How do we identify each of our dolphins?
Project Burrunan provides the first formal population estimates (size and structure) for the only two known resident populations of Burrunan in coastal Victoria, through an abundance modelling technique known as mark-recapture. By photographically identifying individuals on several occasions through time within a population it is possible to model abundance and demographic variables.
Identifications are based on the naturally occurring nicks and notches found on the dolphin’s dorsal fin. Employing this technique, variables, such as population size and trends, survivorship and calving rate can be measured. This provides a baseline for future efforts in conservation and management of these resident population and the species as a whole.
Mark and Recapture is an area of study that is based on the identification of individuals within a population termed ‘marking’ followed by a ‘recapture’ phase where animals are exposed to capture and ‘marking’ again. From one or a series of these ‘recapture’ events, information can be gained on both the individuals recaptured and the population as a whole (Rosel et al. 2011, Urian et al. 2014). It is very important to note that ‘marking’ and ‘recapture’ only involve the collection of photographs of the dolphins dorsal fin (not physically marking and/or capturing).
The use of natural marks through photographical identification has now become a popular method of marking individuals (Slooten et al. 1992, Wilson et al. 1995, Rosel et al. 2011). This modern alternative method of marking has been used extensively on many species including facial recognition (e.g., in primates (Loos et al. 2011), and polar bears (Anderson et al. 2010), and spot pattern matching (e.g., in whale sharks (Brooks et al. 2010)). Having long life spans, low reproductive rates (Wells 1991, Wilson et al. 1999) and high rates of intra-species marking, dolphins make perfect candidates for mark recapture studies (Wells and Scott 1990). As is common in dolphin photo-id studies, this study will use identifications made from the unique series of nicks and notches on the easily damaged, thin trailing edge of the dorsal fin (Würsig and Würsig 1977, Würsig and Jefferson 1990).
From this data, it is possible to determine the range of the animal, its site fidelity and habitat use (Wells 2009). With the addition of a time variable, greater depth of understanding is possible, including the assessment of seasonal feeding habits and breeding cycles (Wells 2009). Adding in multiple individuals and modelling techniques (the focus of this study) population estimates are possible (Wilson et al. 1999).
Dolphin are individually identified and catalogued according to the numbering system. Photographs are also collected from biopsied animals allowing a full genetic fingerprint to be assigned to each individual sampled.