Scientific name: Physeter macrocephalus
Distribution within the Mediterranean:
Conservation status in the Mediterranean:
Conservation status in the world:
Between 15 and 18 meters in adult males and 8 and 17 meters in adult females; between 3,5 and 4,5 meters at birth
Can be confused with:
Blow and diving pattern:
Individual identification characteristics:
The sperm whale (Physeter macrocephalus) is the biggest extant toothed whale. It’s log-like body is laterally compressed with a huge squarish head 1/4 to 1/3 of the total length of the animal. The lower jaw is much narrower than the upper jaw, barely visible from the side. It has a single S-shaped blowhole located in the front of the head facing forward. Flippers are short, wide and triangular-shaped. Dorsal fin is absent, but there is a low and thick dorsal hump, followed by a series of dorsal crenulations on the dorsal ridge. The caudal peduncle is very deep, sometimes with a caudal keel. The skin behind the head and on the sides of the body is wrinkled.
The head is one of the most distinctive characteristics of the species, probably because of it’s asymmetry, clearly seen in it’s interior (the skull and the air passages) and its exterior (the position of the single blowhole). The head is square-shaped seen from the side containing the nasal complex in its interior. This structure is formed by the spermaceti organ, which is enclosed in a muscular case. This organ is made of a spongy tissue filled with an oily substance called the spermaceti oil, and it is bounded to a aerial sacs at both ends. Between the upper jaw and the spermaceti organ there is a structure called junk which is made of connective tissue and spermaceti oil. The oil found in the spermaceti has the same properties as wax and it is different that the oil found in the melon of other odontocete species.
The coloration of the sperm whale is mainly black, brownish or grey, with white areas around the jaw and on the ventral part of the animal. The extension and the shape of these lighter patches vary among individuals. Sperm whales may also have white scars on their bodies, probably caused by other sperm whales or by cephalopods. There is record of at least one completely-white albino individual.
Sperm whales only have teeth in their lower jaw. They have between 18 and 26 pairs of conic, pointed teeth, which can measure up to 20 centimeters. Teeth are sharper in younger individuals, whereas adults may have rounded teeth.
The species has a very high level of sexual dimorphism, being males much larger than females. Males measure between 15 and 18 meters (mean 15), weighting between 43,5 and 55,8 tones, whereas females measure between 8 and 17 meters (mean 11), weighting between 13,5 and 20 tones. Newborns measure from 3,5 to 4,5 meters.
DISTRIBUTION AND MIGRATORY PATTERNS WITHIN THE MEDITERRANEAN SEA
Sperm whale is a cosmopolitan species, that inhabits all the oceans and seas of the world, from the polar waters, to the tropical areas, close to the equator. Despite of this, both sexes show significant differences in their distributions. Whereas females and younger individuals congregate in temperate, tropical waters, adult males carry out long annual migrations between the equator and the polar areas.
The world’s sperm whale population prior to its commercial exploitation it is believed to have reached the 1.100.000 individuals. During the 19th century, commercial catch reduced the population in about 29%. After this reduction, a slow recovery process started and lasted until the 1950s, when a intensive commercial whaling activity started again. In the 1970s the sperm whale population reached its lower levels. From this point onwards, the reduction of the commercial whaling allowed the species to slowly recover. According to different estimates, the actual population of sperm whales reaches the 360.000 individuals, which represent only the 32% of the living individuals in the beginning of the 19th century (Whitehead, 2002). The species is most abundant in the Pacific Ocean, the Antarctic and the Atlantic oceans.
