School of Earth, Atmospheric & Life Sciences
Esteban, L, The use of unmanned aerial vehicles to assess the spatial and temporal dynamics of seals at Martin Islet (NSW), BEnviSci Hons, School of Earth, Atmospheric & Life Sciences, University of Wollongong, 2019.
The increasing anthropogenic influence on natural ecosystems has led to shifts in species geographic range, dispersal patterns and livelihood, thus providing significant challenges for environmental managers. Unmanned aerial vehicles (UAVs) show a unique potential for achieving non-disruptive surveys of marine mammal populations on offshore islands. This technology allows scientists to access and collect information in coastal environments that would otherwise not be possible. UAVs capture quick and continuous high-resolution data across the entirety of a site, often at a reduced cost and risk compared to light aircraft and satellite technologies. Situated 1.35 km off the coast of Port Kembla, Martin Islet is a haul-out site for the vulnerable Australian and Long-nosed fur seals. Haul-out sites are important for these species as they allow seals to rest when foraging and avoid predation. This study aimed to investigate temporal variation, spatial distribution and habitat suitability of the fur seal population at Martin Islet. UAV and boat surveys were completed each month from March to August, with additional, more frequent, aerial surveys conducted every two hours to investigate diurnal haul-out behaviour. An evaluation of in-situ and aerial techniques for surveying seals was conducted to determine the most appropriate methodology for Martin Islet. To statistically evaluate the uncertainty associated with counting seals from visual RGB images and thermal infrared images, seal counts were made by two groups of volunteers on an RGB and thermal image respectively.
The results of this study provide the first empirical evidence of fur seal haul-out behaviour at Martin Islet, revealing temporal seasonal similarities to other haul-out sites within NSW (e.g. Montague Island and Steamers Head). The study found that the seal population at Martin Islet increased in size from a few individuals in March to a peak of approximately 103 seals in late July. A notable increase in observed seal numbers in July and August corresponded with a decrease in seawater temperatures to approximately 16.7°C. This correlates with favoured foraging conditions of the Australian fur seal in the literature. On a diurnal scale, seals hauled out in greatest numbers around midday, while the number of seals in the water peaked at sunrise and decreased over the course of the day.
As a methodological approach to population assessment, our results suggest that UAV-based surveys are more accurate in identifying seals on Martin Islet, particularly as the number of seals hauled out increases. During July and August where the UAV surveys saw a large increase in seal numbers, the boat-based approach recorded just 52% and 37% of the seals observed using a UAV. The variable topography of Martin Islet resulted in a greater degree of uncertainty when identifying seals using thermal imagery compared to RGB imagery. The observer’s ability to distinguish seals from the surrounding landscape was hindered due to the rocky surfaces of the islet heating up in the sun and emitting thermal signals that were often similar in shape to the seals. The range in the number of seals estimated was substantially higher among participants analysing the thermal image (226) compared to the RGB image (29).
The spatial distribution of seals across Martin Islet displayed positive spatial autocorrelation and appears to depend primarily on the topographic characteristics of the islet (R2 = 0.77). Throughout the study, fur seals tended to haul out in groups, favouring the southern coastline of Martin Islet, on relatively uniform areas of low elevation that are easily accessible from the water. Most of the seals hauled out on rock platforms with elevations below 4 meters and within 12 meters of the shore. Three major zones, consisting of statistically significant spatial clusters of seal observations, were identified through the hot spot analysis, covering a combined area of 2016 m2. These zones are situated in the southern corners and north-west portion of Martin Islet. It is hoped that the maps developed in this study are used to guide conservation and help assess potential human and environmental impacts on the islet’s seal population.
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.