ResearchResearch methodsIndividual recognition

Individual recognition

Many cetacean species possess distinctive natural markings (such as pigmentation patterns or nicks on dorsal fins) that vary from one animal to another so that individuals can be recognised at sea. Photographs of these distinctive markings on individuals (so-called photo-identification) form the basis for a widely used research method for studying cetaceans. Long-term photo-identification studies of recognisable individuals provide information on population size and survival (through mark-recapture analyses), movements and reproduction.



Population size

Mark-recapture estimates of population size are based on the simple idea that if a proportion of the whole population is marked in a first sample, then an estimate of this proportion can be obtained by observing the number of marked animals in a second sample. If various assumptions about the nature of the data are made, these two proportions can be equated to give an estimate of abundance. In most studies, records of recaptures (or re-identifications) from a series of samples are built up into so-called capture histories, which are then used to estimate population size.

Clearly, photo-identification can only be used for those species whose individuals possess lasting, recognisable natural markings. But many species do have such markings and abundance has been estimated using mark-recapture methods applied to photo-identification data for a range of cetaceans including bottlenose dolphins, killer whales, humpback whales and blue whales.


Delfín común Calderón gris Delfín mular
Calderón negro Cachalote

An interesting development is to use genetic identity determined from the analysis of DNA obtained from tissue samples as the mark. It is more difficult to collect the data but advantages of genetic markers include definitely not changing over time (which natural markings can), and providing information on the sex of sampled animals. This method has been applied to humpback whales in the North Atlantic.


Survival and reporduction parameters

Mark-recapture methods are also commonly used to estimate survival rates. The idea here is that the marked animals form a cohort that is followed for a period of time; the data on whether or not each animal is recaptured on subsequent occasions providing information on survival. The basic assumptions are the same as for estimating abundance. Survival has been estimated using these methods for humpback whales, gray whales, right whales, killer whales and bottlenose dolphins..

Individual recognition data are also valuable for examining reproductive parameters such as age at first birth, the interval between births and reproductive rate, as has been done for humpback whales. Intensive sampling is necessary to obtain such reproductive information but the effort is worth it because it is extremely difficult to obtain in other ways.



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