Individual whales and dolphins can be identified from photographs.
For example, about 10% of all Hector’s dolphins are individually identifiable, mostly from dorsal fin nicks. Almost all sperm whales and right whales are individually identifiable. Right whales are identified by the patches of ‘callosities’ on the head. These photographs show three different right whales:
The first whale has a ‘clean’ lip. Its bottom lip does not have any callosities (specialised, roughened patches of skin, with white whale lice growing on them).
The second whale does have callosities on its lower lip. The other markings on the head are also very different between these three whales.
The third whale has an unusual colour pattern. About 5% of the right whale population at the Auckland Islands has this kind of colouration.
Hector’s dolphins are identified by nicks out of their dorsal fins, body scars or skin colouration. The small size of Hector’s dolphins makes them vulnerable to shark attacks and some individuals show spectacular shark bite scars.
They also occasionally bite each other, which can result in tooth rake marks. These look like someone has scratched the dolphin’s skin with a fork (see photo below). The skin is quite soft and easy to damage, so it doesn’t take much force to create a tooth rake. Tooth rake scars heal within a few weeks or months, and are not suitable for photo-ID.
Shark bites and other larger scars are used for photo-ID. Some scars result from entanglement in fishing gear.
Information from photographic identification studies provides a wealth of information about whales and dolphins, including:
- The size of the population can be estimated from the number of identifiable (marked) dolphins and the ratio of marked to unmarked dolphins
- At its very simplest, if we have 100 Hector’s dolphins with nicks in their dorsal fins and the ratio of marked:unmarked dolphins of 1:10 then this particular population has about 1000 dolphins in it. Of course the calculations for real populations (which have births, deaths, emigrations and immigrations) are much more complex that this, and rely on “mark/recapture” theory
- It’s possible to estimate the survival rate of a population of whales or dolphins by monitoring how many individuals disappear from the population each year
- This involves correcting for individuals that have lost their marks, have moved to another population or are not seen every year
- Breeding females with ID markings (nicks out of the dorsal fin or colouration markings) are very valuable in any photo-ID study
- For example identifying female Hector’s dolphins has shown that they give birth to a newborn calf every 2-3 years
- In most whale and dolphin populations who hangs around with whom is not random
- Sperm whales have very strong social bonds, with social groups consisting of grandmothers, mothers, daughters and their offspring. Males tend to leave these groups in puberty, coming back to visit these matrilineal groups again when they are old enough to mate
- Most dolphin species have fission-fusion societies, with strong social bonds between a mother and her most recent offspring but otherwise a fairly fluid group composition
- When you encounter a small group of dolphins, this is generally not a nuclear family of ‘mum, dad and the kids’