The Namibian Islands’ Marine Protected Area (NIMPA), covers 25% of the overall coastline of Namibia and one of the largest MPA’s in Africa, is home to a substantial population of around 1 600 Heaviside’s dolphins and 3 500 African dusky dolphins.
These are the findings from the first population estimate of the number of dolphins using this area since the MPA’s establishment in 2009. The findings were recently published in the journals Frontiers in Marine Science and the African Journal of Marine Science.
The Namibian Islands’ Marine Protected Area is located within the northern Benguela Current upwelling system, one of the most productive marine ecosystems in the world. Marine biologists from the Namibian Dolphin Project, and the Namibian Ministry of Fisheries and Marine Resources, conducted the research from 2012 to 2014.
The team used novel survey and statistical methods to generate abundance estimates (i.e. population size) from ship survey data. The dolphins were found through a combination of visual searching from the ship’s deck and through detection of their echolocation calls using acoustic recordings made from a set of hydrophones (underwater microphones) towed 400 m behind the vessel. As most dolphins are very vocal – and use sound throughout the day and night for orientation, finding food and communication, they are ideal subjects for acoustic monitoring. This hydrophone array was able to detect the dolphin sounds as the research ship moved through their habitat.
According to Dr Tess Gridley, co-director of Sea Search and the Namibian Dolphin Project – the ability to identify the presence of animals while underwater has greatly improved detection rates and helps to ensure that animals are not missed: “Although the visual survey team was very effective by day, the towed hydrophone array was also able to detect animals during bad weather and overnight when visual observation wasn’t possible. This made the survey far more cost-effective and efficient. The acoustic survey method also led the researchers to find some Heaviside’s dolphins in far deeper water than expected, providing new insights into how these dolphins use the region around the clock,” she explains. Dr Gridley is a postdoctoral research fellow in the Department of Botany and Zoology at Stellenbosch University and a research associate at the University of Cape Town
Dr Morgan Martin, who conducted the research as part of her PhD-thesis at the University of Pretoria, says the study provides an important baseline on dolphin numbers for Namibia’s only marine protected area, and serves as a model for the remaining Benguela Current ecosystem.
Dr Simon Elwen, co-director of Sea Search and the Namibian Dolphin Project and Research Associated at Stellenbosch University, added that running these kinds of surveys are physically exhausting: “We were operating 24 hours a day monitoring the hydrophone array and navigation, while working on a small ship in fairly rough seas, so we’re really happy with the results of the project now that it has been published”.
Although coastal dolphins in Namibia face relatively few direct impacts – such as bycatch or targeted hunting – there have been major changes in their ecosystem like the collapse of the sardine stocks, and increases in marine mining and surveying: “Although the numbers of dolphins estimated in the NIMPA during these surveys is on a par with populations from other areas – without historic data it is impossible to say if the population has decreased or increased over time and future work is needed to study this,” the researchers conclude.
More about Heaviside’s and dusky dolphins
Heaviside’s dolphins (Cephalorhynchus heavisidii) are only found in the Benguela Ecosystem along the west coast of southern Africa and range from southern Angola to the Cape Point in South Africa. They are one of the smallest dolphins on the planet, being less than 1.7 m long, and are typically observed in groups of three to five individuals. Heaviside’s dolphins are found over the continental shelf, ranging from shallow waters along the coast where they can easily be seen playing in the waves, to around 100 m water depth. They typically have small home ranges, only about 20-50 km along the coast – which means than dolphins in one area, don’t mix with dolphins from other areas.
Dusky dolphins (Lagenorhynchus obscurus) are found in several locations in the southern hemisphere, primarily off New Zealand, South America, and the west coast of southern Africa. Dusky dolphins from these different regions are recognized as separate subspecies (in South Africa we have Lagenorhynchus obscurus obscurus). They range more widely than Heaviside’s dolphins and into deeper waters (out to around 500 m water depth) and are usually seen in larger groups of 10 to 30 and occasionally more.
Due to their limited range with the cold-water Benguela Ecosystem, both Heaviside’s and dusky dolphins are listed amongst the cetacean species (whales, dolphins, porpoises) most vulnerable to climate change. Both species are exposed to many un-quantified human threats including fisheries interaction and bycatch, changes in prey availability and marine eco-tourism.
What we know about sound use in Heaviside’s and dusky dolphins
All dolphins use sound to find objects in their environment, such as fish. This process, called echolocation, involves the animal producing a sound commonly termed a ‘click,’ which hits a target, producing an echo which the animal hears and processes, effectively ‘seeing the world with sound’. Heaviside’s dolphins are one of only 13 species of toothed whales and dolphins that have shifted their echolocation signals to occur in an incredibly high and narrow frequency band around 130 kHz (over six times higher than the 20 kHz upper limit of human hearing). Dusky dolphins echolocate with clicks which have a much broader range of frequencies, from around 10 kHz to 200 kHz which overlaps with human hearing to some extent. Since the two species produce echolocation clicks that are distinct from each other, researchers can determine which species they record underwater even without visual confirmation of the animals.
The two articles can be accessed online:
- “First abundance estimates of Heaviside’s (Cephalorhynchus heavisidii) and dusky (Lagenorhynchus obscurus) dolphins off Namibia using a novel visual and acoustic line transect survey,” in Frontiers in Marine Science https://www.frontiersin.org/articles/10.3389/fmars.2020.555659/full
“Towed passive acoustic monitoring complements visual survey methods for Heaviside’s dolphins Cephalorhynchus heavisidii in the Namibian Islands Marine Protected Area,” in the African Journal of Marine Science https://www.tandfonline.com/doi/abs/10.2989/1814232X.2020.1848925