Short-beaked common dolphin
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SHORT-BEAKED COMMON DOLPHIN Scientific name científic: Delphinus delphis |
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Distribution within the Mediterranean:
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Taxonomy: PHYLUM: Chordata |
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Conservation status in the Mediterranean: Endangered (UICN) |
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Conservation status in the world: Least Concern (UICN) |
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Mean length:
Between 1,7 and 2,7 meters in adults |
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Habitat:
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Diet:
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Identification characteristics:
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Can be confused with:
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Diving pattern: |
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Individual identification characteristics:
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Main threats:
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Until 1994 scientists only recognized one species of common dolphin, but later it was realized that there are at least two species in the genus: the short-beaked common dolphin (Delphinus delphis), that can be found in the Mediterranean Sea and the long-beaked common dolphin (Delphinus capensis). Because of the high morphological variability of this genus, some authors suggested the existence of an endemic subspecies in the Black Sea (Delphinus delphis ponticus). Its taxonomic status hasn’t been confirmed yet.
The short-beaked common dolphin is a medium-sized dolphin, with a slender body and a rather falcate dorsal fin with a pointed tip. Despite its name, it has a moderately long beak, clearly differentiated from the melon. Pectoral fins are slender and pointed.
The coloration pattern is the main distinctive feature of this species. The dorsal and lateral parts of the body are dark, the latter being V-shaped just below the dorsal fin. The belly is white or light colored. It has an hourglass pattern on both sides of the body. The thoracic part is yellowish or ochre and the rear part is greyish.
It has between 80 and 120 small, sharp and pointed teeth in each jaw.
Adult short-beaked common dolphins are about 1,7 and 2,7 meters. At birth it measures about 0,7 and 1 meters. It shows a certain degree of sexual dimorphism with males being slightly longer (5%) than females, not only in body length but also in other body parts such as the cranium. Adult males have a postanal hump (Neumann et al., 2002) that is thought to be related to testes size and to help females choose the healthiest male from a reproductive point of view (Murphy & Rogan, 2006). That would mean that males with biggest humps would have bigger testes and therefore a larger sperm production capacity.
DISTRIBUTION AND MIGRATORY PATTERNS WITHIN THE MEDITERRANEAN SEA
The short-beaked common dolphin is mainly distributed in tropical and temperate waters of the Pacific and Atlantic oceans. It can inhabit shallow and pelagic waters, but it seems to prefer upwelling zones and areas with a steep seabed.
The species inhabits both Mediterranean and Black Seas, where some authors suggest that it could be a different subspecies. Although there are no population density estimates for the whole basin, it is known that its distribution is not homogeneous and that certain areas show a higher density. It is more abundant in the western part of the Mediterranean Sea, especially in the Alboran Sea, a region located between the southern coast of the Iberian peninsula and the northern coast of Africa. The population in this area was estimated to have around 14.736 individuals in 1991 and 1992. It is regularly seen in the Thracian Sea, Aegean Sea and in the westernmost part of the Ionian Sea. Isolated groups can be sighted off the northern coast of Africa up to Tunisia and off the western coast of Corsica and Sardinia. It has also been spotted in the Tyrrhenian and Adriatic Seas and in the Sicily channel. The different areas where the short-beaked common dolphin is more abundant can be seen on the map below.
Some studies on genetic markers have shown that the population inhabiting the Alboran Sea is genetically more related with the adjacent Atlantic populations than with the populations of the eastern part of the Mediterranean Sea (Natoli, 2004). The same studies suggest that genetic flow would be more frequent between the Atlantic populations and the population of the Alboran Sea, than between the latter and the populations of the eastern Mediterranean Sea. This genetic differentiation between the Mediterranean populations could be a consequence of a different distribution, preferring different habitat types.
HABITAT AND FEEDING
Although the habitat of this species has not been studied as deeply as in other cetacean species, it is known to vary depending on the population in the Mediterranean Sea. In this sense, the western population, especially those living in the Alboran Sea, inhabit neritic waters associated with the continental shelf and slope but, and also oceanic waters. They usually form mixed groups with other cetacean species. They can be found swimming with striped dolphins (Stenella coeruleoalba) in the pelagic zone and with common bottlenose dolphins (Tursiops truncatus) in the neritic zone. The eastern populations of the Mediterranean Sea inhabit neritic waters, closer to the coast. They inhabit areas with a water depth of up to 400 meters, showing a preference for waters deeper than 200 meters. They are not frequently spotted in coastal waters. In the southwestern Mediterranean, groups with lactating individuals are sighted in areas close to the continental slope and groups without calves can be found visiting deeper waters more frequently (Cañadas & Hammond, 2008).
