Physeter macrocephalus

Sperm Whale

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Sperm Whale

 

Description

Physeter catodon, the sperm whale, is the largest toothed predator on Earth. Males can be over 18m long, weighing up to 50,000kg.Mature female sperm whales tend to live in social groups of up to 15 mature females and their offspring, whereas mature males live alone or in smaller groups.The sperm whale is listed as vunerable to extinction by the IUCN. Commercial whaling was the biggest threat to this species.

Biology

Sperm whales live in either nursery or bachelor groups. Nursery groups consist of a number of adult females and immature males and females (2) (6). Males leave these groups when they become mature and join bachelor groups, which consist of males of 7 to 27 years of age (6). Older males live in small groups or singly, and visit nursery groups to mate with females during the breeding season (6). Most groups of sperm whales tend to number between 10 and 15 individuals (4). Sperm whales use echolocation to find their prey in the dark ocean depths (6). When foraging, powerful sound waves are emitted from the large head; these can stun and even kill the squid, octopuses and fish on which they feed (4). These whales make deep dives to depths of up to 3,000 meters (almost 2 miles) that can last as long as two hours (2) (6). This is the deepest dive made by any species of mammal (6). Males reach maturity at 10 years of age, but they do not begin to mate until they are around 19 years old and a length of 13 metres. Females become mature at between 7 and 11 years, when they are around nine metres in length. A single calf is born between July and November after a gestation period of around 16 months. The calf is suckled for up to two years (4). Groups of females protect their young by adopting a defensive 'marguerite formation' in which the calves are placed in the centre of the group surrounded by a circle of females, facing tail outwards (4).

Geography

Sperm whales are found throughout the world's oceans in deep waters to the edge of the ice at both poles (Waring et al. 2009 and references therein).

According to Waring et al. (2009), results of multi-disciplinary research conducted in the Gulf of Mexico during the first decade of the 21st Century confirm speculation that Gulf of Mexico Sperm Whales constitute a stock that is distinct from other Atlantic Ocean stocks(s). Sperm whales were commercially hunted in the Gulf of Mexico by American whalers from sailing vessels until the early 1900s. In the northern Gulf of Mexico (i.e., U.S. Gulf of Mexico), systematic aerial and ship surveys indicate that sperm whales inhabit continental slope and oceanic waters, where they are widely distributed. Seasonal aerial surveys confirm that sperm whales are present in at least the northern Gulf of Mexico in all seasons. The best available estimates indicate a population of around 1,500 Sperm Whales in the northern Gulf of Mexico. (Waring et al. 2009 and references therein)

Ecosystem

Habitat and Ecology
The habitat of the sperm whale is the open sea. More specifically, sperm whales can be found in almost all marine waters deeper than 1,000 m that are not covered by ice, except in the Black Sea and possibly the Red Sea (Rice 1989; Whitehead 2003). In some areas, particularly in the western North Atlantic, sperm whales, especially males, can occur in shallower waters (e.g., Scott and Sadove 1997). Females and young are usually restricted to waters at latitudes lower than about 40-50º and to areas where sea surface temperatures are greater than about 15ºC (Rice 1989). Sperm whales are generally more numerous in areas of relatively high primary productivity (Jaquet et al. 1996), although there are some exceptions, such as the Sargasso Sea and the central North Pacific gyre (Barlow and Taylor 2005).

The sperm whale is an animal of extremes in size (up to 18 m), sexual dimorphism (mature males have three times the mass of mature females), ecological imprint (sperm whales take roughly the same amount of biomass from the oceans as humans), and many other attributes (Whitehead 2003). The commercial value of the animal (a function of its size and the quality of sperm whale oil) drove two massive worldwide hunts: the technologically primitive “open-boat” hunt from 1712-~1920 (Starbuck 1878; Best 1983), and modern whaling using engine-driven whaling ships and harpoon guns from ~1910-1988 (Tonnessen and Johnsen 1982). The complex social structure of sperm whales may have been affected by whaling, lowering potential population growth rates, which are very low anyway (Whitehead 2003). On the positive side, sperm whales are very widely distributed (see above), and their primary prey, deep-water squid, are not yet major targets of fisheries.

The generation time (mean age of mothers) for sperm whales can be calculated if one assumes a set of population parameters, specifically age at first birth, mortality rate of mature females, and reproductive rate of mature females. There is uncertainty about these parameters, so two calculations were made using different assumptions:

a) Applying the population parameters most recently used for sperm whales by the International Whaling Commission’s Scientific Committee (International Whaling Commission 1982): age at first birth = 10 years; female reproductive rate in unexploited population = 0.20/year; female adult mortality = 0.055/yr): generation time = 27.3 years.

b) The estimates of mortality used by the International Whaling Commission are particularly problematic, and sperm whales likely have age-specific survival and reproductive rates. Thus it may be more realistic (Whitehead 2002) to use the well-established mortality schedule of killer whales (Orcinus orca; Olesiuk et al. 1990) and an age-specific pregnancy rate taken from the sperm whale data presented by Best et al. (1984; pregnancy rate for mature females = 0.257-0.0038xAge in years): generation time = 27.5

Conservation

Sperm whales were once quite abundant in the Gulf of Mexico, but due to commercial whaling operations, they are seldom seen in this area anymore. Worldwide however, sperm whales populations are more stable than that of many other whales, although they continue to be listed as endangered by USDI (1980). The sperm whale is now the most abundant of the great whales, having been hunted with less intensity that the baleen whales. Worldwide, sperm whales number about 1,500,000.

 

Threats

Sperm whales have a long history of commercial exploitation. Large-scale hunting began in 1712 in the North Atlantic, based at Nantucket in America. They were not widely hunted for their meat, but for ambergris and spermaceti. Ambergris is a substance that collects around the indigestible beaks of squid in the stomach of the whale, and was highly prized for use as a fixative in the perfume industry. Although artificial alternatives are now available, some perfume makers prefer to use ambergris today. Spermatceti was used in the production of cosmetics and candles. Sperm whales still have an economic value today for meat in Japan. Since the 1980s, the International Whaling Commission brought an international moratorium on whaling into force. Despite this measure, Japan continues to hunt sperm whales, and relatively small numbers are taken each year with hand harpoons at Lamalera, Indonesia. Further threats include entanglement in fishing gear and collisions with boats. Although whaling has, with the exceptions outlined above, largely ceased, the after-effects of such prolonged and intensive hunting are still being felt today. It is thought that the selective hunting of the largest, breeding males will have decreased pregnancy rates, and the loss of the largest females from nursery groups would have decreased the survival of the groups.

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