Hellenic Trench IMMA
56 568 km2
Qualifying Species and Criteria
Sperm whale – Physeter macrocephalus
Criterion A; B (1, 2); C (1, 2); D (1)
Cuvier’s beaked whale – Ziphius cavirostris
Criterion A; B (2); C (1, 2); D (1)
Marine Mammal Diversity
Criterion D (2)
Stenella coeruleoalba, Grampus griseus, Delphinus delphis, Tursiops truncatus, Monachus monachus
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The Hellenic Trench is a long bathymetric feature in the eastern Mediterranean consisting of a continuous steep continental seaward slope, often bounding offshore linear trenches, troughs and basins, which reach 5 km in depth. The area is the core habitat for the eastern basin distribution of the Endangered Mediterranean sperm whale (Physeter macrocephalus) subpopulation. This eastern Mediterranean distribution includes some 200-250 animals threatened by potentially unsustainable ship-strikes. Additionally, the Hellenic Trench features a sub-area which is the largest among five high-density areas of Mediterranean occurrence for Vulnerable Cuvier’s beaked whales (Ziphius cavirostris) that have suffered repeated mass stranding events in the area.
Description of Qualifying Criteria
Criterion A – Species or Population Vulnerability
The IUCN Red List classifies the Mediterranean subpopulation of sperm whale as Endangered and infers that their numbers are declining from significant anthropogenic threats. These whales are likely to be genetically isolated from the Atlantic population. Furthermore the subpopulation of Cuvier’s beaked whale in the Mediterranean Sea is listed as Vulnerable (VU) C2a(ii) IUCN Red List of Threatened Species, with a decreasing population trend (Cañadas and Notarbartolo di Sciara, 2018).
Criterion B: Distribution and Abundance
Sub-criterion B1: Small and Resident Populations
The area is a core habitat for sperm whales in the eastern Mediterranean, with most of the Eastern Mediterranean subpopulation being resident in or visiting the area. Although the eastern Mediterranean basin subpopulation is smaller than that in the western basin, the entire Mediterranean population is small, such that numbers of sperm whales using the area are a substantial portion of the whole Mediterranean population. While there are many photographic recaptures of individuals within the individual western and eastern Mediterranean basins there are only three cases of animals moving from the western basin to the eastern basin, suggesting that though not isolated the western and eastern subpopulations are predominantly separate.
Sub-criterion B2: Aggregations
Within the Hellenic Trench whales were observed to have a very pronounced peak in density over the 1,000m isobath on the slopes of the landward side of the trench. This combination of slope, aspect and depth appears to represent a significant zone for aggregating sperm whales in the eastern Mediterranean making the Hellenic Trench the core habitat for the subpopulation of the entire eastern basin. Such consistent and predictable aggregations can enable effective management actions to reduce the risk of ship strikes (such as through minor routing changes) which are recognised by the IMO as an effective way to address the ship strike issue.
Criterion C: Key Life Cycle Activities
Sub-criterion C1: Reproductive Areas
The area appears to be the core habitat for sperm whale in the eastern Mediterranean for calving and nursing. Calves (≤2 years old) were present in 79% of social unit encounters, accounting for 17% of social unit members within the area. Observations of 15 newborns indicate a mid-summer calving season which would imply a mating season from late winter to late spring. Though no observations of mating within the area have been reported, this mating season lies outside the normal fieldwork season, which may explain the lack of such observations so far.
Calves have been observed during sightings of Cuvier’s beaked whales and also have repeatedly stranded along the Hellenic Trench, including newborns bearing foetal folds, the most recent being on the Island of Rhodes in September 2016. During the last mass stranding of Cuvier’s beaked whales in southern Crete in April 2014, a fully formed foetus was found in a necropsied stranded female animal. These data show that the area constitutes a reproductive area for the species.
Sub-criterion C2: Feeding Areas
The area is an important feeding ground for the eastern Mediterranean sperm whale subpopulation. Solitary males, loose male aggregations and members of long-term resident social units of sperm whales were observed feeding in the area, with foraging and feeding activities being confirmed acoustically.
Cuvier’s beaked whales utilizing a portion of the area is important for their feeding activities. Cuvier’s beaked whales have similar same feeding habits as sperm whales and both species inhabit the area because of the presence of their prey. Long lasting deep feeding dives have been repeatedly observed along the Hellenic Trench and stomach contents of stranded animals contained primary meso- and bathy-pelagic species that are also found in sperm whale stomachs of stranded sperm whales in the very same area.
Criterion D: Special Attributes
Sub-criterion D1: Distinctiveness
The Mediterranean population of sperm whales are genetically and culturally different from their conspecifics in the Atlantic Ocean. In addition, at the local level of the eastern Mediterranean, social units are likely to be resident and may not migrate out of this basin. Distinctive communication sounds (codas) produced by males and social units in the Hellenic Trench have not been recorded in the western Mediterranean so to date. This is an indication that cultural evolutionary processes may have affected coda dialects differently in the eastern Mediterranean basin compared to the western Mediterranean, as the repertoires of codas are acquired by cultural transmission, a process that occurs at the level of core social units. The Mediterranean population of Cuvier’s beaked whales are also genetically different from conspecifics in the Atlantic Ocean.
Sub-criterion D2: Diversity
The area includes the regular presence of seven marine mammal species including Sperm whale, Cuvier’s beaked whale, Striped dolphin, Risso’s dolphin, Common dolphin, Common bottlenose dolphin, and Monk seal. It is possible that Rough toothed dolphins also use the area as they have been sighted in neighbouring areas with similar habitats. Therefore, the Hellenic Trench area is considered to have an important diversity Mediterranean marine mammals.
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