The diet of the short-beaked common dolphin varies depending on the area, because of the different habitats in which they are found in the different regions of the world. Feeding generally occurs in shallow waters, but foraging dives to 200 meters have been recorded (Bearzi et al., 2003 & Perrin et al., 2009). They show a preference for epipelagic and mesopelagic fish prey and different squid species. Their diet in the Mediterranean is mainly composed of small sized, shoaling fish species such as the European anchovy (Engraulis encrasicolus), European pilchard (Sardina pilchardus), the round sardinella (Sardinella aurita) and the garpike (Belone belone), but also by some cephalopod species and sporadically some crustaceans.
Although having a great dietary diversity, some studies showed that the short-beaked common dolphin isn’t an opportunistic feeder. It focuses its efforts on catching high energy density prey species to meet the high energy requirements of its metabolism and its energetically expensive life style.
REPRODUCTION AND LIFE HISTORY
As in other cetacean species, there are many differences in reproduction and life history between the populations of short-beaked common dolphin from the different regions of the world. Its maximum estimated age ranges between 25 and 30 years. Gestation lasts for 10 to 12 months. Length at birth varies from 82 to 104 cm. The calving interval varies from 1 to 3 years depending on the region. Weaning occurs at 6 months in the individuals of the Black Sea and later in other populations. Body length at the moment sexual maturity is attained varies between 150 and 200 centimeters, at the age of 3 to 12 years for males and 2 to 7 years for females. Reproductive seasonality varies depending on the region. In some regions this occurs between spring and summer, in others between summer and autumn and in others throughout the whole year. It appears that populations inhabiting colder waters show a greater synchrony in reproduction. The reproduction parameters of the different population are shown in more detail in the table below. There is not much information about the Mediterranean population of the short-beaked common dolphin concerning reproduction and life history.
| Region | L.A.B. (cm) |
L.S.M. (cm) |
A.S.M. (years) |
Lactation (months) |
C.I. (years) |
M.L. (cm) |
Reproduction season | Maximum Age (years) |
| Mediterranean | – | – | – | – | – | ♀ 217 – 222 ♂ 220 – 222 |
– | – |
| Black Sea | 80-90 | ♀ 150 – 170 ♂ 170 – 180 |
♀ 2 – 4 ♂ 3 |
19 | 1,3 – 2,3 | ♀ 178 – 200 ♂ 191 – 219 |
– | 22 |
| N Pacific | 82 | 170,7 | 8 | 16,5 | – | ♀ 199 ♂ 211 |
May – June | 27 |
| E. Trop. Pacific | 87 | 186,5 | 7,9 | 19 | 2 | ♀ 200 – 213 ♂ 221 – 235 |
Whole year | 25 |
| NW Atlantic | 92,7 | 200 | 8,3 | 16,5 | 2 | ♀ 215 ♂ 236 |
July | 30 |
| NE Atlantic | 104,1 | 200 | 9 – 13 | 16,5 | 3,5 | ♀ 222 – 230 ♂ 231 – 250 |
– | 25 |
| L.A.B. Length At Birth – L.S.M. Length at Sexual Maturity – A.S.M. Age at Sexual Maturity – C.I. Calving Interval – M.L. Maximum Length | ||||||||
ECOLOGY, BEHAVIOUR AND IDENTIFICATION
The short-beaked common dolphin forms medium-sized groups, although large, boisterous aggregations containing hundreds or even thousands of individuals can be seen. These large groups would be composed by smaller subunits of 20 to 30 more closely related individuals. The mean size of groups in the Mediterranean Sea varies depending on the region. The smallest composed of somewhat more than 10 individuals and the largest of around 600 animals. Some studies show that in the southwestern Mediterranean Sea, the biggest aggregations occur close to the continental slope (Cañadas & Hammond, 2008).
This species often interacts with other species of cetaceans and they sometimes travel together. Interactions with individuals of the genus Globicephala and Lagenorhynchus are usual. Mixed groups with Risso’s dolphin (Grampus griseus), sporadic groups with the common bottlenose dolphin (Tursiops truncatus) and more usually with the striped dolphin (Stenella coeruleoalba) have been sighted in the Mediterranean Sea. Associations with the latter species especially occurs in those areas where both species share habitats. Interactions with the other species are more frequent in the zones where the density of short-beaked common dolphin has been reduced.
It is quite easy to recognize the species at sea, because it is a medium-sized chunky dolphin with a large and very light thoracic frontal part. The light coloration and the fact that it is easily seen above the water surface, make it easier to be distinguished from other species of cetaceans such as the striped dolphin (Stenella coeruleoalba). Dorsal fin is triangular, falcate and with a pointed tip. Its coloration varies from totally black to very light in the center. The hourglass pattern on both sides of the body also makes its identification easier.