In the Mediterranean Sea, the sperm whale is distributed throughout the whole basin, from the Strait of Gibraltar, to the most eastern edge. Despite of this, there may be more individuals in the western basin, being the Ligurian Sea, the Strait of Gibraltar, the Tyrrhenian Sea, the Gulf of Lion and the Balearic Islands, hot-spots for the species, than in the eastern basin, where it can be mainly found in the Ionian and Aegean seas (Gannier et al., 2002; Frantzis et al., 2003; Reeves and Notarbartolo di Sciara, 2006; Lewis et al., 2007 & Notarbartolo di Sciara and Birkun, 2010). The Strait of Gibraltar, the Balearic Islands, the Ligurian Sean and the Hellenic Trench, where stable populations of sperm whales can be found, are areas of special interest for the species (Frantzis et al., 2014; Carpinelli et al., 2014 & Rendell et al., 2014). Although it hasn’t been deeply studied yet, the Tyrrhenian Sea has the potential to be an important area for the sperm whale (Aïssi et al., 2014; Mussi et al., 2014 & Pace et al., 2014). Although migratory movements have been recorded in the Mediterranean Sea, there is no evidence of migratory routes heading outwards, thus the mediterranean population would be genetically isolated from those in the Atlantic ocean (Druout et al., 2004; Engelhaupt et al., 2009 & Notarbartolo Di Sciara, 2014). There are documented migratory routes between the Ligurian Sea and the Balearic Islands, between the Strait of Gibraltar and the Balearic Islands, between the Strait of Gibraltar and the Ligurian Sea, between the Ligurian Sea and the Tyrrhenian Sea or between the eastern and the western basins for males, but none for females (Drouout et al, 2004; Frantzis et al., 2011; Carpinelli et al., 2014; Mussi et al., 2014 & Rendell et al., 2014). Some males have been resighted in areas such as the Strait of Gibraltar or the Ligurian Sea over successive years, females have been resighted in the eastern basin. The resighting rate suggest a great degree of site fidelity by the sperm whales of the Mediterranean (Frantzis et al., 2011; Mussi et al, 2014 & Pace et al., 2014).
HABITAT AND FEEDING
Sperm whales mainly inhabit deep waters of all oceans, although they prefer waters between 300 and 1.500 meters of depth (Mann et al., 2000). Females usually inhabit offshore waters deeper than 1.000 meters, whereas males, may also approach the coast, to shallower waters related to the continental slope (Gannier et al., 2002; Azzellino et al., 2008 & Perrin et al., 2009). In any case, sperm whales are mainly found in areas of high primary production (Mann et al., 200; Gannier et al., 2002 & Gannier and Praca, 2007). In the Mediterranean Sea, sperm whales prefer deep continental waters close to the continental slope, although they can also be found in deeper offshore waters (Notarbartolo di Sciara and Birkun, 2010). Some authors suggest that the habitat used by the species is not only determined by characteristics such as the slope and its orientation, depth or primary production, but also by the activity of the animals (Pirotta et al., 2011). In this sense, feeding areas would be deeper than areas used to socialize.
The sperm whale is an opportunistic predator that feeds on the most common prey species in the different feeding areas (Evans and Hindell, 2004). It usually feeds on different species of cephalopods, although its diet may also contain other invertebrate or deep-fish species (Jefferson et al., 2008). Females mainly feed on cephalopods such as the giant squid (Architeuthis, spp.), the Humboldt squid (Dosidicus gigas), some species of the families Histioteuthidae, Ommastrephidae, Onychoteuthidae, Gonatidae, Pholidoteuthidae, Octopoteuthidae, and also demersal and mesopelagic fishes (Perrin et al., 2009). Males feed on similar species as females, but they tend to eat bigger individuals or species such as the colossal squid (Mesonychoteuthis hamiltoni) and more demersal fish species including sharks and rays (Perrin et al., 2009). In the Mediterranean Sea, sperm whales feed on similar species as in other regions of the world. 90 % of their diet is composed by cephalopods of the family Histioteuthidae (Histioteuthis bonnellii), and other families such as Ommastrephidae, Onychoteuthidae, Gonatidae, Pholidoteuthidae, Octopoteuthidae and Cranchiidae (Gannier and Praca 2007 & Praca and Gannier, 2008).
REPRODUCTION AND LIFE HISTORY
Sperm whale is a good example of a K-selected species. These species have low birth and death rates and long life spans, living up to 50-70 years. They have few calves, but they take very good care of them in order to increase their survival rate (Mann et al., 2002). Reproduction take place in spring. Females of the same population synchronize their estrous periods. Gestation lasts for 14 to 16 month and there sex ratio is 1:1 at birth. Weaning occurs at the age of 5 years, although calves start eating solid food when they are less than one year old. Although males reach sexual maturity in their teens, they don’t show a reproductive behaviour until their late twenties. They continue growing until their forties. Females reach sexual maturity at the age of 7 to 13 years. The mean duration of the calvin interval is 5 years. Females become physically mature at the age of 40, males at the age of 50 (Mann et al., 2002 & Perrin et al., 2009).
