Red List of South African Species

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Least Concern (LC)
Assessors: Claire Relton
Contributors: Doug Butterworth Ken Findlay Simon Elwen Michael Meÿer Herman Oosthuizen Stephanie Plön Matthew Child
Reviewers: Claire Relton

Rationale

This subspecies is fully protected in South African waters under national legislation and an IWC moratorium on commercial whaling. Thus, although there are no reliable data available with which to assess the population status of Dwarf Minke Whales, there is no reason to suspect that there is a decline in the local population of this subspecies. The Dwarf Minke Whale is therefore listed as Least Concern because, although it has very low numbers naturally, there are no specific identified threats that could cause major population decline. However, further monitoring and an estimate of population size and trend is necessary. Taxonomic and distributional resolution and delimitation is also necessary. This subspecies should be reassessed once such data are available.

Regional population effects: Dwarf Minke Whales have a broad distribution across the southern hemisphere. There are no barriers to dispersal, thus rescue effects are possible.

Distribution

The Dwarf Minke Whale occurs in both tropical and warm temperate waters of the southern hemisphere. Their distribution extends along both the east and west coasts of South Africa (Best 1985) and Australia (Arnold et al. 1987, Bannister et al. 1996, Arnold 1997), and they have been recorded off Mozambique, New Zealand (Dawson and Slooten 1990), New Caledonia (Garrigue and Greaves 2001), Brazil (Baldas and Castello 1986, Zerbini et al. 1996) and in the Beagle Channel (Chile/Argentina; Acevedo et al. 2005). Dwarf Minke Whales may primarily occur within coastal habitats (Best 1985, Zerbini et al. 1996, Perrin and Brownell 2002), however Zerbini et al. (1997) recorded them over the continental shelf off Brazil within fairly shallow waters (4–200 m deep). Their range extends from approximately 7°S to 58°S (Best 1985, Perrin and Brownell 2002). The range limits of B. acutorostrata and B. bonaerensis in the southern hemisphere are fairly ambiguous, because parts of their range are thought to overlap. Dwarf Minke Whales are present at higher latitudes and are considered to be a more subtropical species compared to Antarctic Minke Whales, but are significantly less common than Antarctic Minke Whales. Off the east coast of South Africa, B. acutorostrata are generally present closer inshore than B. bonaerensis (Skinner and Chimimba 2005).

During the summer months, most records of Dwarf Minke Whales are from sub-Antarctic waters. Their large-scale movements and migration patterns remain uncertain, as some whales are present year-round, whereas others may exhibit migratory behaviour. Acevedo et al. (2007) describes a seasonal north/south movement of Dwarf Minke Whales off South America, and similarly in the western South Pacific, Arnold (1997) observed the seasonal occurrence of this species between May and September, with peaks in June and July. These individuals are suggested to spend summer in Antarctic waters south of New Zealand, between 55° and 62°S (Pastene et al. 2010). Pastene et al. (2010) proposed that there may be multiple populations of Dwarf Minke Whales in the southern hemisphere, which exhibit seasonal spring/summer migrations due south to Antarctic waters.

Population trend

Trend

According to the IWC, there are no reliable data available with which to assess the population status of Dwarf Minke Whales, as the older available quantitative sighting data of Minke Whales in the southern hemisphere does not distinguish B. acutotostrata from B. bonaerensis. However, B. bonaerensis is considerably more numerous compared to the Dwarf Minke Whale. Only recently has it been possible to distinguish Dwarf Minke Whales from Antarctic Minke Whales at sea, using colouration patterns and the identification of shoulder patches, and sightings are too few to provide a population estimate.

