Stenella coeruleoalba

STRIPED DOLPHIN

Red List Category & Criteria: Vulnerable A2bcde

Geographic Range:

This species occurs in temperate and subtropical waters of all oceans. It is present throughout the Mediterranean Sea but has not been recorded in the Black Sea.

Population:

Population structure
Morphological and genetic studies strongly suggest that the Mediterranean and eastern North Atlantic populations are genetically differentiated, with little or no gene flow across the Straits of Gibraltar. Maximum body length of eastern North Atlantic striped dolphins is 5-8 cm longer than that of Mediterranean individuals (Calzada and Aguilar 1995). Skull size is also smaller in Mediterranean specimens than in their neighbouring Atlantic counterparts (Archer 1997). Mitochondrial DNA analysis, from Gibraltar to Greece, yielded 59 haplotypes (n=166), none of which was shared between the two areas, thus supporting strong differentiation; analyses of nuclear DNA support this result (Gaspari et al. 2007, Gaspari et al. in prep).

Subpopulation structure 
Genetic analyses of Striped Dolphins from Gibraltar to the coast of Israel, both at nuclear and mitochondrial levels, show that this species is genetically structured within the Mediterranean with low gene flow across the basin. Furthermore, there is evidence for sex-biased dispersal (Gaspari et al. in prep). On a smaller geographic level, Gaspari (2004) found evidence of genetic differentiation between inshore and offshore populations in the Ligurian Sea. Moreover, inside the Mediterranean there is some clinal variation in body size suggestive of population structure and/or restriction in gene flow between areas (Calzada and Aguilar 1995). This appears to be confirmed by significant differences in tissue pollutant levels between Spain and Italy (Monaci et al. 1998).

Population trend
the population declined in the early 1990s. Current population trend is unknown, although the population may have recovered to pre-1990 levels, at least in some areas (Gómez de Segura et al. 2006).

Habitat and Ecology:

The Striped Dolphin is an oceanic species. It shows a preference for highly productive, open waters beyond the continental shelf. 


Life History Parameters
Age at maturity:
Females: 12 years (western Mediterranean population) (Calzada et al. 1996)
Males: 11.3 years (western Mediterranean population) (Calzada 1996)


Longevity:
Females: 32 years (Di Mèglio and Romero-Álvarez 1996, Calzada et al. 1997)
Males: 28 years (Di Mèglio and Romero-Álvarez 1996, Calzada et al. 1997)

Percentage of living population that is reproductively mature:
Unknown

Average age of parents in the population:
Females: 22 years (Calzada et al. 1996, Calzada et al. 1997)
Males: 20 years (Calzada et al. 1996)

Gestation time:
Slightly over 12 months (Aguilar 1991)

Average interbirth interval:
2.7 years (in Japan) (Kasuya 1985)

Maximum potential annual rate of population increase:
0.09 (Schmitz and Lavigne 1984)

Major Threat(s):

In 1990–1992 a die-off devastated much of, if not the entire, Mediterranean population; 1,000 carcasses were examined in Spain, Italy and France alone, but the toll was undoubtedly much higher because these countries represent only a fraction of the Mediterranean coastline known to have been affected by the process (Bortolotto et al. 1992, Aguilar and Raga 1993). Immediately after the event, the mean school size was found to be less than one third of original levels, which may be taken as an indication, but not as a proof, for a proportional reduction in overall population size (Forcada et al. 1994). The primary cause of the die-off was a morbillivirus infection (Domingo et al. 1990, Van Bressem et al. 1993). The epidemic started in regions containing unusually large numbers of inbred dolphins that were possibly more susceptible to diseases (Valsecchi et al. 2004). PCBs and other organochlorine pollutants with potential for causing immunosuppressive effects were suggested to have enhanced its lethality because the individuals that succumbed to the disease were those carrying the highest PCB tissue concentrations (Aguilar and Borrell 1994). 

A second outbreak occurred in 2006–2007 affecting the coasts of Spain (Fernández et al. 2008, Raga et al. 2008), France and the Italian Ligurian Sea (Garibaldi et al. 2008). The mortality associated with that event was moderate, with only about 200 carcasses reported, probably because older dolphins were still protected by the immunity developed during the 1990–1992 epidemic (Raga et al. 2008). Moreover, differently to the previous outbreak, PCB levels in the individuals that succumbed to this event were not significantly different from those in the surviving population, which suggests that pollutants were of no relevance to the 2006–2007 event (Castrillon et al. in press). Recurrent epidemics may have profound cumulative effects on the population dynamics of Mediterranean Striped Dolphins (Van Bressem et al. 2009a).

