The striped dolphin (Stenella coeruleoalba) is a cetacean of the family delphinidae, that belongs to the suborder of the odontoceti or toothed whales. It is the most common cetacean in the Mediterranean Sea. It has a very slender body, but a little bit more robust than other species of the same genus. The dorsal fin is rather falcate. The beak is rather short and clearly differentiated from the melon. The pectoral fins are long and slim.

Its coloration pattern shows clear differences in a dorso-ventral way. The belly is whitish or light colored, while the dorsal part is dark grey. It has a light grey stripe that goes from the thoracic area to the dorsal ridge of the animal (dorsal part behind the dorsal fin). This stripe may be absent in some individuals. There is a dark stripe, mostly black, originating at the beak, encircling the eye and running back to the anus. There is another dark stripe that starts at the eye and widens to the pectoral fins. There usually is an accessory third dark stripe between the other two.

It has between 38 and 59 slender, pointed teeth in the upper jaw and between 37 and 55 in the lower jaw.

It’s maximum length is 2,56 metres, although the mean length varies from 1,8 to 2,7 metres. In the Mediterranean, the mean length increases progressively from north to south. The southern individuals are 3 cm longer than the northern ones. The biggest individuals are 5 to 8 cm shorter than the striped dolphins of the NE Atlantic. There are no size differences between the eastern and the western dolphins in the Mediterranean Sea. Striped dolphin also shows sexual dimorphism in the Mediterranean, where males are 2 cm larger than females. The size of the newborns varies from 90 to 100 cm (from 90 to 95 cm in the Mediterranean) and their weight is around 11 kg. The maximum weight ever registered for an adult was 156 kg.

DISTRIBUTION AND MIGRATORY PATTERNS WITHIN THE MEDITERRANEAN SEA

The striped dolphin usually lives in subtropical and warm waters all over the world. It is the most abundant and the best known cetacean in the Mediterranean Sea, where it spreads over almost the whole basin. However, its distribution isn’t even. In fact, we can divide the Mediterranean Sea into two major areas based on the density of striped dolphins. It is very common in the western region, extending from the Gibraltar Strait to the Italian Peninsula and Sicily. Its abundance in the eastern part, from the Italian Peninsula to the Turkish coasts, is much lower. No presence of striped dolphins has been recorded in the Black Sea. In the western region of the Mediterranean, where most of the research has been conducted, the species is more common in the northern part than in the southern. Different studies show that the striped dolphin is especially abundant in those areas with high primary production. In this sense there are two main regions for this species in the Mediterranean Sea. On one side, the Liguro-Provençal basin, which thanks to the upwelling of deep waters, is rich in nutrients and zooplankton, and is one of the most productive areas in the Mediterranean Sea. On the other side, the Alboran Sea, although the high productivity is explained by the inflow of nutrient rich Atlantic waters through the Gibraltar Strait. The high productivity in both regions favors the presence of small fishes and cephalopods, which are the main prey of the striped dolphin. There are two other areas where the species is not as common as in the first ones. These are the waters that surround the Balearic Islands, from the Iberian Peninsula to the northern coast of Africa. The low density may be explained by the absence of oceanographic processes that would favor primary production. Some authors believe that it might due to the most lethal outbreak that this species has suffered in the last decades -The 1990 to 1992 Morbillivirus outbreak-. This outbreak started and was more important in this zone. Recent studies (Gómez de Segura et al., 2006) suggest that the striped dolphin population that lives between the Balearic Islands and the Iberian Peninsula might be recovering, and that its density could be greater than at the beginning of the 1990s.

There are no recent estimates on the number of individuals that live in the Mediterranean Sea, but studies conducted in 1991 and 1992, just after the morbillivirus outbreak of 1990, estimated the western Mediterranean striped dolphin population to be 117,880 individuals -excluding the Tyrrenian Sea-. 14,000 individuals live in the Ligurian Sea (1), 30,800 in the Provençal Sea (2), 17,700 in the Alboran Sea (3), 5,800 in the Balearic Sea (4) and 18.800 in the waters between the Balearic Islands and the northern coast of Africa. More recent studies conducted in the same area, suggest that the population may be composed by a larger number of individuals. There are no specific numbers of the eastern Mediterranean striped dolphin population.

Although it is only an hypothesis and although better research on the populations is needed, some authors believe that the size variation between northern and southern individuals, may be due to the difference of seasonality in both regions, that could have led to a certain degree of genetic isolation between both areas.