ECOLOGY, BEHAVIOUR AND IDENTIFICATION
The sperm whale has a very specific social structure formed by two different social units which are geographically isolated for most of the time: breeding schools and male bachelor schools (Gannier et al., 2002).
- Breding schools: They are formed by around 12 females and their offspring. These units are usually formed by related individuals. These groups may include two or more matrilines and some individuals may switch groups. Different units can travel together (Perrin et al., 2009). These units may confer some advantages to the species, such as the communal care of calves, protection against predators and collaboration in feeding (Mann et al., 2000). Survival of calves is of crucial importance for females. Calves cannot perform dives as deep as the ones their mothers do during feeding, so they remain at the surface with other females of the group (Connor et al., 1998 & Mann et al., 2000). This units remain in tropical waters close to the equator.
- Male bachelor schools: When males are between 4 and 21 years old, they leave the breeding unit where they have been living in order to join a male bachelor group (Perrin et al., 2009). These groups move towards the poles, although they come close to the Equator in winter. As males grow older, they reach higher latitudes during their migrations and congregate in smaller units. Older males travel alone. An underlying social structure would be responsible for maintaing closer the most related males (Mann et al., 2000). Males first return to the breeding areas to mate in their late twenties. Even though the time they remain in these areas is unknown, there has never been seen more than one male in the same breeding unit at the same time (Mann et al., 2000 & Perrin et al., 2009).
Although the largest migrations, between the Equator and the poles, are carried out by males, especially the older ones, females may also fulfill migratory movements up to 2.000 km.
In the Mediterranean Sea breeding units are smaller (between 3 and 7 individuals). They are distributed across the southern basin south of the 41st parallel north and their presence would be rather unusual in the northern areas. Males have a broader distribution across the hole basin, visiting both northern and southern areas (Gannier et al., 2002; Frantzis et al., 2003 & Frantzis et al., 2011).
Some recent studies show that long-term relationships between bachelor males are stablished around the island of Ischia, although it is not known if these relationships are stablished between related animals or if different males join in order to feed (Pace et al., 2014). Mediterranean population is believed to be isolated from other populations from outside the basin (Frantzis et al., 2011 & Notarbartolo Di Sciara, 2014).
Sperm whale has two main behaviours, which are social resting at or near the surface and foraging at depth. The first one occupies one quarter of sperm whale’s time, whereas the second behaviour occupies the remaining three quarters (Mann et al., 2000):
- Social resting at surface: These are low activity periods used to rest at surface or to socialize with other individuals. Different breeding units may join during social resting in order to establish social relationships. In this sense, a strong physical contact may be observed sometimes, when individuals use their jaws and flippers to interact with other animals, or they just swim together. Furthermore, there are some groups showing an intense aerial activity that includes movements such as fluke-ups, tail slaps, spyhops and even breaches. All these behaviours have been traditionally seen as socializing behaviours. Bachelor male groups also have social resting periods, although they show less activity than breeding groups (Mann et al., 2000).
- Foraging at depth: Sperm whales usually search their prey during deep dives ranging from 400 to 1.200 meters of depth and lasting for 40 to 50 minutes (although they can sometimes last for 2 hours). Periods between dives are usually not longer than 10 minutes (Mann et al., 2000; Watkins et al., 2002; Drouot et al., 2004; Miller et al., 2004 & Watwood et al., 2006). No correlation has been found between the duration of dives and the periods the animals spend at surface. At the end of a dive, they usually surface almost at the same place where the dive started. There is high acoustic activity during dives, which is thought to be related to echolocation and the detection of prey items. Two main sounds are produced during dives, clicks and creaks (Drouot et al., 2004; ). Clicks start during descent and they would mainly be used to detect prey items or as communication between individuals (Miller et al., 2004 & Watwood et al., 2006). On the other hand, creaks are only produced when individuals are at the deepest point of their dive, when they are capturing prey (Drouot et al., 2004 & Miller et al., 2004). The analysis of the sounds produced by sperm whales during foraging dives, helped researchers to find out that those dives usually occur in depths between 400 and 1.200 meters and clicks start when animals reach depths of 400 meters, usually 300 to 500 meters before the emission of creaks starts (Drouot et al., 2004 & Watwood et al., 2006)
There is only another cetacean species in the Mediterrenean Sea, that reaches the size of sperm whale, the fin whale (Balaenoptera physalus). Despite of this, they are easy to distinguish from each other thanks to their blows. The blow of the sperm whale, which is between 2 and 5 metres high, is directed left and forward, at an angle of 45° to the surface of water, whereas the blow of the fin whale is 4 to 6 metres high and column-shaped. When resting at surface, sperm whales float motionless with only a small part of the body out of the water. Unlike fin whales, sperm whales usually raise their flukes out of the water before starting a dive.