Poxviruses with the potential to cause mortality of neonates and young calves are also circulating in this population (Van Bressem et al. 2009b). Marine brucellae also infect Mediterranean Striped Dolphins (Van Bressem et al. 2001). These bacteria may limit recruitment by compromising the normal functioning of male and female reproductive systems, inducing abortions and killing neonates and sexually mature individuals (Van Bressem et al. 2009a). Finally, infection by the protozoan Toxoplasma gondii seems to be common in Mediterranean striped dolphins (Domingo et al. 1992, Cabezon et al. 2004) and has been suggested as another factor contributing to the death toll during morbillivirus epidemics.

Tissue levels of organochlorine compounds, some heavy metals, selenium and possibly other pollutants such as the brominated flame retardants, polycyclic aromatic hydrocarbons and perfluorinated compounds are high and often exceed threshold levels above which detrimental effects commonly appear in mammals (Aguilar 2000, Marsili et al. 2001, Kannan et al. 2002, Pettersson et al. 2004). Blubber concentrations of DDT and PCB, the two main organochlorine pollutants, show a slowly declining trend in the last two decades (Aguilar and Borrell 2005) but are currently still high. Besides the alleged immunodepressive effect (see above), high PCB levels may have been implicated in the development of unusual luteinized cysts in the ovaries of four of 56 (7.1%) Mediterranean Striped Dolphins (Munson et al. 1988), although other factors like the morbillivirus infection, stress and food depletion could have contributed to the abnormal surge or release of luteinizing hormone. 

The Spanish driftnet fishery in the Alboran Sea reportedly killed 148–170 dolphins per season in the early 1990s (Silvani et al. 1999); this fishery was halted in 1995 but the nets were transferred to Moroccan boats, which continue operating and are estimated to kill in the order of 1,500–2,000 Striped Dolphins per year (Tudela et al. 2003). The Italian driftnet fishery (spadare fishery) has been claimed to kill 5,000–15,000 dolphins, mostly Striped Dolphins, per year (Di Natale 1992); although fishing effort is declining, current catch levels are thought to be still high. The French thonaille driftnet fishery has been estimated to kill 180–472 Striped Dolphins per season (Imbert et al. 2001). Reports from other fisheries are sparse and bycatch data are not collected systematically, but what evidence there is suggests that incidental catches of Striped Dolphins are widespread and likely to represent a significant toll at least in pelagic purse-seines, drifting long-lines and gill nets (Di Natale and Notarbartolo di Sciara 1994). To this should be added a certain number of direct catches for human consumption or for use as bait, which still continue in several Mediterranean countries (SGFEN 2001). 

The diet of Striped Dolphins includes commercial fish and cephalopod species (Pulcini et al. 1992, Blanco et al. 1995), so the widespread depletion of fishery resources in the Mediterranean has the potential to affect Striped Dolphin numbers.

Global warming may have significant direct and indirect effects on cetacean populations in the Mediterranean (Gambaiani et al. 2009). Climate variability and change affect biomass in a number of ways, including shifts in species distribution. Azzellino et al. (2008b) showed a direct effect of sea surface temperature on striped dolphin distribution in the Ligurian Sea.

Conservation Actions:

Control of pollution, particularly that by organochlorine compounds, has become more effective in the last two decades and the levels of those pollutants are decreasing. However, existing laws and control should be further enforced and Striped Dolphin populations should be monitored to assess trends and geographical variation of known pollutants in tissue levels. 

The population size was estimated in the western Mediterranean immediately after the 1990 die-off. Abundance should be monitored, particularly to assess recovery from the die-off(s). 

Occurrence of ovarian cysts should be monitored in the population and their potential impact on reproduction should be investigated. 

Diet should be studied through stomach content and isotopic analyses to assess overlap with commercial fisheries. 

Stranded dolphins should be examined for viral, bacterial and Toxoplasma gondii infection and for the presence of antibodies against these agents.

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