For over three decades, photo-Identification has been used for the study of population size and trends of small cetaceans. Photo-identification is a method used in Capture-Recapture population sampling studies, which use the special markings on the body of an animal to distinguish it from other individuals. For dolphins, scars and marks, especially on the trailing edge of the dorsal fin and skin lesions on the dorsal fin and back are used to distinguish between individuals.
Observations and photographs of dolphins may be taken from a variety of platforms, such as coasts, boats and airplanes. In the case of the Northern Aegean Dolphin Project, MOm’s research vessel “IFAW-Odysseia”, a 13m-long wooden caique is used as the field/research station. Surveys are conducted only under appropriate weather conditions, i.e., sea state less than 3 Beaufort and good visibility (e.g. low swell), during which "IFAW-Odysseia" is traveling with a speed of 8 knots, so as to allow for sufficient opportunity to detect dolphins. GPS coordinates are recorded during the survey, along with information on the sea state and the sighting conditions.
The minimum number of people to conduct a survey is not less than three; one observing from the front part of the boat and the other two, focusing on the left and right side. Upon encountering a group of dolphins the teams record the time, coordinates, distance from boat, species, group size and age differences. The ultimate goal of the survey is to travel in parallel to the dolphin group and take as clear and as perpendicular photographs of the dorsal fins of all the individuals comprising the group as possible. Of course, one must always keep in mind not to disturb the study objects for too long; usually an encounter of no more than an hour is the rule (of course varying in relation to the size of the group).
The next step is to match photos to individuals, create and then update the photo-identification catalog (the initial cataloguing of an identified individual is called the capturing process, and later re-sightings are defined as the recapture events) and keep a record of the recapture history of each individual. This information, along with data taken on board, such as environmental co-variates, expressed behavior and group size, are incorporated in specific models in order to assess population size, survival rates, recruitment numbers, movements etc.