INDIVIDUAL IDENTIFICATION CHARACTERISTICS
Individual identification marks are useful in order to recognize the different individuals and its use can be applied to population or distribution studies, among others. Different photo-identification studies throughout the world mainly use one characteristic to distinguish the individuals (Whitehead, 1990; Matthews et al., 2001; Ciano, 2006 & Alessi et al., 2014):
Marks on the flukes: These are marks, mainly on the posterior margins of the dorsal flukes of the individuals, and they include notches, missing parts or holes, giving a characteristic outline for every fluke. Teeth-marks, usually seen as a white lines, may also be used for photo-identification. These marks are stable over time, what make them useful for long-term studies (Dufault and Whitehead, 1995).
Some studies also used photographs of the dorsal humps, which allow researchers to distinguish adult females (those individuals with a callus on the dorsal hump) (Whitehead, 1990). Other recent studies in the Mediterranean Sean also analyzed the number and distribution of marks on the dorsal hump and the flanks of the animals (Alessi et al., 2014). However, the main characteristic used for Photo-identification are the flukes.
CONSERVATION STATUS AND MAIN THREATS
The sperm whale is a species distributed throughout all the worlds oceans and this may be one of the reasons why whaling highly affected its populations. Some studies calculated its population to be around the million of individuals by the end of the nineteenth century. Whaling activities drastically reduced those numbers, thus the actual sperm whale’s population would be formed by around 300.000 individuals. The biggest population is found in the Pacific Ocean and is composed by 80.000 animals, followed by the Antartic Ocean’s population with around 12.000 individuals (Whitehead, 2002). Although, the population trend is unknown, some studies suggest that, after whaling on the species ceased, sperm whale’s population would be recovering (Whitehead, 2002).
The International Union for the Conservation of Nature (IUCN) listed the species under the status of vulnerable. The exact number of animals of the Mediterranean population, which is widely distributes throughout the basin, is unknown, although scientist think that only a few hundreds live there (Notarbartolo di Sciara and Birkun, 2010). Some studies carried out in the Ionian Sea estimated the local population in 68 individuals (Lewis et al., 2007). Other studies based on photo-identification techniques carried out, on one hand in the Ligurian sea during 18 years and the balearic islands (Rendell et al., 2014), and in the Hellenic Trench for 13 years (Frantzis et al., 2014), estimated the sperm whale’s population to be formed by 400 individuals in the first case and by 167 in the second. The authors of the first study conclude that they may have underestimated the size of the studied population, because the new animals discovery rate shows a continuous increase and no signs of stabilization, while the authors of the second study believe that they have already identified almost all animals in the area. If we extrapolate these results to the rest of the basin, the whole population would be composed by 2.500 individuals, 45% of which would be mature adults. It should be noted, that there are big areas which haven’t been thoroughly explored yet, such as the Tyrrhenian Sea which might be important for the species (Aïssi et al., 2014 & Rendell et al., 2014). For all these reasons, and bearing in mind that the species shows a decreasing population trend in that area, the IUCN listed the Mediterranean sperm whale’s population under the status of endangered. Some of the threats the species has to face are:
- Bycatch: The use of driftnets in the Mediterranean Sea expanded in the 80s. This is a quite simple but non-selective fishing method that consists in placing several floating nets for several hours. This nets can measure up to 50 km long and be 20 to 30 metres high. All animals that try to cross them, get trapped there (Bearzi, 2002; E.J.F., 2007 & Pace et al., 2008). Spanish, Italian, French and Moroccan fisheries have used this method for several years greatly affecting the cetacean populations of the Mediterranean Sea. Around 10.000 cetaceans where caught every year, at the moment this fishing method reached its maximum activity (Bearzi, 2002). Although this fishing method was banned by the European Union some years ago, it still is legal in countries that don’t belong to the Union (E.J.F., 2007). The most affected species are the striped dolphin and the short-beaked common dolphin, but sperm whales also get caught in these traps (Notarbartolo di Sciara, 1990 and 2014; Bearzi, 2002; Tudela et al., 2005; E.F.J., 2007 & Pace et al., 2008), probably due to the shape of its head (Tudela, 2004). A total of 229 sperm whales got caught in driftnets between 1973 and 2003. Although these numbers doesn’t seem to be high, they acquire a significant importance, bearing in mind that the Mediterranean population is small and that it have a growth rate of 0,86 % (Tudela, 2004; www.pbs.org, 2004; E.J.F., 2007; Pace et al., 2008 & Reeves et al., 2013).
