Clicking throughout the year: sperm whale clicks in relation to environmental conditions off the west coast of South Africa

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.

Killer whale acoustic patterns respond to prey abundance and environmental variability around the Prince Edward Islands, Southern Ocean

“…Humpback whales ( Megaptera novaeangliae ) are known to produce units within their songs [ 42 ], some of which are similar to killer whale social calls, and we differentiated these from killer whale social calls by their repetitive pattern and downsweeping from 1 kHz to low frequency (less than 500 Hz). Sperm whales ( Physeter macrocephalus ) also produce echolocation clicks [ 43 ], but these are usually found further offshore (greater than 500 m water depth) of the PEIs [ 44 ].

Figure 2 Different calls of killer whales detected around the PEIs.

…”

Summer circumpolar acoustic occurrence and call rates of Ross, Ommatophoca rossii, and leopard, Hydrurga leptonyx, seals in the Southern Ocean

“…Seal calls were incidentally recorded whilst listening for these large baleen whales, and no efforts were made at sea to identify the seal calls to a species level. No acoustic data were collected during the 2000/2001, 2004/2005 and 2009/2010 IWC-SOWER cruises (Ensor et al 2001(Ensor et al , 2005Sekiguchi et al 2010).…”

Acoustic occurrence, diel‐vocalizing pattern, and detection ranges of southern right whale gunshot sounds off South Africa's west coast

“…More calls of Antarctic blue and fin ( Balaenoptera physalus ) whale were detected by AAR1 during the time of recording overlap with AAR2 (Shabangu et al 2019); these whales also vocalize in shallow water depths above 50 m (e.g., Watkins et al, 1987; Oleson et al, 2007). However, different acoustic occurrence patterns were observed for Antarctic minke whales, Balaenoptera bonaerensis (Shabangu, Findaly, et al, 2020) and sperm whales (Shabangu & Andrew, 2020) where more calls were detected by AAR2 than AAR1 during time of recording overlap, since these whales vocalize in deeper waters (e.g., Watwood et al, 2006; Shabangu, Findlay, et al, 2020). The above results support previous findings that the sound source and hydrophone depth relative to the thermocline determine whether signals can be detected (Siderius et al, 2007; Song et al, 2010).…”