Results and Status, including trends (brief)
This assessment presents a brief overview of the key results and status of twelve species of marine mammals, one seal and 11 cetaceans, that are regularly present in the Mediterranean Sea and face several threats due to heavy anthropogenic pressures throughout the entire Mediterranean basin.
Results and Status, including trends (extended)
Mediterranean monk seal – Conservation efforts initiated over the past few decades seem to have at least partially stymied the population’s decline, as the current overall abundance of eastern Mediterranean subpopulation is said to be substantially higher than the 350 monk seals estimated in 2004 (Güçlüsoy et al. 2004) and 2010 (Aguilar and Lowry, 2010)Although there have been improvements in the methodologies used to study monk seals (e.g., the remote use of infrared photo cameras in caves), it is unlikely that the estimated increase in population size was substantially influenced by differences in methodology as the methods used to calculate abundance (although different by location) have been largely similar across time (e.g., Pires and Neves 2001, Pires et al. 2008 and Karamanlidis et al. 2009).. It is unclear when this recent small increases and signs of recovery began and if it will continue (Karamanlidis and Dendrinos, 2015).
Currently there are no population estimates for monk seals at the Mediterranean level; genetic analysis suggests that there may be two separate populations – genetically isolated – within the Basin, one in the Ionian Sea and one in the Aegean Sea. Previously listed as Critically Endangered by the IUCN Red List, the Mediterranean monk seal has been recently reassessed as Endangered, following an observed increase in individuals at localized breeding sites.
Fin whale – Comprehensive basin-wide estimates of density and abundance are lacking for all the species of cetaceans across the Mediterranean Region. Nonetheless, these parameters have been previously obtained for fin whales over large portions of the Central and Western Mediterranean Basin, highlighting seasonal, annual and geographical patterns. Line-transect surveys in 1991 yielded fin whale estimates in excess of 3,500 individuals over a large portion of the western Mediterranean (Forcada et al., 1996), where most of the basin’s fin whales are known to live. Panigada et al. (2011, 2017) reviewed the existing density and abundance estimates in the Central and Western parts of the Basin and reported on a series of aerial surveys conducted in the Pelagos Sanctuary and in the seas around Italy, providing evidence of declining numbers in density and abundance since the 1990’s surveys. These recent estimates provided values of 330 fin whales in July 2010 in the Pelagos Sanctuary area. Panigada and colleagues also reported on density and abundance estimates on a wider area, including the Pelagos Sanctuary, the Central Tyrrhenian Sea and portion of the sea west of Sardinia, with an estimated abundance of 665 fin whales in summer 2010. Laran et al. (2017) estimated approx. 460 (95% Confidence Interval 130-1 620) and 1,130 (95% CI 560-2 420) fin whales in the Pelagos sanctuary in winter 2011-2012 and summer 2012 respectively.
Sperm whale – There are no robust information on sperm whale population estimates for the entire Mediterranean Sea, while there are estimates obtained through photo-identification, line transect acoustic studies in localized specific areas. Given the values obtained in some Mediterranean areas (e.g. the Hellenic Trench, the Balearic islands, the Central Tyrrhenian Sea and the Ionian Sea), it has been suggested that the entire population may be around a few thousand animals, with possibly less than 2500 animals sexually mature and in a reproductive status (Notabartolo di Sciara et al., 2012). Laran et al. (2017) estimated approx. 560 (95% CI 120-2 650) and 370 (95% CI: 80-1 700) sperm whales in the North Western Mediterranean Sea in winter 2011-2012 and summer 2012 respectively.
Cuvier’s beaked whale – No density or abundance estimates for this species are available for the whole Mediterranean Sea. The only available robust sub-regional estimates come from line-transect surveys in the Alborán Sea and from photo-identification studies in the Ligurian Sea. The most recent corrected estimates number 429 individuals (CV=0.22) from the Alborán Sea and around 100 individuals (CV=0.10) in the Ligurian Sea (Podestà et al., 2016). The lack of other estimates throughout the whole Mediterranean Sea precludes any inference on the numerical consistency of the entire population.
