Site fidelity, residency and habitat use of the Risso's dolphin Grampus griseus in the Gulf of Taranto (Northern Ionian Sea, Central-eastern Mediterranean Sea) by photo-identification

Abstract

A photo-identification tool was applied to identify and create a catalogue of photo from Risso's dolphins providing evidence of its site fidelity, residency and habitat use in the Gulf of Taranto (Northern Ionian Sea, Central-eastern Mediterranean Sea). Sightings carried out from 2013 to 2016, occurred in a depth range from 436 to 1000 m with a mean depth value of 762 +/- 217 m. A total of 215 specimens of G. griseus were observed including 206 adults and juveniles, 7 calves and 2 new-borns. The evidence of calving and nursering females was inferred from calf association without any other sex information. Group size varied between 2 and 30 dolphin specimens, with a mean value of 13 +/- 7 specimens. A total of 54 Risso's dolphins were identified as distinctly marked. Among these, 34 specimens (63%) were sighted only once and 20 specimens (37%) were re-sighted up to 4 times. Rates of re-sighting within single year were relatively high, reaching 23, 18 and 43% during 2013 (3 re-sighted/13 dolphins), 2014 (4 re-sighted/22 dolphins) and 2016 (12 re-sighted/28 dolphins), respectively. The site fidelity reached 0.50, with a mean value of 0.13 0.18. The monthly residency rate varied between 0.11 and 0.44, with a mean value of 0.17 0.09. Among a total of 9 Risso's dolphins considered as "multi-year residents", 7 specimens were re-sighted in two consecutive years and 2 were re-sighted in a period of two non-consecutive years. Although results suggest the need for further studies implementing the photo-identification catalogue for the Risso's dolphin and allowing possible reconstruction of its migration patterns on the Mediterranean and global scale, the Taranto Valley canyon system seems to be a critical habitat to enforce specific conservation measures for the effective protection of the species.