HABITAT AND FEEDING

The striped dolphin is an opportunistic feeder and its diet is mainly composed by pelagic or bathypelagic fishes, squids and even certain sort of crustaceans, which form great shoals in the water column. The diet of the species is variable between regions, but it mostly feed on species that inhabit deep waters -from 200 to 700 metres- and that show a daily vertical migratory pattern. Most authors think it feeds in the evenings. Examinations of stomach contents suggest that the diet of the striped dolphin of the Mediterranean would be mainly composed by different squid species of the families Ommastrephidae, Histioteuthidae, Enoploteuthidae or Onychoteuthidae and different fish species of the families Gadidae, Sparidae and Gonostomatidae. Recent studies show that the diets composition may vary with age (Gómez-Campos et al., 2011). The younger and immature animals would feed on a wider variety of species, while adult animals would have a less varied diet. The same studies suggest that the diet of the striped dolphin would be mainly composed by small pelagic fishes, such as the european pilchard (Sardina pilchardus) or juvenile hake (Merluccius merluccius). The importance of cephalopods and crustaceans could be less important than previously thought. There are no important differences in diet between the western Mediterranean populations.

The striped dolphin is a pelagic species and its habitat is usually located in oceanic waters beyond the continental shelf, above all, those with more than 600 metres depth, although they can also be seen in shallower waters. It usually is more abundant in those areas that present deep water upwelling phenomena. These events lead to a higher primary production, making these zones optimal for feeding. Two examples are the Ligurian and Alboran seas, located in the western region of the Mediterranean Sea. Those are the two most productive areas of the basin and that’s why the striped dolphin’s abundance is higher there.

REPRODUCTION AND LIFE HISTORY

There are appreciable differences in reproduction and life history between the Mediterranean population of striped dolphins and the populations of the rest of the world. The mating system is a good example. In the north-western Pacific, there are two mating periods. One in winter and the other at the beginning of summer. In the Mediterranean there only is one maximum, usually at the beginning of autumn. Gestation lasts for 12 to 13 months and lactation for a year and a half. After lactation, females experience a resting period of 2 to 6 months till the next mating period. The total length of a reproductive cycle is around 3 years. Males become sexually mature between 7 and 15 years old, and females between 5 and 13 years old. Both sexes are socially mature by the age of 17 years. In the Mediterranean sea, males reach sexual maturity by the age of 11,3 years, and females by the age of 12. Physical maturity comes a bit later, between 13 and 18 years old in males and between 15 and 20 years old in females. Maximum estimated age is 57 years.

Sex ratio in adult individuals in the Mediterranean Sea is 1:1, that means, one male for every female. But sex ratio isn’t always the same, at birth males are more numerous than females, 1,4:1. Parity is achieved just after weaning.

ECOLOGY, BEHAVIOUR AND IDENTIFICATION

Striped dolphin herds usually number between 10 and 30 individuals, although there can occasionally be up to 500 individuals in a group. Their number and composition are different depending on the geographical area. For example, the social structure of the Japanese population is complicated and is structured in 3 different types of groups that vary depending on the age of their members. They can be divided in immature groups, mature groups and mixed groups. The first ones consists of pubertal dolphins and their mean size is 277 individuals. Mixed groups are composed by both mature and juvenile individuals and their mean size is 375 dolphins. Mature groups consist of sexual mature individuals, and can be divided in two subgroups (1) mating groups, composed by many sexually mature males and females, and (2) non-mating groups, with many non-mating females and a few social mature male. Their mean sizes vary between 225 and 764 individuals, respectively. After weaning, dolphins would remain in the mature group for 1,5 to 2 years. Afterwards, they would move into the immature group, where they would remain for one or two years after attaining sexual maturity, in the case of males, and 3 years before, in the case of females. At that time, they would move into the mature group.

In the Mediterranean Sea, groups are usually composed by less than 20 individuals, although herds of more than 50 individuals are not infrequent (Forcada et al., 1994). Calving would take place in groups consisting of more than 30 dolphins. Group composition and size where directly affected by the epizootic outbreak of 1990 – 1992.  Mean size of the herds decreased from 25,3 to 7 individuals. Since then, there has been seen a trend towards the increase of group mean size.

The striped dolphin is a really easy identifiable species at sea, thanks in part to its prominent and dark dorsal fin. In spite of this, the striped dolphin may be confused with the short-beaked common dolphin (Delphinus delphis), since they can form mixed groups, usually in the southern region of the Mediterranean basin. The coloration pattern is helpful to distinguish both species, above all the dark stripes that can be seen at both sides of the striped dolphin, and the grey blaze under the dorsal fin.

It is a very active species both in and out of the water. Breaches are a common behaviour, (including 7 metre high breaches) and it also performs spectacular pirouettes such as backwards somersaults and a complex movement known as roto-tailing. This movement consists in whipping the tail in a circle as the dolphin performs a high arcing leap out of the water. Some of these behaviours may be understood as part of the social communication. It is frequent to see them riding bow waves in the Mediterranean, where the mean velocity is 11 km/h. The maximum velocity ever recorded for this species is 60 km/h. Its dives can last for 5 to 10 minutes and achieve a minimum depth of 200 metres.