Short-beaked common dolphin – Common dolphins used to be very common in the Mediterranean Sea, and during the 20th century the species was subject to a large decline, drastically reducing its population levels. No population abundance estimates are available for the Mediterranean Sea, apart from localized areas, such as for example the Gulf of Corinth and the Alborán Sea, thus making it difficult to assess the entire population.
Long-finned pilot whale – Two populations have been described in the Mediterranean Sea, one living in the Strait of Gibraltar and one in the area between the Alborán and the Ligurian Seas. The Gibraltar population has been estimated at less than 250 individuals, while there are few estimates for the other population, which seems to be declining (Verborgh et al., 2016). Laran et al. (2017) estimated approx. 300 (95% CI 90-950) and 650 (95% CI: 160-2 540) long-finned pilot whales in the North Western Mediterranean Sea in winter 2011-2012 and summer 2012 respectively.
Risso’s dolphin – There are no population estimates for Risso’s dolphin in the whole Mediterranean Sea, with information coming only from localized areas. Distance sampling was used to estimate winter and summer abundance of Risso’s dolphins in the north-western Mediterranean (N=2550 (95% CI: 849–7658) in winter and N=1783 (95% CI: 849–7658) in summer). Systematic photo-identification studies allowed to estimate, through mark-recapture methods, an average population of about 100 individuals (95% CI: 60–220) summering in the Ligurian Sea (Azzellino et al., 2016). Laran et al. (2017) estimated approx. 2 050 (95% CI 700-5 850) and 1 410 (95% CI: 550-3 740) Risso’s dolphins in the North Western Mediterranean Sea in winter 2011-2012 and summer 2012 respectively.
Killer whale – The most recent abundance estimate for this species is 39 individuals in 2011, representing one of the lowest levels compared to other killer whales population elsewhere in the world (Esteban et al., 2016).
Striped dolphin – Comprehensive basin-wide estimates of density and abundance are lacking for this species across the Mediterranean Region; nonetheless, ship and aerial surveys have provided abundance and density values for striped dolphins over large portions of the Central and Western Mediterranean Basin, highlighting seasonal, annual and geographical patterns. The overall higher density, and hence abundance, was observed in the North-Western Mediterranean Sea and estimated at 95,000 individuals (CV=0.11) (Panigada et al., 2017), with values clearly decreasing during the winter months and towards the Southern and Eastern sectors, reflects the general knowledge on the ecology of these species, described as the most abundant one in the Basin. Several estimates of abundance and density for this species have been provided for many areas of the Mediterranean, especially in the west. Laran et al. (2017) estimated approx. 57 300 (95% CI: 34 450-102 050) and 130 000 (95% CI: 76 750-222 100) striped dolphins in the North Western Mediterranean Sea in winter 2011-2012 and summer 2012 respectively. No baseline data are available for the whole basin however.
Rough-toothed dolphin – The very small number of authenticated records over the last 20 years (12 sightings and 11 strandings/bycatch) render any population estimate impossible and statistically unacceptable.
Common bottlenose dolphin – There are no density and abundance estimates for the entire Mediterranean Sea, with the only statistically robust estimates obtained from localized, regional research programmes in the Alborán Sea, the Balearic area, the Ligurian Sea, the Tunisian Plateau, the Northern Adriatic, Western Greece and Israel in the Levantine Basin. Laran et al. (2017) estimated approx. 13 410 (95% CI: 5 530-32 590) and 3 860 (95% CI: 1 040-15 020) common bottlenose dolphins in the North Western Mediterranean Sea in winter 2011-2012 and summer 2012 respectively.The IUCN assessment for the Mediterranean population implies that less than 10,000 common bottlenose dolphins are present in the Basin.
Harbour porpoise – This cetacean is not regularly present in the Mediterranean basin except in the Aegean Sea, where individuals from the Black Sea subspecies are occasionally observed and in the Alborán Sea, where individuals from the North Atlantic Ocean are rarely seen. No density and abundance estimates are available.