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. Diel-vocalizing 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.
Highlights
The seasonal acoustic occurrence and diel-vocalizing patterns of sperm whales Physeter macrocephalus off the west coast of South Africa have not previously been investigated owing to the lack of acoustic research effort in this region
The observed yearround acoustic presence of sperm whales in South African waters indicates residency to a certain extent, and such occurrence is likely due to the high primary productivity in the west coast bioregion associated with the upwelling Benguela Current
The random forest (RF) model results show that environmental conditions significantly influenced the seasonal acoustic occurrence of sperm whales
Summary
The seasonal acoustic occurrence and diel-vocalizing patterns of sperm whales Physeter macrocephalus off the west coast of South Africa have not previously been investigated owing to the lack of acoustic research effort in this region. Recent quantitative analysis of sperm whale population trends indicate that there are limited signs of recovery (Branch & Butterworth 2001, Whitehead 2002) whereby population recovery might be heavily dependent on female survivorship rates (Chiquet et al 2013). Both the International Union for the Conservation of Nature Red List of Threatened Species (Taylor et al 2008) and the Regional Red List (Elwen et al 2016) currently classifies them as Vulnerable. There are no reliable estimates of the sperm whale population in the southern African region (Best 2007)
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