Natalie dos Santos
"Acoustic Tracking Sea Turtles on South African Foraging Grounds"
Project Update:
Acoustic tracking sea turtles on South African Foraging Gounds.
Natalie dos Santos (PhD Student, Nelson Mandela University) 14 October 2025
This first acoustic telemetry-based study of sea turtle movements in South African waters aims to determine the spatiotemporal movements (residency patterns, home ranges, and diel and seasonal activity) of green turtles and hawksbills on Aliwal Shoal, an unrecognised turtle hotspot on our east coast.
To date, the scholarship has covered the transport and operational costs of seven sea days to deploy five acoustic receivers on the reef, begin receiver detection range tests, and catch and tag two sea turtles.
We received five Vemco VR2W acoustic receivers loaned by the South African Acoustic Tracking Array Platform (ATAP) in mid-July. After scouting ideal deployment sites around Aliwal Shoal’s shallow pinnacles, we deployed all receivers on 31 July, bringing the total number in the area up to nine. A highlight was making it safely through the notoriously bad Umkomaas surf launch in my little 5.4 m research boat laden with nearly 300 kg of railway lines, floats, and receivers! We chose receiver locations to maximise turtle detections while keeping the equipment deeper than 18 m to minimise damage and noise interference from strong surge and currents.
Between bouts of bad August and September weather, we dived the receivers to fine-tune their locations, check the moorings, and carry out some maintenance. On 9 October, we caught our first turtle – the first ever caught and tagged on Aliwal Shoal! This juvenile green turtle (495 mm straight carapace length, 20.8 kg) was flipper- and acoustic-tagged, photo-identified for our long-term database (initiated in 2020), and sampled for genetic, stable isotope, and barnacle analyses. The individual was new to our photo-ID database and may represent a new recruit to the foraging population.
A few days later, on 12 October, we caught and tagged our second turtle, a subadult hawksbill (654 mm straight carapace length, 39.5 kg). Known locally as ‘Sam,’ this individual is a well-documented resident seen regularly by divers since 2020. Both tagged turtles have since been spotted foraging and resting on the reef by divers. I find it incredibly exciting to know that as I type this, we are already collecting new data on sea turtle movements, and that the new skin samples in my freezer will soon reveal where these turtles originate – perhaps from the Seychelles or the remote French Scattered Islands.
We also started conducting receiver detection range tests on 12 October, placing ‘sentinel’ tags 50–400 m away from each receiver for about four days at a time to measure their detection efficiency. We selected sentinel tag locations where the detection radii of multiple receivers overlap, so that one sentinel tag can simultaneously test multiple receivers. With three sentinel tags deployed at a time, we'll spend seven days shuffling them around the reef in total, and expect to complete range tests next month.
We plan to deploy all 15 acoustic tags on sea turtles by the end of the year, with most of the fieldwork scheduled for November and December once the weather improves. The project has been an absolute blast so far – we’ve learned so much, enjoyed every minute, and are very grateful for the support from the BLSTF.
​PROJECT DESCRIPTION:
Background
Sea turtles are long-lived, ecologically important species that have faced significant overexploitation, making them conservation dependent. However, conservation efforts worldwide have focused primarily on nesting beaches due to the challenges of in-water research, resulting in a lack of basic knowledge of sea turtles’ lives at sea, where they spend most of their time and perform key ecological functions. Understanding these is critical for setting more effective conservation targets informed by sea turtles’ primary ecological roles (Bjorndal and Bolten 2003).
In South Africa, knowledge of in-water sea turtle ecology remains limited, as research and conservation efforts have historically focused on the recovery of loggerheads and leatherbacks on nesting beaches, overlooking juveniles and subadults, adult males, and other species (green turtles and hawksbills) that remain at sea. However, technological advances like photo-ID and citizen science have helped us recently obtain the first estimates of in-water relative abundance, demographics and spatial distribution of sea turtles along the east coast of South Africa. We found that green turtles are the most abundant turtle species here, followed by hawksbills and loggerheads, and that reefs support resident aggregations of green turtles of all size classes, immature hawksbills, and subadult and adult loggerheads.
Findings of particular interest emerged from Aliwal Shoal, a subtropical algal-dominated reef world renowned for its biodiversity and shark diving. Research here revealed new insights into the foraging behaviours of year-round resident turtles, the effects of human disturbance (e.g., boat strikes and a clear drop in turtle sightings during peak boat traffic), and a potential hotspot for Critically Endangered hawksbills, possibly linked to preferred food availability or more complex reef topography ideal for sheltering. These findings have raised key research questions about turtles’ responses to human disturbances, home ranges, ecological impacts on benthic communities (given that they spend much of their time foraging), and the factors driving hawksbill abundance at Aliwal Shoal.
This project will use acoustic telemetry, a cost-effective, long-term alternative to satellite tracking, to study green turtles and hawksbills in the Aliwal Shoal Marine Protected Area (MPA), currently unrecognised for its relatively high sea turtle densities. This site is part of South Africa’s Acoustic Tracking Array Platform (ATAP), a network of ± 350 receivers along the coastline into Mozambique, making it ideal for studying both residency patterns and potential broader movements.
The project aims to determine the spatiotemporal movements (residency patterns, home ranges, and diel and seasonal activity patterns) of green turtles and hawksbills in the Aliwal Shoal MPA using acoustic telemetry, and to compare these between species and size classes. It is hypothesised that all sea turtles will exhibit year-round residency with small home ranges, two daily activity peaks, and no seasonal activity patterns.