The short-beaked common dolphin is a fast swimmer and very acrobatic aerially. It is usual to see it breach out of the water or slap the surface with its flippers or its flukes and even fulfilling spectacular somersaults. A very characteristic behaviour, known as pitch poling, consists in breaching out of the water and landing on one of the sides of its body. It is an active and energetic bowrider. Maximum dives last 8 minutes, although normal dives last from 10 seconds to 2 minutes.
INDIVIDUAL IDENTIFICATION CHARACTERISTICS
Besides all typical characteristics of the species, each individual shows specific traits that allow for their individual identification. There are two main characteristics used by many research groups, that can be used to identify the individuals of the short-beaked common dolphin.
- Shape and coloration of the dorsal fin: The coloration pattern of the dorsal fin varies significantly from one individual to the other. It ranges from totally black to mostly white especially in its central part.
- Notches in the posterior margin of the dorsal fin: Shape and location of the notches of the posterior margin of the dorsal fin is a useful characteristic in individual identification for many dolphin species. Although the short-beaked common dolphin individuals don’t have as many notches as the individuals of other species, the shape and position of the notches are a good way to identify the different animals.
- Other mechanisms: In addition to the last two, there are other characteristics that can make individual identification easier, such as the presence of deformities like humps o abnormal fins, or an unusual body pigmentation.
CONSERVATION STATUS AND MAIN THREATS
The IUCN (International Union for Conservation of Nature) listed the short-beaked common dolphin under the status of least concern. It is considered to be a very abundant species that has large populations in almost all the areas that have been studied. In the eastern Pacific the population is estimated to have 2,963,000 individuals and there are around 352,000 animals off the western coast of the United States. In the Atlantic ocean there are not as many individuals as in the Pacific, but the different populations are still large: around 63,400 individuals in the waters off the European continental shelf, about 273,000 in oceanic waters, 61,000 in the western part of the Bay of Biscay and 121,000 in the north-western Atlantic. Recent studies in the Black Sea have estimated the population to be composed of several thousand, or even 100,000 or more individuals.
The short-beaked common dolphin of the Mediterranean is a good example of how environmental factors and human activity can result in the alteration of the density and distribution of that species. Until the 1960s the short-beaked common dolphin was one of the most abundant cetaceans in the Mediterranean and it was very common in many areas where it is rare nowadays. There is no recent information about the size of the population in the Mediterranean. The last studies conducted during the years 1991 and 1992, suggested that there were 14.736 individuals in the population of the Alboran Sea -in the southwestern Mediterranean Sea. Bearing in mind all these aspects and knowing that the population is decreasing, the IUCN has listed the short-beaked common dolphin of the Mediterranean as an endangered species. The following are some of the causes that could explain the decrease of the population:
- Competition with other species: Although there is no scientific evidence, some authors believe that the increase of the population of striped dolphin (Stenella coeruleoalba) during the second half of the 20th century, could have contributed to the decrease of the population of the short-beaked common dolphin. Although the habitats of both species overlap in some areas of their distributions, their diet differs, sharing only a few prey species. Other authors believe that the decrease of the population could have been caused by other phenomena that could create great changes in marine communities, ultimately affecting the distribution of the cetaceans of the Mediterranean Sea.
- Environmental changes: Although they probably aren’t the best explanation for the decline of the short-beaked common dolphin population (Bearzi et al., 2003), their effects, together with other problems detailed below, have contributed to the decrease of the number of individuals of the species in the Mediterranean Sea. Environmental changes -caused by anthropogenic activities or natural causes- don’t directly affect the different species of cetaceans, but they do so in an indirect way. The first causes changes in density and distribution of their prey species (Selzer et al., 1988 & Bearzi et al., 2003) which would later affect the distribution of cetaceans.
- Pollution: The Mediterranean Sea is a rather closed sea and its coastline is one of the most urbanized and altered by human action. In addition, industrial and agricultural activities are very important and they affect the species that inhabit the basin, for example, the concentrations of pollutants in cetaceans is much higher than expected. These compounds accumulate in the organisms due to a process known as bio-accumulation, which consists in the accumulation of chemical substances in a higher concentration than they are found in the surrounding environment. The compounds are firstly assimilated by filter-feeder microorganisms and then are transmitted to higher trophic levels due to predation. As they do so, their concentrations are always higher. This means that the organisms at the top of the food web show the higher concentration of pollutants.
