Over the last two weeks a team of nine researchers from Sea Search Research and Conservation, the Dyer Island Conservation Trust, the Mammal Research Institute’s Whale Unit (University of Pretoria) and Center for Statistics in Ecology, Environment and Conservation (University of Cape Town) collaborated on an exciting project to collect acoustic and behavioural data, photo ID’s and biopsy samples from dolphins and whales in the Overstrand area.
The main focus of the study was the endangered Indian Ocean humpback dolphin, Sousa plumbea. This is a very shy species and can be particularly elusive, so time was also spent sampling bottlenose and common dolphins, as well as Brydes’, humpback and southern right whales.
This work forms part of a larger project which is investigating the environmental factors limiting the range ends of small cetaceans around the Western Cape, with the longterm goal of predicting the impacts of climate change on these animals.
Data collected from humpback dolphins will also contribute to the broader Sousa Project, which is an intergroup collaboration on the scientific research on Indian Ocean humpback dolphins in South Africa. Through data collection and sharing, the project aims to assess in detail the conservation status of humpback dolphins along our coastlines.
Humpback dolphins utilise exactly the same areas of the coast that man does, so besides for shark predation, threats are usually as a result of human activities. Power boats and jetskis disturb their behaviour, shark nets put up to protect bathers indiscriminately catch dolphins too and, because they favour estuarine areas, they are especially vulnerable to exposure to pesticides and other pollutants that make their way from agricultural areas down the river system and into the sea – often being accumulated in the prey that humpback (and other dolphin species) consume.
Two rigid inflatable boats were used for the work, covering different areas simultaneously, so that there was maximum coverage of the study area. Dependent on the swell forecast for the day, one boat usually launched from Kleinbaai Harbour, surveying the waters from Danger Point towards Pearly Beach, while the second boat launched from Gansbaai Harbour, focusing on Walker Bay.
One vessel was set up primarily for acoustic work and the other for biopsying, while both vessels collected behavioural and photo ID data.
Acoustic monitoring is used in conjunction with the behavioural sampling and allows scientists to describe the dolphins’ vocalisations, measure the frequency thereof and to infer what sounds are made during certain behaviours or as a result of natural or man-induced stressors.
Biopsy samples are used for genetic and isotope work, answering questions about how populations may be different from other populations within one species, and how these things can change over time, while isotope analysis provides information about what the dolphins have been feeding on.
Photo ID determines who’s who in the population and provides answers on population size, calving intervals and success. Previous data collected by the Dyer Island Conservation Trust and Dyer Island Cruises has already allowed for identification of about 30 different humpback dolphins from the Greater Dyer Island area!
When high swells reduced the work efficiency locally, one of the vessels was towed to Struisbaai to work in the lee of Cape Agulhas in an area previously identified as a humpback dolphin hotspot.
The team had 8 encounters of humpback dolphins, 16 of bottlenose dolphin and over 20 whale encounters of three species (Bryde’s, southern right and humpback). They managed to get 6 recordings from humpback dolphins, and identify individuals which were previously seen off Strandfontein in False Bay. They also obtained biopsy samples from bottlenose, common and humpback dolphins. One individual dolphin was photographically identified in Walker Bay, Kleinbaai and Struisbaai during the trip covering a distance of at least 120 km in just a few days.
Future plans are to continue with these fieldtrips every couple of months as they provide valuable information that can aid conservationists and environmental managers in decision making on marine spatial planning in order to help to protect whales and dolphins and preserve important habitats for them in the long term.
The research team members were: Dr Simon Elwen, Dr Tess Gridley, Bridget James, Monique Laubscher, Barry McGovern, Meredith Thornton, Sandra Hoerbst, Dr Els Vermeulen and Chris Wilkinson. All work was conducted under permit from CapeNature and the Department of Environment Affairs in terms of Section 83 of the Marine Living Resources Act.
Meredith Thornton: Dyer Island Conservation Trust