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Summer and winter surveys of deep waters of the Hellenic Trench, Greece, provide insights into the spatial and temporal distribution of odontocetes

Kirsten F. Thompson*, Thomas Webber, Leonidas Karantzas, Jonathan Gordon, Alexandros Frantzis

*Corresponding author:

ABSTRACT: The Mediterranean Sea provides habitat for globally threatened cetaceans. The Hellenic Trench is an Important Marine Mammal Area providing core habitat for sperm (Physeter macrocephalus) and Cuvier’s beaked whales (Ziphius cavirostris). Surveys have characterized distributions of these species in near shore areas (<2000 m deep). Sparse survey effort in deeper waters during winter has allowed speculation that it is not an important habitat in winter. We used passive acoustic monitoring from towed arrays to document cetaceans during summer/winter covering 18366 km of trackline off the Peloponnese and Crete. We confirm the acoustic presence of five odontocetes: sperm whales (49 encounters, mean depths (x̄) -3360 m, range (R) -1250 to -4210 m), Cuvier’s beaked whales (four encounters, x̄ -3070m, R -1970m to -3770 m), Risso’s dolphins (Grampus griseus) (five encounters, x̄ -3340m, R -2250 m to -4440 m), striped dolphins (Stenella coeruleoalba) (two encounters, x̄ -2980m, R -2490 m to -3470 m) and rough-toothed dolphins (Steno bredanensis) (one encounter, -3650 m). We also encountered 224 unidentified delphinids, including a potential blackfish (-3020 m). We confirm these species are present in the Hellenic Trench throughout the year. Given the impact of multiple threats on sperm and Cuvier’s beaked whales in the Hellenic Trench, we suggest that policymakers follow the precautionary approach in managing human activities in the area. Ongoing seismic surveys, hydrocarbon extraction and uncontrolled shipping could have potentially negative effects on these species at the population level and we advise caution in permitting such activities in future.