Mossel Bay Cetacean Project

Direct estimation of the total abundance of populations using the Mossel Bay area falls outside the scope of this study. Daily visual estimates of cetacean group sizes (the number of individuals in a pod) will show patterns in relative abundance throughout the year. Photographic identification data of bottlenose and humpback dolphins will be collected opportunistically as part of ongoing boat work and contribute to future estimates of population size.

The behaviour of cetaceans in the bay is one of the central goals of the study. We are investigating variations in the behaviour (for example: travelling, feeding, resting, if the group is closely bunched or loosely dispersed) of dolphins across days and seasons, with variations in group size, area used and presence of boat traffic and other species, including white sharks. The data will be collected from shore and is thus ideal as the presence of observers has no effect on the animal’s behaviour.

Of particular interest are inter-specific behaviours: bottlenose dolphins are well known to harass other species of dolphin, including the humpback dolphin. These interactions may play an important role in habitat selection and behaviour of humpback dolphins in the bay.

Where the animals are going, when, how they are using different areas in the bay is what’s important. By using a surveyor’s theodolite we are able to accurately plot the position and track the speed and direction of movements of groups of animals up to several kilometres from shore. Identifying areas of high use and the animal’s behaviour therein will allow us to generate baseline data in a ‘pristine’ environment and better predict and measure any impacts from fish farm installations and other human activities. The identification of important, localised, small scale habitat use is one key to understanding the ecology of large vertebrate predators in the bay and will help provide the information needed to manage the conservation of the ecosystem as a whole.

Competition between predator and prey and competing predator species can have a profound effect on the habitat use patterns of animals. Bottlenose dolphins are known to harass several other species of dolphin and porpoise affecting their behaviour and distribution. By simultaneously tracking groups of humpback and bottlenose dolphins we will be able to investigate behavioural interactions between these species.

Dolphins also feature on the menu of the Great white shark so researchers will be on the lookout for predator avoidance or harassment.

The behaviour and movements of all cetaceans (with a focus on humpback dolphins) will be investigated in relation to desalination plant activities such as pipe flushing and the release of brine and over a longer period, in relation to changes over time from the initialization of activities at the plant. The recent expansion of surveying activities to neighbouring Vlees Bay will allow for a comparison between habitat use of cetacean species at the “impacted” site (Mossel Bay) and “control” site (Vlees Bay).

Aquaculture is a relatively new and growing industry in South Africa. It is a multi-million dollar global industry that is increasingly supplementing wild caught fish stocks in the human diet. Although this serves to help relieve the pressures on overexploited wild fish stocks, fish farming is not without costs and environmental impacts.

The majority of data on fish farm-marine mammal interactions relates to keeping seals out of farms and there is almost no data on interactions with cetaceans, which can be complex. Fin fish farms can have a negative impact on marine mammals, mostly through entanglements, especially if farms are mismanaged, but they can also have positive influence by increasing numbers of local wild fish stocks around farms which benefits the animals that prey on them, such as dolphins. This study will provide the first data on marine mammal-fish farm interactions in South Africa and will be highly relevant to the growth of this budding industry.