
range of 83 km in winter for sperm whale clicks produced at 1100 m. Random forest models classified daylight regime, sea surface height anomaly and month of the year as the most important
predictors of sperm whale acoustic occurrence. The continuous acoustic occurrence of sperm
whales suggests that the study area supports large biomasses of prey to sustain this species’ food
requirements year round. This is the first study to describe the seasonal acoustic occurrence and
diel-vocalizing patterns of sperm whales off the west coast of South Africa, extending knowledge
of the species previously available only through whaling records.