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Research Goals
The goal of this specific research project is to fill knowledge gaps on the spatiotemporal use of foraging grounds by sea turtles, to essentially gain a better understanding of their ecological roles in marine ecosystems along South Africa’s east coast. This work, alongside concurrent studies investigating the 1 natal origins, abundance, and foraging ecology of sea turtles in local neritic habitats, forms an integral part of my overall PhD project investigating sea turtle ecological roles. The goals of my research in general are to improve knowledge of the understudied in-water biology and ecology of sea turtles to inform more effective management and conservation strategies for these endangered species.
Methods
Field data collection
In-water capture – Green turtles and hawksbills on Aliwal Shoal will be captured by hand using standard "turtle rodeo" methods described by Ehrhart and Ogren (1999). Turtles will be caught while freediving or SCUBA diving, swum to the surface while 'hugging’ them to secure the front flippers and avoid injury, and brought onto the boat for processing (± 45 minutes). We will target eight green turtles (ideally two juveniles, two subadults, two adult males, and two adult females), and seven hawksbills (ideally three juveniles and four subadults – adults are rare here).
Acoustic tagging – Turtles will be tagged with Innovasea V16-4x high-power acoustic tags weighing less than 30g in air, with a ± 1365-day battery life. For green turtles, fibreglass cloth with two cable ties will be epoxied to one of the last lateral scutes of the carapace, and the tag will be secured with the cable ties and covered in epoxy putty and anti-fouling paint. For hawksbills, two or four small holes (<3mm) will be drilled through one of the back outer marginal scutes using a new sterile stainless steel drill bit, where the tag will be attached with small cable ties and covered in liquid epoxy and anti-fouling paint. Attachment methods maximise retention times for each species as green turtles’ soft shells are prone to tear-outs when drilled, whereas hawksbills’ hard shells are not (Smith et al. 2019).
Other processing – Turtles will be measured for straight and curved carapace length and width, weighed using a harness and spring scale, and assessed for body condition based on plastron shape. Skin samples will be collected from the outer edge of the front flipper using a sterile 6mm biopsy punch and stored in 99% alcohol (for mitochondrial DNA analyses and stable isotope analyses for concurrent studies). Turtles will also be flipper tagged and photo-IDed to see if they are new or known to our existing database of individuals and their resightings since 2020. After processing, turtles will be released at their initial capture location. Ethical clearance from Nelson Mandela University’s Animal Research Ethics Committee has been obtained, and required permits are renewed annually in February/March.
Acoustic receiver deployment and range tests
An array of five Innovasea VR2W 69kHz acoustic receivers will be strategically deployed on and around Aliwal Shoal’s shallow pinnacles (~ 2.1 km2 sea turtle hotspot) for the ± four-year project duration. This will strengthen the existing array of four receivers in the Aliwal Shoal MPA. Receivers will be moored to the substrate at depths of 18-30m following ATAP’s standard protocols (Cowley et al. 2017). Range tests will be conducted following Daly et al. (2014) by positioning an acoustic tag at various distances (5 400m) and depths (10m and 25m) from each receiver for three days. Detection probabilities will then be calculated. Detection data will be downloaded during annual retrieval and replacement of receivers, ensuring continuous data collection. Tagged turtles will also be detectable across ATAP’s ± 350 receivers along the South African coastline and into Mozambique (Murray et al. 2022).
Statistical analyses
Statistical methods will follow adaptations from Pillans et al. (2022), Daly et al. (2023), and Towner et al. (2022). Residency will be determined using residence indices representing the proportion of time spent in an area (calculated as the number of days detected/detection span) and plots of daily detections over time. Home ranges will be determined by kernel utilization distribution (KUD) depicting 2 the probability of an animal occurring at a location within its home range as a function of relocation points (data obtained from receiver detections). Seasonal movements will be determined by calculated mean 50% and 95% KUD areas for tagged individuals in each season. Diel patterns will be determined using a similar approach but for day and night. Statistical analyses will be conducted using the latest versions of R and RStudio.
Expected Outcomes
Expected outcomes of this project include the first comprehensive study of sea turtle movements on Aliwal Shoal and the first acoustic telemetry-based sea turtle movement study in South Africa (and one of the first in Africa). The results will address specific research questions and fill critical knowledge gaps about the ecological roles of sea turtles on foraging grounds. The findings will also be used to inform management recommendations, such as skipper speed limits to reduce the risk of boat strikes and disturbance to sea turtles in the Aliwal Shoal MPA, to aid their protection.
Dissemination of Results
I plan to disseminate the results through a scientific paper for publication in a peer-reviewed journal and through presentations at local and international scientific conferences. These results will also form a key part of my PhD dissertation, which will be made available online. I will also share the findings through public talks at dive centres, local aquaria, and universities to ensure that recreational divers, citizen scientists, aquaria staff, and students interested in turtles or acoustic telemetry are educated or updated on the project’s findings and their implications. To reach a wider audience, I will write a popular science article for local news outlets and share project updates and interesting findings on social media, including the SA Turtles page (@sa_turtles) and my personal page (@natsdossantos), with the goal of inspiring people to appreciate and care for sea turtles and marine ecosystems.
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