Kogia sima (Owen, 1866)

English: Dwarf sperm whale
German: Kleiner Pottwal
Spanish: Cachalote enano
French: Cachalot nain

1. Description

Kogia spp. are porpoiselike, and robust, with a distinctive underslung jaw, not unlike sharks. They have the shortest rostrum among cetaceans and the skull is markedly asymmetrical. Dwarf sperm whales reach a maximum size of about 2.7 m total length and a body mass of 272 kg. Colouration in adults is dark blueish grey to blackish brown on the back with a light venter. On the side of the head, between the eye and the flipper, there is often a crescent-shaped, light-coloured mark referred to as a "false gill" (McAlpine, 2002).

Kogia must be treated as feminine because it has a Latin feminine ending. Simus, -a, -um, is a Latin adjective, and therefore it must agree in gender with the generic name with which it is at any time combined. Thus the correct spelling of the scientific name of the dwarf sperm whale is Kogia sima (Rice, 1998), as opposed to Kogia simus in most publications to date.

2. Distribution

According to Caldwell and Caldwell (1989), there are two problems in trying to establish ranges for Kogia. First, members of this genus are only rarely identified at sea (and then usually not to species), and second, it is only recently that the two species have been clearly recognised as separate. As a consequence, most reliable records of either species are based on stranded individuals or occasionally on ones taken in small fisheries for small cetaceans.

Rice (1998) summarises that K. sima lives mainly over the continental shelf and slope off tropical and temperate coasts of all oceans. Range includes the western Atlantic from Virginia south to Rio Grande do Sul in Brazil, including the Antilles; the eastern Atlantic from the Mediterranean Sea south to Cape Province; The Indian Ocean from Cape Province north to Oman, east at least as far as Lomblen in Indonesia, and south to South Australia; the western Pacific from Chiba prefecture on the east coast of Honshu, and the Mariana Islands, south to Hauraki Gulf in New Zealand; and the eastern Pacific from Vancouver Island south to Valparaiso in Chile (Rice, 1998).

Although it was assumed that populations were continuous around the world, new molecular genetic results from Susan Chivers (pers. comm.) indicate that specimens of K. sima sampled from the Atlantic and Pacific ocean may represent different species, suggesting that there is little interchange between these two ocean basins.

3. Population size

Because of the lack of sightings at sea, which may be more because of its behaviour than true abundance, and the fact that Kogia is only rarely encountered in commercial fisheries where such records are often kept, there are no real estimates of abundance for either Kogia species (Caldwell and Caldwell, 1989). Mullin et al. (1994) sighted dwarf sperm whales in the Gulf of Mexico over water depths between 400 and 600m. The species accounted only for 1% of the animals seen and occurred in 12% of the herds observed during the aerial survey. Dolar (1999) estimated the population size in the eastern Sulu Sea at 650.

K. sima seems to be especially common off the southern tip of Africa and in the Gulf of California (Sea of Cortez), Mexico, where it occurs particularly close to shore. Most records are from strandings, which are relatively common in some places, though these may simply represent areas of most research rather than a true picture of distribution. Lack of records of live animals may be due to inconspicuous behaviour rather than rarity. (Carwardine, 1995; Jefferson et al. 1993).

Recent strandings have been reported from Sable Island, Nova Scotia (Zoe and Hooker, 2000), the Gulf of Mexico (Delgado et al. 1998), British Columbia, Canada (Willis and Baird, 1998), the Azores (Goncalves et al. 1996), Ecuador (Felix et al. 1995), the Antilles (Debrot and Barros, 1992), the coast of France (Duguy, 1990) and Japan (Sylvestre, 1988), supporting the notion of a world-wide distribution.

4. Biology and Behaviour

Habitat: The Dwarf Sperm Whale is an inconspicuous animal and generally lives a long way from shore (Jefferson et al. 1993). Rarely seen at sea, except in extremely calm conditions, it is the smallest of the whales and is even smaller than some dolphins. Predominantly a deep-water species, possibly concentrated over the edge of the continental shelf (closer to shore than the Pygmy Sperm Whale). Appears to prefer warmer waters (Carwardine, 1995).  

Behaviour: Rises to the surface slowly and deliberately and, unlike most other small whales (which roll forward at the surface), simply drops out of sight. Probably does not approach boats. May occasionally breach; leaping vertically out of the water and falling back tail-first or with a belly flop. Some records suggest that, when resting at the surface, it floats lower in the water than the Pygmy Sperm Whale. Probably dives to depths of at least 300m (Carwardine, 1995).
One of the few reported behavioural observations at sea stems from Scott and Cordado (1987) who report sighting a mother and calf after a purse-seine set was deployed on yellowfin tuna, Thunnus albacares , associated with a mixed school of spotted dolphins, Stenella attenuata , and spinner dolphins, S. longirostris . They were accidentally encircled. While inside the net, the female released into the water a cloud of reddish material, presumably faeces, 6-8 times during the course of the set. The mother released the faeces whenever a dolphin approached the calf; she then appeared to hide herself and the calf in the middle of the opaque cloud.

Schooling: Group sizes tend to be small, most often less than 5 individuals (although groups of up to 10 have been recorded (Jefferson et al. 1993).

Reproduction: In at least one area, there appears to be a calving peak in summer (Jefferson et al. 1993).

Food: Dwarf sperm whales appear to feed primarily on deep-water cephalopods (Jefferson et al. 1993) as well as on fish and crustaceans (Caldwell and Caldwell, 1989).

5. Migration

Duguy (1994) suggests that the species does not migrate extensively, since it can be observed year-round off African coasts.

6. Threats

Direct catch: Some small scale catches of dwarf sperm whales have been reported (Caldwell and Caldwell, 1989 and refs. therein). K. sima was encountered in a small harpoon fishery for pilot whales at St. Vincent in the Lesser Antilles, in Japan and occasionally in an aboriginal industry on Lomblen Island in Indonesia, and has been reported from fish markets in Sri Lanka.

Incidental catch: Caldwell and Caldwell (1989) suppose that it is unlikely that Kogia is significantly affected by humans. When taken in commercial fisheries the numbers are so few that either species is considered rare. However, Jefferson et al. (1993) believe that substantial numbers are taken each year in gillnets in the Indian Ocean, and possibly elsewhere. Zerbini and Kotas (2001) report on by-catch in the Brazilian driftnet fishery. Because of their small size and habit of often lying at the surface, apparently oblivious to approaching vessels, a few Kogia are probably run down and injured or killed (Caldwell and Caldwell 1989).

Pollution: Both species have been reported with plastic bags in their stomachs that may have prevented digestion of food and ultimately brought death. Perhaps the textural or visual quality of the plastic was similar to that of squid and thus enticed the whales to devour it (Caldwell and Caldwell 1989).

7. Remarks

This species has a world-wide distribution and is poorly known. Basic data on population sizes and impacts of threats on the population are lacking. Not listed by the IUCN or by CMS.

Both kogiid species also occur in southern South America. Recommendations iterated by the scientific committee of CMS for small cetaceans in that area (Hucke-Gaete, 2000) also apply. For recommendations on south-east Asian stocks, see Perrin (1996).

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