Conservation Genetics, Photo-ID and Foraging Ecology of Bryde’s whales

The Mammal Research Institute’s Whale Unit is currently conducting boat-based fieldwork in Walker Bay in the Western Cape. The main species of interest is the Bryde’s whale, a sleek rorqual with three ridges on its rostrum and an upright, curved dorsal fin towards the back of its body. Usually shy, elusive and fast-moving, they travel long distances in search of their prey, meaning that data collection on Bryde’s whales is both exciting and challenging!

Conservation Status

There are two types of Bryde’s whales that occur off the southern African coast. They belong to the same species but are separated into two subspecies – some are smaller and occur inshore while others are larger, offshore animals. The offshore ones are usually found off the continental shelf and are often covered in many oval, whitish scars. This scarring is due to mouthfuls of blubber being removed by cookie-cutter sharks that sneak up on the whales from behind! The inshore whales are fortunately spared this gory experience, but they do however have to contend with threats due to living in close proximity to humans, such as overfishing, entanglement in fishing gear and unwanted attention from recreational vessels.

The inshore population is estimated to be in the region of 600 individuals, which is the smallest population size of all large whale species occurring in South African waters! Its international conservation status is listed as Data Deficient, while the national status is Vulnerable. This is not good news for Bryde’s whales, meaning that the projects that the Whale Unit is conducting are of great importance.

Resident Whales

Unlike southern right and humpback whales that undertake a long migration from the Southern Ocean up along the east and west coasts of Africa, inshore Bryde’s whales are resident along the southern African coast all year round. Living in warmer waters, and having a high metabolism, means that they need to hunt frequently, mirroring the movement of their prey. Their distribution along the coast thus coincides with the seasonal movement of their main prey species, sardines and anchovy. They tend to move northwards along the east and west coast during autumn and winter, returning to the central Agulhas Bank during spring. Little is known about their movements along the coast north of Cape Town, but all good research constantly poses even more questions as new knowledge is discovered along the way!

Biopsy Sampling

Some work has already been conducted on the genetics of Bryde’s whales off South Africa, but more data are needed. Knowledge on the population structure is of great importance for the conservation management of the species. In order to collect samples for this study, researchers use a crossbow with a modified bolt ending in a tiny biopsy tip (instead of an arrow head). This biopsy collects a small skin sample which will be used for genetic analyses. The sample will provide a unique ID for each whale, but it is not always possible to collect samples from every whale encountered, so ID photographs will be taken simultaneously. The head will be photographed, if possible, in order to confirm the species, as well as both sides of the dorsal fin, for individual identification. Any nicks and scars will be noted in order to create a unique ID for as many individuals as possible.

These data will be compared to the existing Bryde’s whale catalogue and will be used to describe the movement of whales along the coast. With enough information collected over many years, scientists can also describe calving intervals, associations between individuals, estimate (minimum) ages and describe local residency patterns.

Trophic ecology

There is already some knowledge on the seasonal movements of these whales, and about their main prey species, however detailed information on their feeding and trophic ecology remains to be studied. Trophic ecology is the study of energy flow in food webs, describing who eats who i.e. the level at which both the whale and its prey are situated along the food chain. It is not possible to see exactly what whales are feeding on at all times, so skin samples (and baleen samples from stranded animals) will be used for stable isotope analysis, which will show if the whale has been feeding on pelagic shoaling fish or krill, for example, and will also indicate if there have been any changes in prey species over time.

Environmental Change

Due to their year-round residency, Bryde’s whales are not only vulnerable due to competition with man for its food, but may well show some effects of environmental change. For this reason the project will also analyse body size and shape (which is an indicator of body condition, and thus the health of an individual). This will be done by flying a drone to collect aerial photographs for later analysis.

Read more about our Whale Unit and Stable Isotope Laboratory by following these links:

https://www.mammalresearchinstitute.science/whale-unit

https://mammalresearchinstituteup.squarespace.com/stable-isotope-laboratory

Meredith Thornton, Science Communicator

Mammal Research Institute