Developing methods to use static acoustic click detectors for long term monitoring of coastal delphinids along the Cape south coast, South Africa

Abstract

This thesis investigated the use of a static acoustic data logger known as a Cetacean and Porpoise Detection (C-POD) device for monitoring two species of dolphins that occur in Mossel Bay, South Africa: the Indo-Pacific bottlenose dolphin, Tursiops aduncus, and the Indian Ocean humpback dolphin, Sousa plumbea. These two species have a near shore distribution which brings them into frequent contact with human activities such as boating, fishing, desalination plants and the onshore component of gas extraction facilities which can potentially affect their behaviour and populations dynamics. Although both species are relatively well studied within South Africa in terms of biology and abundance, this has been limited to the KwaZulu-Natal coast and Algoa Bay with little information available for areas west of Algoa Bay, and almost no information available locally on their acoustic behaviour. This study aimed to: 1) calculate the detection range and effective detection radius (EDR) of C-PODs for T. aduncus and S. plumbea taking into account environmental and group parameters that may affect these and 2) distinguish T. aduncus from S. plumbea based on temporal variation in specific echolocation click parameters. Field work took place in Mossel Bay from a land-based platform during the period May 2013 to August 2014. A surveyor‟s theodolite was used to collect positional data on dolphins and behavioural information was collected through visual observations. A C-POD was deployed near the land based site and serviced and downloaded on a monthly basis for the same survey period. Visual detections were matched to acoustic detections (echolocation clicks) made within a 60 sec and 30 sec time window. Data were analysed using the software programme R. General Estimating Equations (GEE) were used to 1) model the detection probability and EDR of a C-POD for both species during both time windows; 2) model differences between the two species for seven click parameters produced by the C-POD. The impact of background noise on C-POD detections was also assessed. As cetaceans are not the only form of sound in a marine environment distinguishing dolphin clicks from all other noise sources can be a challenge. Sources of background noise include clicking shrimp, movement of sediment, boat traffic and chorusing by reef fish and crustaceans during reproductive or territorial displays and storms which can either mask the clicks produced by dolphins or fill up the memory card. Data collected from two different deployment locations at different depths were subsequently compared after significant ix interference from other noise source was detected at the first deployment location. Mean number of detected dolphin clicks after filtering increased from 6463.33 to 97820 clicks per deployment and mean number of minutes in which detections were recorded increased from 0.23 min to 2.79 min per deployment. Average detection probability for bottlenose dolphins was 0.357 (95% CI: 0.352 - 0.363) for the 60 sec window and 0.257 (95% CI: 0.253 - 0.262) for the 30 second window. For humpback dolphins the average detection probability was 0.084 (95% CI: 0.082 - 0.087) for the 60 sec window, and 0.043 (95% CI: 0.042 - 0.044) for the 30 sec window. EDR for bottlenose dolphins was 1161.38 mm (95% CI: 1150.14m – 1172.62 m) for the 60 sec window and 1035.761 m (95% CI: 1024.89 m – 1046. 64 m) for the 30 sec window. EDR for humpback dolphins was 765.25m (95% CI: 755.64 m – 774.85 m) for the 60 sec window and 751.00 m (95% CI: 741. 40 m – 760. 60 m) for the 30 sec window. Detection probability and EDR were higher for bottlenose dolphins than humpback dolphins over both time windows. For both species, during both windows, detection probability was significantly influenced by group size (p < 0.05). Four out of the seven parameters produced by the C-POD yielded significant pairwise differences (p < 0.05) between the species. Average inter-click interval was the only co-variable that was significant in the GEE model. The average ICI for bottlenose dolphins was 55.91 ms (95% CI: 51.23 – 60.60) whilst the average ICI for humpback dolphins was longer at 119.76 ms (95% CI: 119.76 – 174.00). The interaction between average ICI and modal frequency was also significant (p < 0.05) indicating that a combination of co-variables might be required in order to differentiate between the two species. Despite the small sample size and the challenges associated with acoustic studies in a noisy marine environment this study provided valuable information regarding the use of static acoustic data loggers such as C-POD in South Africa. As newer versions of C-POD are frequently introduced many of the challenges encountered in this study will be minimized.