Acoustic behaviour of southern right whales in relation to numbers of whales present in Walker Bay, South Africa

Abstract

Estimating numbers of whales present in an area from recorded call rates could be a useful conservation tool. We recorded southern right whale Eubalaena australis vocalisations and presence in Walker Bay on the south coast of South Africa. In all, 45 sessions with synchronous acoustic and visual data were analysed to determine call rates directly (overall call rate, OCR) and in relation to the number of whales sighted (call rate per whale, CPW) and number of groups sighted (call rate per group, CPG).The OCRs were examined in the presence of varying numbers of whales, using a loglinear model to investigate the dependence of the call rate on whale density. The number of whales present exerted a strong quadratic effect on the OCR, which peaked at around 15 whales and decreased to a low rate as whale presence approached a maximum, for all calls combined and for four of their constituent 13 call types: a quadratic trend was present to varying degrees among the remaining call types. Both quadratic and linear trends were absent when OCR was assessed against number of groups present, possibly because group size increased with increasing density of whales. A linear regression on the CPW and CPG suggested that there was a negative inverse relationship with the number of whales and groups present respectively. These findings are important in that, while they preclude the estimation of absolute numbers from call rates, they imply that under the conditions prevailing in Walker Bay, southern right whales were aware of the presence, arrival and departure of other animals in their vicinity and adjusted their vocal behaviour accordingly.Keywords: acoustics, call rates, Eubalaena australis, log-linear model, southern right whale, vocalisations, relative abundance IntroductionThe ability to locate and estimate the density and movement patterns of whales present in an area is an important prerequisite for many aspects of their 2 conservation, but such factors are often difficult to ascertain. Whales spend much of their time submerged, and may cover long distances daily. Visual sightings are severely hampered by rough weather conditions, and are almost impossible at night.Communication among individual cetaceans in the contexts of reproductive display, prey detection, predator avoidance and navigation, is primarily acoustic (Clark 1982, MacLennan and Simmonds 1992, Parks 2003a. Therefore, using biological sounds to detect whales (Mellinger et al. 2011) is suited both to the underwater medium and the properties of the calls. While this method can only detect vocalising individuals, acoustic detection of whales has several advantages over visual detection, as has been demonstrated for fin whales Balaenoptera physalus (McDonald and Fox 1999). It can be applied in remote situations and for extended periods of time (Clark et al. 2010, Mellinger et al. 2011, in situations where boat and aerial surveys would be impractical or expensive. Additionally, remote acoustic detection can cover a greater range than visual detection, can occur under...

Vocalizations produced by southern right whale (Eubalaena australis) mother-calf pairs in a calving ground off Brazil

“…Over the past decade, extensive effort has been made to understand the vocal behavior of the highly endangered North Atlantic right whale Eubalaena glacialis -NARW Parks et al, 2011;Mussoline et al, 2012;Matthews et al, 2014;Bort et al, 2015). In contrast, since the first repertoire descriptions of southern right whales' (Eubalaena australis -SRW) by Clark (1982Clark ( , 1983 little attention has been given to this species' acoustic ecology in the Southwest Atlantic (Hofmeyr-Juritz and Best, 2011;Tellechea and Norbis, 2012).…”

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

“…Twelve call types of SRWs have been defined to date in South African waters (Hofmeyr‐Juritz, 2010; Hofmeyr‐Juritz & Best, 2011). Additionally, SRWs produce a short, distinctive broadband explosive sound termed the gunshot sound (Clark, 1983; Hofmeyr‐Juritz & Best, 2011; Webster et al, 2016). The gunshot sound is termed so “because of its acoustic similarity to the sound of a rifle being fired” (Parks et al, 2005).…”

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However, Best (2006) and Mate et al (2011) showed that the coastal waters of the South African west coast are also used as an austral spring/summer feeding ground by the portion of the population that remains there year-round.Seasonal acoustic occurrence of SRWs in South African waters is currently based on short-term shallow coastal (adjacent to the coast and within the 100 m isobath) research (Figure 1; Hofmeyr-Juritz, 2010; Vinding-Petersen,-2016). An expansion of such acoustic work to more offshore waters (Figure 1) and more long-term programs is useful for a more comprehensive understanding of the species' seasonal occurrence patterns.Twelve call types of SRWs have been defined to date in South African waters (Hofmeyr-Juritz, 2010;Hofmeyr-Juritz & Best, 2011). Additionally, SRWs produce a short, distinctive broadband explosive sound termed the gunshot sound (Clark, 1983;Hofmeyr-Juritz & Best, 2011;Webster et al, 2016).

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“…An expansion of such acoustic work to more offshore waters (Figure 1) and more long-term programs is useful for a more comprehensive understanding of the species' seasonal occurrence patterns.Twelve call types of SRWs have been defined to date in South African waters (Hofmeyr-Juritz, 2010;Hofmeyr-Juritz & Best, 2011). Additionally, SRWs produce a short, distinctive broadband explosive sound termed the gunshot sound (Clark, 1983;Hofmeyr-Juritz & Best, 2011;Webster et al, 2016). The gunshot sound is termed so "because of [Correction added on 20th May 2022, after first online publication: The copyright line was changed.…”

Automatic classification of whistles from coastal dolphins of the southern African subregion

“…Although clearly advantageous, the application of PAM for research and monitoring of cetacean species in African waters has lagged behind other more affluent regions. There have been some recent advances in this field (Hofmeyr-Juritz and Best, 2011;Gridley et al, 2012;Gridley et al, 2014;Gridley et al, 2015), however, basic data on the call repertoire and vocal characteristics of most cetacean species encountered in southern Africa are still missing (Best, 2007;Elwen et al, 2011). Such data are necessary for the implementation of successful PAM programs.…”