Spatial and temporal patterns of habitat use of Heaviside's dolphins in Namibia

Abstract

This thesis reports the findings of a fine-scale habitat selection study of Heaviside s dolphins (Cephalorhynchus heavisidii) at two sites in Namibia; Walvis Bay and Lüderitz. Walvis Bay and Lüderitz are the two largest embayments along the Namibian Coast, and therefore the two industrial ports in Namibia are located there. These bays are also inhabited year-round by Heaviside s dolphins, and, at Walvis Bay, a resident population of common bottlenose dolphins (Tursiops truncatus). Heaviside s dolphins are endemic to the western coast of southern Africa ranging within the Benguela current ecosystem from Table Bay, South Africa, to southern Angola. They are not well-studied in the northern half of their range to date, and little is known about geographical variation in the ecology of this species. Furthermore, their coastal distribution and year-round residency near developing industrial ports could potentially put them at risk from human impacts, as industrial use of these areas increases. This study was conducted in order to obtain baseline information on fine-scale spatial and temporal patterns of distribution and important habitat parameters for this species in the northern half of its range in order to inform future management of commercial and industrial activities in these areas. Two complementary methods were used to obtain data on dolphin presence: visual surveys from a small boat were conducted at both study sites over multiple years, and continuous acoustic monitoring was done at Lüderitz over a short period concurrent with one year s surveys. Visual surveys provided the flexibility to cover a wider area and to obtain presence and absence data, including positons of groups, while acoustic monitoring allowed for a continuous observational presence even during nighttime hours, allowing for the ability to detect diel patterns in area use. The Namibian Dolphin Project has been conducting small boat surveys for cetaceans in Walvis Bay since 2008 and in Lüderitz since 2010, with a focus on Heaviside s dolphins and common bottlenose dolphins. Surveys were conducted in a non-systematic way, with the aim of covering the entire study area while maximizing opportunities for data collection from groups of animals, and was thus rather more focused on high-density areas for these two species. Effort-corrected encounter data from these surveys were mapped and linked spatially to habitat parameters using ArcGIS in order to identify areas of frequent dolphin presence for both species. Generalized Additive Models (GAMs) were used to discover the parameters relevant to habitat selection at each site for Heaviside s dolphins and at Walvis Bay for bottlenose dolphins. Habitat selection at Lüderitz was not examined for bottlenose dolphins, because there were too few encounters there. Acoustic monitoring for dolphins at Lüderitz was conducted continuously over a two-month period in 2014 using five click-logging instruments called C-PODS moored at different sites from the harbour mouth westward across a series of small bays towards the open ocean, which make up the core study area for visual surveys at this study site. The data obtained give information on relative presence, both spatially between the deployment sites, and also temporally. Another aspect of the acoustic data which was examined was the prevalence of very rapid series of echolocation clicks used when an echolocating animal acoustically investigates a specific target, oftentimes a potential prey item. Spatial and temporal variation in the prevalence of these rapid click trains, hereafter referred to as potential feeding buzzes, has implications regarding patterns of foraging. Strong habitat preferences over a fine scale were found at both study sites, with areas of near-constant presence close to areas with very little presence. There are similarities between the most frequented areas at each study site: Pelican Point, in Walvis Bay, and Diaz Point, in Lüderitz. Animals in Lüderitz, however, made use of sheltered inshore waters, and were found much closer to the coastline, and in shallower waters, than animals in Walvis Bay. This may be due to the presence to bottlenose dolphins close inshore in Walvis Bay as the two species showed remarkably little overlap in distributions given the small scale of the study site. Patterns of acoustic detections between C-POD deployment locations closely matched encounter rates from the small boat surveys around those locations, though with lower detections at night on all C-PODS, implying a diel movement offshore. Highest detections overall were at midday, except at Guano Bay (the most westerly site) where early morning detections were highest. Analysis of inter-click intervals showed a similar diurnal pattern in the proportion of potential feeding buzzes to overall acoustic activity. The continuous presence close to shore and strong habitat preferences of dolphins within both of these industrial ports put them at risk from anthropogenic activities that occur there. Continued monitoring of these populations in the future will help to identify and mitigate these risks. Identification of areas with the most frequent occurrence of dolphins at each study site should be used to inform management decisions in the future.