- Collisions with vessels: The Meiterranean Sea is subjected to an intense maritime traffic which has been increasing since the mid eighties. About 220.000 vessels cross the basin every year. The most affected areas are the Strait of Gibraltar, and the northeastern area of the basin (De Stephanis and Urquiola, 2006 & Vaes and Druon, 2013). Great cetaceans such as the fin whale (Balaenoptera physalus) and the sperm whale are the most affected animals by this intense maritime traffic. Scientist estimated that between 1 and 2 great cetaceans die to a collision with a great vessel, although this numbers may be and underestimation (Panigada and Leaper, 2010; David et al., 2011 & Mayol, 2012). In this sense, 6% of the stranded individuals in Italy and Greece and 6% of the photo-identified individuals in Greece show marks caused by collisions with vessels (Notarbartolo di Sciara, 2014). The factor that mainly determines the death potential of every collision is the speed of the vessel. Speeds higher than 14 knots are mortal for cetaceans (Laist et al., 2001). Due to the small size of the Mediterranean sperm whale population, and to the fact that number of registered collisions probably is smaller than the number of real collisions, this turns out to be a very important threat for the conservation of de Mediterranean sperm whale population (Panigada and Leaper, 2010 & Mayol, 2012).
- Acoustic pollution: The underwater noise produced by human activity can be both caused unintentionally, as a consequence of other activities such as maritime traffic or offshore drilling, and intentionally as in the case of sonars or seismic surveys (Notarbartolo di Sciara and Gordon 1997).
The main sources of underwater noise in the Mediterranean are the intense maritime traffic, the seismic surveys, the offshore drilling activities, the coastal construction works and the underwater explosions (Notarbartolo di Sciara et al., 2012). The worst consequence of this variety of noise sources is that noise pollution is present all day long in the basin, and that there aren’t quiet areas in the Mediterranean anymore (Notarbartolo di Sciara et al., 2012). The exact effect that noise pollution causes to cetaceans is still unknown with certainty. Although different studies show contradictory results, some authors believe that underwater noise could negatively affect sperm whales (Gordon et al., 2003 & Notarbartolo di Sciara et al., 2012).
- Marine debris: The Mediterranean region is one of the most heavily populated and developed regions of the planet. A consequence of this development is that a great amount of waste, mainly plastics, ends up in the sea every year (Notarbartolo di Sciara et al., 2012). Sperm whales are prone to mistake floating plastics for their usual prey and may ingest them (Notarbartolo di Sciara et al., 2012 & De Stephanis et al., 2013). This may interrupt their digestion, decrease their appetite and, in the most extreme cases, decrease their growth rate or even cause them a painful death (Notarbartolo di Sciara et al., 2012). An good example is the case of a sperm whale calf found in the coast of Mikonos, Greece. The animal died as a consequence of the ingestion of a big amount of plastic bags, fishing net pieces, plastic strings, etc. (Notarbartolo di Sciara et al., 2012).
- Other threats affecting the Mediterranean sperm whale population include xenobiotic pollution, overfishing, habitat degradation and climate change (Panigada and Leaper, 2010 & Notarbartolo Di Sciara, 2014).
There are many international conventions protecting the striped dolphin, such as the appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the appendixes I and II of the Convention on Migratory Species (CMS) or the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic Area (ACCOBAMS). The fact that the marine sanctuary of Pelagos, located in the Ligurian Sea, is one of the most important areas for the sperm whale’s distribution in the Mediterranean, may contribute to it’s conservation in a positive way. Despite of this, there are many important habitats to the sperm whale’s distribution that aren’t protected. In addition, there are some factors such as the increase of acoustic pollution, due to the increase of the number of seismic surveys, that may affect the conservation of the species in the Mediterranean.
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