Life History
Dwarf minke whales are sexually mature at a smaller size than Antarctic minke whales. Females are likely to attain sexual maturity at 6-6.5m length (Kato et al., 2000), whereas female Antarctic minkes are not mature until about 8m in length.
It is possible that this annual winter aggregation in the Great Barrier Reef is for breeding purposes, and we have documented several occurrences of apparent courtship behaviour (e.g. synchronous and belly-to-belly swims).
Courting and mating is thought to occur between June and August and birthing probably peaks in May. There are fewer sightings of dwarf minke whales on the northern Great Barrier Reef from late July onwards but we do not know whether this is because most whales have left the area or whether they just approach vessels and swimmers less frequently at this time.
We have no information for dwarf minke whales, but in other minke whales births are thought to occur about 10 months after conception.
Calves of dwarf minke whales are about 2m long at birth, distinctly smaller than in other minke whales. There does not appear to be a distinct calving area. Strandings of newborn or very young animals have been recorded from near Melbourne, Victoria to Fraser Island, in southern Queensland on the east coast, and around Perth on the west coast of Australia. However, we have seen very young dwarf minke whales at reefs north of Port Douglas so the calving area is likely to be extensive.
In other minke whales, the calves suckle for only a short time, with weaning at about 4-6 months.
The breeding cycle of other minke whales is just over a year, which is more rapid than the larger baleen whales (e.g. humpback and right whales which may have 3-4 year breeding cycles).
Population size
The population size is unknown for any population of dwarf minke whales. The Action Plan for Australian Cetaceans considers it insufficiently known to assess the conservation status for dwarf minke whales. Recent work by Sobtzick (2010) has estimated the size of the interacting dwarf minke whale population in the northern GBR – see our Photo-ID study page for more information.
Hunting
The dwarf minke whale was originally described by Dr. Peter Best in 1985, mainly based on animals examined at the shore whaling station in Durban, South Africa. However the number of dwarf minke whales taken was small and they were probably generally ignored by whalers because of their small size. There were no confirmed records of dwarf minke whales from Japanese and Russian commercial whaling in the Antarctic, although a few records (referring to whales with a white flipper band) might have been dwarf minke whales. In 1989 and the early 1990’s, a small number of dwarf minke whales were harpooned at about 55-62°s, as part of the Japanese scientific whaling program. This is further north than the main commercial whaling grounds which may, with the small size of dwarf minkes, explain why they apparently escaped the attention of commercial whalers. At present, there appears to be no commercial whaling for dwarf minke whales and the Japanese scientific whaling program has not specifically targeted them since 1993.
Other forms of minke whales continue to be hunted, either through scientific whaling programs (in the Antarctic and northwest Pacific around Japan) or as commercial and/or and indigenous hunts (off Norway and Greenland).
We have seen scars on dwarf minke whales in the Great Barrier Reef that appear to have been caused by traditional harpoons, suggesting that these whales have visited South Pacific island nations that engage in traditional hunting.
Figure 1: Dwarf minke whale showing multiple scars from cookie cutter shark bites, and a long, raked scar potentially caused by a traditional harpoon.
Studies on the whales’ migration patterns (e.g. through satellite tagging) will assist in identifying potential risks and threats to these whales for the 8-10 months each year when they are outside the Great Barrier Reef Marine Park.