INDIVIDUAL IDENTIFICATION CHARACTERISTICS

Size, shape and coloration allow us to differentiate the different cetacean species. All species also show enough inter-individual variability in some morphological traits, that allow us to distinguish each member of a concrete population. The striped dolphin shows three morphological traits helpful to identify the different individuals.

Scars and marks: During their lifetime, cetaceans may get marks and scars due to interactions with other individuals of the same or of other species. These marks, above all the ones located on the most dorsal parts, are a good way to identify individuals of striped dolphins in the Mediterranean Sea in photo-ID studies.

Dorsal fin shape and scars: The size, shape and scars of the dorsal fin (above all those on the posterior margin) are unique for each dolphin and they allow the individual identification of the striped dolphins. The location, size and shape of the scars remain stable over time, so they can be used in photo-ID studies.

Coloration pattern: The coloration pattern of the striped dolphin shows a certain degree of variability between the different individuals. Although this criterion does not allow a complete distinction between two individuals, it may be used as a first selection in order to classify the animals in different groups in photo-ID studies.

Although these criterions are normally used in mark and recapture or photo-ID projects, it is difficult to conduct such studies in the case of the striped dolphin, for three reasons: (1) The big size of the population can make difficult matching individuals with photos of a possible catalog. (2) The dorsal fin marks and scars are not as obvious as they are in other toothed whale species. (3) Due to the two previous reasons it is very difficult to spot a previously sighted dolphin.

CONSERVATION STATUS AND MAIN THREATS

The striped dolphin is a widely distributed species, above all in tempered and warm waters regions. Different studies conducted during the last decades, allowed investigators to estimate the number of individuals living in each area. It is specially abundant in the Pacific Ocean, where the different populations show following numbers: 1,470,854 individuals in the east tropical Pacific, 570,000 individuals in the waters surrounding Japan, 19,000 individuals in the western coast of the United States of America and 13,143 in the Hawaiian Islands. In the Atlantic ocean, there live around 94,462 individuals along the eastern coast of the United States of America, around 3,325 individuals in the Gulf of Mexico and 74,000 individuals in the Bay of Biscay. The Mediterranean population has been estimated to be 117,880 individuals. The world populations counts a little bit more than 2,000,000 individuals and that is one of the reasons why the IUCN (International Union for Conservation of Nature) listed the striped dolphin under the status of no concern. Despite all these numbers, the population trend is unknown. In the Mediterranean Sea, the species is listed under the status vulnerable, because there have been -and still are- some threats that have reduced its population during the last decades. Some of these threats are:

• Water pollution: The Mediterranean Sea has been significantly affected by human activity developed on its coasts during the last decades. Both industrialized northern countries and agricultural southern countries have contributed to the water pollution by discharging chemical pollutants, organochlorines (DDTs and PBCs) and heavy metals (mercury and selenium) all over the Mediterranean basin. All these pollutants show higher concentrations than normal in the environment. Many of them accumulate in many organisms, reaching their highest concentrations in the species located at the top of the food web. This process is known as bioaccumulation. These chemical compounds accumulate in the blubber of the striped dolphins throughout their life, and females transmit them to their offspring. The levels of DDTs and PBCs detected in the Mediterranean striped dolphins are the highest levels of pollutants ever detected in a wild mammal. The effects of these compounds are uncertain, but they could be related to the development of cysts in the ovaries, that would end to an anomalous ovulation. They could also be related to physiological dysfunctions, affecting growth, reproduction and immune system. Some authors think that the high concentrations of PCBs and DDTs, that weaken the immune system or may impact the normal liver function, could have contributed to the 1990 – 1992 morbillivirus outbreak.
• Fishing interactions: The fishing industry of the Mediterranean sea is highly developed and its activity is really important in certain areas, causing an impact on the striped dolphin population. Non-selective fishing methods, such as driftnets or purse-seine fishing, are still used, and they are associated with bycatch and the death of many individuals. The most dangerous fishing gear are the ones used in the swordfish (Xiphias gladius) and the albacore (Thunnus alalunga) fisheries. There are three countries that carry (or carried) out these fishing methods, Italy, Spain and Morocco. The Spanish government made illegal driftnet fishing in 1991, but the fishing fleet used it until 1994. At that time the bycatch estimations varied from 145 to 183 individuals every year. The bycatch estimations for the Moroccan fleet vary from 1.500 to 2.000 individuals every year (Tudela et al., 2005), and for the Italian fleet, they were around 8.000 small cetaceans every year when driftnet fisheries reached their peak (Bearzi 2002). The nets used by Italian and Moroccan boats exceed the limitation of 2,5 kilometres established by the European Union, with longer nets measuring up to 14 km.
  There is another problem affecting the striped dolphin, the competition for resources. The main prey species of the Mediterranean dolphin is also of great commercial interest. In some regions, the European Pilchard (Sardina pilchardus) isn’t the main prey species of the striped dolphin anymore, due to the decrease of its population as a result of the intense fishing activity. In these regions, the european pilchard has been replaced by the hake (Merluccius merluccius) as the main prey species of the striped dolphin. The depletion of these fish species could affect the total number of striped dolphin individuals on a long-term basis.
  Another effect of the fishing industry on the striped dolphin is the direct catch, that is still carried out for human consumption or to be used as bait in some countries.
• Morbillivirus: An epizootic outbreak caused by a morbillivirus similar to the ones affecting other carnivore species, caused the worst die-off of the striped dolphin in the Mediterranean Sea from 1990 to 1992. Although its origin is unknown, different authors believe that the virus was very similar to another morbillivirus that affected the harbour porpoise (Phocoena phocoena) in the Irish Sea. The individuals affected, showed different infectious signs such as interstitial pneumonia, lymphoid depletion, necrosis of lymphocytes in lymph nodes and spleen, etc. The presence of pollutants in the striped dolphin has been considered to be a factor that could have helped the fast spread of the virus. Dolphins examined in 1990 had PCB concentrations three times higher than usual. These compounds are immune system depressors in mammals. Other factors such as the poor nutritional status, the presence of ectoparasites on the death bodies, suggest that dolphins could have been previously weakened. The causes for this prior weakening are still unknown, but some authors believe that it could be caused by the high temperatures of the winter of 1989 – 1990. These high temperatures would have caused a decrease on the productivity, making prey species less available.
  The outbreak lasted for three years, 1990, 1991 and 1992, affecting different regions of the Mediterranean every year. The first cases appeared on the coast of Valencia at the beginning of July 1990. The outbreak slowly spread to the south, to the north and to the Balearic Islands. The maximum die-off in Spain was reached between August and October. By the end of August the first dead dolphins arrived in France, where the maximum die-off was reached during October. At that moment the outbreak also arrived in Italy. At the beginning of November, the number of individuals that stranded dead along the coast decreased significantly. This first outbreak especially affected the waters between the coast of Valencia and Catalonia and the Balearic Islands. In 1991 the epizootic broke again on the southwestern coast of Italy, Sicily and Greece. In Italy it all started in June and lasted to the end of September. The maximum die-off was reached in august. There is not much data about the 1992 outbreak, but it only affected the waters surrounding Greece.
  The epizootic especially affected individuals from 11 to 20 years old, although the mortality of calves and newborns was also high. The cause of death of the latter was not the morbillivirus infection per se, but the deaths of their mothers. During the 1990 and 1991 outbreaks, 1,107 dead bodies were found on the Spanish, Italian and French coasts, but there are two reasons that make authors believe that the outbreak was much bigger. (1) The striped dolphin lives in oceanic waters. Due to its pelagic nature, only a small fraction of the dead individuals would have reached the coast. (2) Efficient response networks were only developed in Spain, France and Italy, but not in Africa or Greece. Because of these two reasons, many authors suggested that the real effect of the outbreak would have been much greater, causing the death of several thousands of individuals. 
  During the following decades, there has been two more morbillivirus outbreaks, although their effects were much smaller.  In 2008, 200 bodies stranded on the coasts of the Iberian Peninsula, the Ligurian Sea (Italy) and France. In 2011, a greater than usual number of strandings of striped dolphins on the coasts of Valencia, could have been caused by a morbillivirus infection.
• Other threats: Factors such as climate change could influence the distribution of the species, making its conservation more difficult.

There are many international conventions protecting the striped dolphin, such as the appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the appendix 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). Despite all these protection agreements, there is only one protected area for marine mammals in the Mediterranean Sea, located in the Ligurian Sea, which is not enough for the conservation of the species. Due to the oceanic nature of the striped dolphin, more Marine Protected Areas should be created in Spanish or French waters, in order to ensure the conservation of the species. Although direct catch is forbidden and the use of driftnets is regulated, there should be a stricter control, in order to avoid cetacean mortality directly caused by them. Another big concern is that the population increased its numbers after the epizootic outbreaks just at the same moment that its available habitat is decreasing, due to human activity. For all these reasons, the scientific community thinks that it is indispensable to keep getting information and data on the biology and demography of the population, the effects of the pollutants, the composition of diets and on the possible effects of the fishing industry on the striped dolphin, in order to establish better measures for the conservation of the species.

IMAGES

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