The short-beaked common dolphin of the Mediterranean sea shows a concentrations of pollutants -above all DDTs- much higher that its conspecifics of other areas (Borrell et al., 2001), although the concentration of DDTs and other pollutants is lower than in other cetaceans species of the Mediterranean Sea (Fossi et al., 2003). These chemical compounds have adverse effects on the cetaceans such as growth disorders, depression of the immune system, reproductive impairment, etc, that could end up being a stress factor for the populations (Borrell et al., 2001). Some authors also believe that pollutants could have contributed to the widespread of some epizootic outbreaks in cetaceans, such as the Morbillivirus outbreak of the Striped dolphin (Stenella coeruleoalba) in the Mediterranean (Aguilar & Raga, 1993) or the short-beaked common dolphin in the Black Sea (Birkun et al., 1999).
There ares ome areas like the Alboran Sea, which is the Mediterranean region with a higher density of short-beaked common dolphins, that aren’t as polluted as other areas. The exact effect of the pollutants in the decline of the Mediterranean population of short-beaked common dolphin is still unknown, but comparison of the toxicological information obtained in each region should be helpful to clarify this aspect. - Direct catch and bycatch: Until the 1950s the short-beaked common dolphin was deliberately killed in several areas of the Mediterranean and this still occurs in a few places. Its flesh was valued for human consumption or used as bait. The species was seen as a direct competitor with the fisheries of the different areas. Some research methods also ended up with the death of the animal.
Bycatch was and still is a very important threat to the conservation of many species of the world. The use of driftnets in swordfish fisheries (Xiphias gladius) and some species of tuna in the Mediterranean Sea, is often associated with the bycatch of many fish species (sunfish (Mola mola), some species of sharks, etc), marine turtles and even cetaceans, above all the striped dolphin (Stenella coeruleoalba) and the short-beaked common dolphin. In the latter case, although bycatch is probably not the only cause of the species decline, it could play a very important role. The impact of the driftnet fisheries on the species is very important, because it especially affects extremely young calves (Silvani et al., 1999). Some studies estimate that Spanish driftnet fisheries caught 183 short-beaked dolphins in 1993 and 145 in 1994. These numbers represent about 1% of the population. The Spanish government forbade the use of driftnets in 1991, although they where illegally used until 1994. Some authors estimate that when driftnet fisheries reached their peak, there were over 8,000 cetaceans caught only in the Italian seas and 10,000 in the whole Mediterranean basin (Bearzi, 2002). Although this fishing method is forbidden in the Mediterranean Sea, it is still used in some countries. It is estimated that the Moroccan fleet catches around 1,800 individuals every year, reaching the same levels as those reached at the beginning of the 1990s (Tudela et al., 2005). The International Whaling Commission advises that an anthropogenic removal greater than 2% of the best available population estimate may cause the decline of the population and that different measures should be taken in order to avoid the bycatch. The bycatch of Short-beaked common dolphin in the Alboran Sea conducted by the Moroccan fleet is estimated do be slightly higher than 10% of the population (Tudela et al., 2005 & EJF, 2007). The exact direct effects of driftnet fisheries are still unknown, but it is believed that bycatch could be unsustainable for many marine mammal populations of several regions. - Overfishing: As in many other areas of the world, the fish stocks in the Mediterranean Sea have been decreasing in the last decades. The impact is much bigger on the species of small pelagic fishes such as the European pilchard (Sardina pilchardus) or the European anchovy (Engraulis encrasicolus), which are caught for human consumption or to be used as fish meal to farm higher-trophic level species (Lleonart 2005 & Piroddi et al., 2011). Many of these species have a key ecological role in the Mediterranean Sea (Coll et al., 2012) and they are an important part of the short-beaked common dolphin’s diet (Bearzi et al., 2003). The overfishing isn’t thought to be the only reason of the decline of the short-beaked common dolphin population, but its contribution has been significantly important, and in some areas it is the main cause of the decreasing trend of the population (Bearzi et al, 2008 & Piroddi et al,. 2011). Overfishing is an issue of concern in all the areas of the Mediterranean Sea where short-beaked common dolphin has been studied.
There are several international conventions protecting the short-beaked common dolphin, amongst them the appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the appendices 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). There is only one big Marine Protected Area in the Mediterranean Sea, the Ligurian Sea, located between the northern coast of Corsica and Italy. This region is far away from the short-beaked common dolphins main population distribution. In the final months of 2013 the Spanish government approved the creation of 4 new marine Sites of Community Interest (SCI) -2 of them in the Mediterranean Sea- which should ensure the conservation of the habitats and the species of these area. 6 more areas will be created before the end of the same year. The creation of these sites is part of the LIFE INDEMARES project and is the result of a longterm research and the collaboration between different organizations, which started in 2002. Although similar initiatives have been started in other regions of the Mediterranean Sea, they may not be enough to ensure the conservation of the species, and other measures should be taken in order to prevent a future decline of the short-beaked common dolphin population. Among others, these measures should include the reduction of the overfishing and of habitat degradation (Bearzi et al., 2003).
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