Each animal is IDed by his or her dorsal fin.
(The dorsal fin is the large fin located on the back).

The dorsal fin of each animal is unique in shape, scars, notches etc.
It is like a fingerprint; No dorsal is the same.
Most cetaceans develop scars, or missing notches in the dorsal
during their lifes making it very easy to ID them. Some examples:

Some reasearchers hold entire databases of a population of
animals that is studied by them. Most known Killer Whale pods in
the world can be found on such a chart.

_{ID chart of the J-pod Killer Whales (click to enlarge)}

IDing individual animals is essential for research.
Thanks to this things like the exact age for the animals
in that population, the amount of births, the behaviour and
migration can be monitored accuratly.