Sperm whale occurrence, site fidelity and population structure along the Hellenic Trench (Greece, Mediterranean Sea)

• Published in: Aquatic Conservation: Marine and Freshwater Ecosystems
• Main Authors: Frantzis, Alexandros, Alexiadou, Paraskevi, Gkikopoulou, Kalliopi Charitomeni
• Other Authors: Blue Planet Shipping Ltd., Hellenic General Secretariat for the Youth of the Greek Ministry of Education, OceanCare, Switzerland, Oracle Corporation, Vodafone Group Foundation, WIND Hellas
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Sperm whale
• Online Access: http://dx.doi.org/10.1002/aqc.2435; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Faqc.2435; https://onlinelibrary.wiley.com/doi/pdf/10.1002/aqc.2435

ABSTRACT Twelve summer surveys conducted each year between 1998 and 2009 along the Hellenic Trench have been analysed to provide information about a previously unknown sperm whale population unit. Sperm whales were detected acoustically 238 times; 178 of these led to a visual encounter with social units (96), male aggregations (45), solitary males (32) or unclassified groups (5). The overall detection rate was 10.9 detections per 1000 km of acoustic effort. A pronounced peak in sperm whale density was observed along the 1000 m depth contour: 74% of visual encounters (corrected for effort) were within 3 km of this contour. Density decreased both in shallower waters and deeper waters further offshore. One hundred and eighty‐one whales were photo‐identified. Fifty‐seven percent of 136 social unit members and 36% of 25 males segregated from social units were encountered in multiple years. Social units were resighted in up to six different years spanning 9 years (15 if opportunistic photo‐identifications are included). Several males were resighted in three different years usually spanning up to 4 years. This indicates a high level of site fidelity. The mean group size was 2.47 (range = 1–5) for males and 8.21 (range = 4–13) for social units. Maximum group size reached 15 whales when casual visitors were included. The 16 identified social units were generally stable although some individuals moved between social units, and some social units split or mixed. Female to male sex ratio was 1.55:1 within social units and 1.06:1 overall. Calves (≤2 years old) were present in 79% of social unit encounters, accounting for 16.6% of social unit members. Observations of 15 newborns indicate a mid‐summer calving season. This study indicates that the Hellenic Trench is core habitat for the eastern Mediterranean sperm whale sub‐population. This population, which is very small, is believed not to exceed a few hundred individuals. Given the endangered status of the entire Mediterranean population, managing threats in this area and ...