It is likely that B. acutorostrata was not subjected to substantial overexploitation during commercial whaling activities directed at Minke Whales, because its range falls predominantly outside of the Antarctic Minke whaling areas. Between 1987 and 1993, more than 1,700 Minke Whales were captured by Antarctic pelagic fleets, and only 16 of these were recorded to have been Dwarf Minke Whales (Nishiwaki et al. 2006). These individuals were predominantly taken from the northern limits of the whaling operations, between 55° and 62°S. Best (1985) suggested that only 3–4% of Minke Whales landed off Durban were Dwarf Minke Whales, and very few individuals are thought to have been caught off Brazil (Zerbini et al. 1997).

Within the assessment region, although numbers are likely to be low, there is no reason to suspect a decline since no major threats have been recognized at present.

Threats

Although the northern subspecies were hunted in the northern hemisphere under both commercial and scientific permits, this subspecies has not been hunted in the assessment region since 1993/94. As it occurs mainly outside the whaling grounds for Antarctic Minke Whales, it has never been substantially exploited.

This subspecies occasionally occurs inshore, where it is vulnerable to entanglement in fishing gear and shark nets. Although entanglement of Minke Whales in South African waters is considered uncommon compared to Humpback Whales (Megaptera novaeangliae) and Southern Right Whales (Eubalaena australis), occasionally Minke Whales have been recorded trapped in shark nets off the coast of Durban, KwaZulu-Natal (IWC 2001, Meÿer et al. 2011). Between 1981 and 2009, 10 Minke Whales were recorded caught in shark nets off Durban and, of these, three were positively identified as Dwarf Minke Whales (IWC 2001, Meÿer et al. 2011).

The effects of anthropogenic climate change, resulting in decreased sea ice in the polar regions, may severely diminish the food resources of Minke Whales. The exact implications of these effects remain unclear; however, systematic monitoring of the compounding threats of climate change is warranted (Moore 2008).

Similar to other cetaceans, collision with vessels, plastic marine debris and noise pollution, including the threats of seismic survey operations, may become accumulating minor threats to this species, but no quantitative species-specific data exists. Although apparently largely fish-eating, there is no documented conflict with commercial fisheries in or outside of the assessment region.

Uses and trade

This northern subspecies were heavily hunted in the past, and harvesting is ongoing in parts of the North Atlantic and North Pacific. There is currently no trade in this subspecies of Minke Whale in the assessment region as it has been largely excluded from the Antarctic catches.

Conservation

The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) listed B. acutorostrata on Appendix I, with the exception of West Greenland where it is listed in Appendix II. The Dwarf Minke Whale is fully protected in South African waters, and no specific conservation efforts have been identified. This subspecies would benefit from the development of methods to clearly distinguish it from Antarctic Minke Whales, during ship-based surveys. This could lead to a more comprehensive, systematic monitoring programme for Dwarf Minke Whales.

Recommendations for managers and practitioners:
  • Design a monitoring scheme that can distinguish this subspecies from Antarctic Minke Whales, B. bonaerensis.
  • Ensure that accidental catches (in fishing or shark nets), strandings or collisions of Dwarf Minke Whales are reported, and managed by the relevant authorities, and in the case of mortality, make certain that the remains are made available for ongoing scientific investigation of the life history and genetics of this species.
Research priorities:
  • Investigation into the distribution, abundance and taxonomic status of Dwarf Minke Whales within the southern hemisphere.
  • General ecology, including diet, distribution, reproduction, movement patterns and behaviour of this subspecies.
  • Effect of the cumulative impacts of anthropogenic influences, such as pollution, commercial fishing practices and ecotourism on Dwarf Minke Whales.
  • The identification of individuals using shoulder patches and colouration has proved to be a valuable means of assessing population structure and distribution of this species in the waters off Australia (Arnold et al. 2005), and may be equally useful in other parts of its range.
Encouraged citizen actions:
  • Submit location sightings to conservation authorities, which will help in determining the spatial and temporal distribution of the population.
  • Save electricity and fuel to mitigate CO2 emissions and hence the rate of climate change.
  • Report any stranding reports to the relevant local authorities.
  • When participating in whale/dolphin watching tours, ensure regulations are followed.

References 

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