Abstract
Passive acoustic monitoring (PAM) is a powerful method to study the occurrence, movement and behavior of echolocating odontocetes (toothed whales) in the wild. However, in areas occupied by more than one species, echolocation clicks need to be classified into species. The present study investigated whether the echolocation clicks produced by small, at-risk, resident sympatric populations of Indian Ocean humpback dolphin (Sousa plumbea) and Indo-Pacific bottlenose dolphin (Tursiops aduncus) in Menai Bay, Zanzibar, East Africa, could be classified to allow species specific monitoring. Underwater sounds of S. plumbea and T. aduncus groups were recorded using a SoundTrap 202HF in January and June-August 2015. Eight acoustic parameters, i.e. -10 dB duration, peak, centroid, lower -3 and lower -10 dB frequencies, and -3 dB, -10 dB and root-mean-squared bandwidth, were used to describe and compare the two species’ echolocation clicks. Statistical analyses showed that S. plumbea clicks had significantly higher peak, centroid, lower -3 and lower -10 dB frequencies compared to T. aduncus, whereas duration and bandwidth parameters were similar for the two species. Random Forest (RF) classifiers were applied to determine parameters that could be used to classify the two species from echolocation clicks and achieved 28.6% and 90.2% correct species classification rates for S. plumbea and T. aduncus, respectively. Both species were classified at a higher rate than expected at random, however the identified classifiers would only be useful for T. aduncus monitoring. The frequency and bandwidth parameters provided most power for species classification. Further study is necessary to identify useful classifiers for S. plumbea. This study represents a first step in acoustic description and classification of S. plumbea and T. aduncus in the western Indian Ocean region, with potential application for future acoustic monitoring of species-specific temporal and spatial occurrence in these sympatric species.
Highlights
Passive acoustic monitoring (PAM) is a powerful technique to study the occurrence, movement and behavior of odontocetes in the wild [1,2,3,4]
The Random Forest (RF) model resulted in a 73.2% (5th and 95th quantiles 72.4%-74.0%) correct classification rate for S. plumbea and T. aduncus echolocation clicks submitted to the model, compared to 50% expected by random chance alone
The RF model resulted in a correct classification rate of 28.6% (5th and 95th quantiles 25.7%-31.4%) for S. plumbea, compared to an expected rate of 27.6%, and 90.2% (5th and 95th quantiles 90.2%-90.2%) for T. aduncus, compared to an expected rate of 72.4%
Summary
Passive acoustic monitoring (PAM) is a powerful technique to study the occurrence, movement and behavior of odontocetes (toothed whales) in the wild [1,2,3,4]. Some sympatric species of odontocetes produce similar clicks which can limit the effectiveness of PAM for species-specific studies, as acoustic species classification can be challenging [14]. Acoustic parameters of odontocete clicks vary depending on their sound production morphology [14], with some species having the ability to optimize their clicks within the context of the specific habitat [15]. These parameters are further influenced by sound propagation [11], off-axis effects [16] and differences in recording systems [14]. Classification of clicks for delphinids, especially sympatric species, has proved difficult due to overlaps in some acoustic parameters among species [14, 23, 24]
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