On Tuesday 3 October, the Mammal Research Institute’s Whale Unit started their annual southern right whale aerial survey which aims to monitor the population of right whales along the South African coastline. This survey has been carried out annually since the first survey in 1979, making this year the 39th consecutive annual right whale survey and making the resulting dataset one of the longest running datasets on any marine mammal worldwide.
The survey is conducted along the coastline between Nature’s Valley and Muizenberg, Cape Town, although on occasion it is extended westwards, rounding Cape Point and extending up the west coast. During the survey, an airbus H120B helicopter is used to search for whales from a searching altitude of approximately 300m and at a distance of between 500 and 800 m from the shore. All of the whale species encountered during the survey are recorded, but photographs for the photo-identification of individuals are only taken of southern right whale cow/calf pairs and individuals with distinctive brindle colouration or markings. Individual southern right whales can be identified by the pattern of callosities (patches of thickened skin which are colonized by whale lice and barnacles) on their heads and some individuals can be identified by the pigmentation markings on the dorsal surface of their bodies. For this reason, vertical photographs of both the heads and backs of the individuals are taken. While photographing animals, the helicopter descends to an altitude of between 150 to 200 m where it usually spends less than 5 minutes photographing each individual in the group before ascending to searching altitude again.
The first day of the survey saw the team covering the coastline between Nature’s Valley and Mossel Bay where they encountered 7 cow/calf pairs and 27 “unaccompanied adults”, or adults without calves. They also recorded the odd humpback whale, Bryde’s whale and group of bottlenose and humpback dolphins. On the second day of the survey the team surveyed Saint Sebastian Bay and encountered 24 cow/calf pairs and 8 unaccompanied adults. These numbers are reported to be a little under average but still much better than that of the last two years when numbers were very low, possibly due to a strong El Nino warming effect which may have affected food availability. However, the reasons behind the low abundance of right whales is still being investigated and this hypothesis is speculatitive.
The photographs from the survey will be compared to the Whale Unit’s right whale photo-identification catalogue of just over 2300 recognizable adults. The sighting histories of these whales are analysed in order to determine vital information on this population of right whales such as their abundance, population growth rate, survival, calving intervals as well as the age at which a female calf first returns with her calf. One of the main objectives of the surveys has been to monitor the recovery of right whales from whaling. Intensive hunting in the late 1700s, 1800s and early 1900s reduced the estimated global population of 70000 – 80000 southern right whales to a mere 60 reproductive females by the time they were fully protected in 1935. Thankfully, they have been making a remarkable recovery since then with a population increase of about 7% every year. The South African breeding population is currently estimated at 5000 to 6000 individuals, while the global population size is just under 15000 individuals.
You can check up on the progress of the right whale surveys as they move westwards along the coast by following their posts on their facebook page: https://www.facebook.com/MRIWhaleUnit/.
Written by Danielle Conry
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