INDIVIDUAL RECOGNITION

There are many questions that can only be answered if we can recognise individual whales. Do the same whales return to particular reefs from year to year? How long do particular whales stay at one location? Do the same whales occur together from day to day? Cataloguing individually recognisable whales may also give some indication of numbers of whales in a particular area.

Whales can be recognised by several means. In the short term (days to weeks) the most useful features are scars. Some of these are complex and unique. Other types of scars are found more widely

Oval scars

These are oval (usually with longest dimension along the body of the whale) scoops from the blubber, usually several centimetres long and varying in depth. Fresh scars on the back are particularly distinctive because the white of the exposed blubber contrasts strikingly with the dark colour of the back. They can be coded according to position and relation to the colour pattern of the whale.

We do not know definitely what causes these scars. Some are probably the bites of cookie-cutter sharks Isistius brasiliensis. These small (to 0.5m) sharks have the teeth of the lower jaw fused into a plate which can act like an ice cream scoop, removing a plug of blubber or flesh from the sides of dolphins, whales and large oceanic fishes. They occur in open ocean, and we have collected them off the Great Barrier Reef in the Coral Sea. At least some of the scoops and the half-moon bites seen especially on the tail flukes are likely the bites of cookie-cutter sharks.

Parallel slits/scratches

On the head, back and sides of the whales we have seen 4-5 parallel slits/scratches of variable length. They are probably of multiple origins; possibilities include tooth rake marks or the scars from the finlet bases of tunas.

Bite marks and tooth scrapes

We are documenting quite numerous scars on various parts of the whales' anatomy which appear to be caused by sharks. Both large and small diameter jaws are involved.

Parasites

We have seen two types of parasites on dwarf minke whales.

Pennella is a gigantic copepod (a relative of shrimps, free-living species of which are usually about the size of a grain of rice) which grows up to one-third of a metre long. The head is buried in the blubber of the whale and long horns of the head form anchors to hold the parasite in place. The main body hangs off the side of the whale, looking as if someone tagged the animal or it had gotten snagged by a hook and piece of fouled fishing line. Pennella is probably a true parasite, feeding on the blood of the whale.

Xenobalanus is a barnacle which has a reduced shell, embedded in the tail or dorsal fin of the whale. The main part of the body hangs off the trailing edge of the tail or fin, where it can feed on tiny organisms swept past in the water. We have seen 20-30 of these barnacles hanging off the tail flukes of a dwarf minke whale.

Nicks in the dorsal fin and flukes

In the northern hemisphere, where animals can not be observed underwater, the most commonly used feature to identify individual minke whales has been nicks in the dorsal fin. In our experience, the fin of dwarf minke whales has few obvious nicks, although there are sometimes obvious nicks in the tail flukes.

Colour pattern

The most complex to code but potentially the most long lasting feature is colour pattern. Scars heal, parasites drop off but, based on work overseas on other minke whales and our work on dwarf minke whales, colour pattern remains stable enough to recognise individuals over time.

We are using all features to identify individual whales and have successfully re-identified 15-20 in each of the last two seasons (1999 and 2000). Some animals have been spotted more than once in a single season. Three minkes initially identified in 1999 were re-identified in 2000.