Definition in marine biology: the method of study of marine mammals using photographs to identify individuals.
We will particularly concentrate on dolphins and how we rely on the presence of unique natural features, specifically on the dorsal fin (the size and location of notches and cuts), to identify individual animals.
Natural markings data can be very effective for estimating survival rates of marine mammals. The method is useful because it allows enumeration of almost the whole population without fear of double counting individuals or of seriously underestimating population size.
Natural markings studies are invaluable for estimating survival and birth rates, for identifying migration routes, also for detailed studies, including abundance estimation of a small population. Individual identification is an important tool for studies of behavior, ecology, and population biology. Pigment patterns, shape of the dorsal fin and other specialties on the body also help with recognising of individual animals.
Dorsal fin is the part of the body where there are the most unique markings, such as scars and cuts by which we can distinguish particular individuals. These are made by mutual interactions or can be caused by other organisms and diseases, sometimes also unpleasant encounters with humans.
Dorsal fins are like fingerprints on humans, each is different. It’s also the part of the body which is always visible when surfacing. There are no bones or cartridge, just connective, fat tissue and a dense vein interlace which has a function of thermal regulation.
Photo identification enables us to:
- estimate the number of individuals in the population;
- observe the population state: distribution, annual fertility rate, migrations, behaviour, social structure, habitat structure and the frequency of individual dolphins in individual areas;
- make an identification catalog of individual dolphins;
- recognise individuals by their natural markings;
- follow the process of wound healing;
- observe growing up of the calves;
- observe and estimate the offspring.
Once we have successfully obtained all or most of the photos of dorsal fins of all the individuals in a specific pod – photos must be taken of both sides of the dorsal fin, we compare photographs, new with the old, to determine which specific individuals we encountered and/or add new ones to the catalogue. This way we determine population size and follow their numbers in time.
There can be some problems with this method; when smaller proportions of animals are identified, estimates of population size can be badly compromised for a variety of reasons. One problem is that the population being estimated is not always well defined, with some animals from elsewhere temporarily entering the population and others temporarily absent. A severe problem for large populations, in which only a small proportion can be identified, is that false positives in the matching procedure, even if they occur only rarely, can lead to a substantial underestimation of population size. So it is rarely a cost-effective or reliable method for estimating the size of large populations of marine mammals.
Other identification methods include temporary researcher-applied markings, scarring and branding, and/or attachment tags which all require some sort of direct interaction with dolphins which is more or less disturbing for the animals. Whereas photo-id is a distance sampling technique where we only take photos from a short, but safe distance (for the animals), which is relatively non-disturbing for dolphins.
Photo-id also enables us to observe movements and socialising of individuals. Whether they live in relatively stable pods (like pilot and killer whales), or in fluid groups (characteristic of many smaller dolphins) and how large those groups are. Photo identification studies also show whether some dolphins are resident in small areas year-round, or make long distance migrations.
References:
Encyclopedia of marine mammals
Weir et.al. 2008. A Long-Term Opportunistic Photo-Identification Study of Bottlenose Dolphins (Tursiops truncatus) off Aberdeen, United Kingdom: Conservation Value and Limitations. Aquatic Mammals. 34, 4: 436-447
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