Aspects of the distribution of Cuvier's beaked whale (Ziphius cavirostris) in relation to topographic features in the Pelagos Sanctuary (north-western Mediterranean Sea)

Abstract

Cuvier's beaked whale is a poorly-known species. It has been considered common since 1980 in the Pelagos Sanctuary (north-western Mediterranean Sea), but it has hardly been studied, chiefly due to difficulties in sighting. Stranding data indicates that the beaked whale is present all along the Ligurian coast. As with any deep-diving odontocete, Cuvier's beaked whale feeds mostly on deep-sea squid, but also on some fish and a small number of crustaceans. As a consequence, it is thought to be found mainly in waters deeper than 1000 m, where the sea bed has a particular slope. The aim of this work is to analyse a large quantity of sightings in order to define the favoured habitat of the beaked whale. Topographic features such as depth, depth gradient and bathymetric anomaly were analysed due to their direct influence on the prey of Cuvier's beaked whales. Data were registered between Genova and Imperia, from 2000 to 2006. Two hundred and forty-seven sightings were recorded, a total of 532 whales. The mean herd size observed was 2.3±1.5 (range=1–11). For 40 sightings, the group composition was divided into maturity categories, using results obtained by photo-identification. Seventeen groups consisted of purely immature animals, and 4 groups consisted of only mature animals. The 19 mixed herds were composed mainly of 4.0±2.2 individuals (range=2–8) and consisted of 58% mature individuals. The 17 immature groups consisted of 2.1±0.9 individuals. Mature animals were usually found alone. Forty-eight per cent of beaked whales were seen where the depth was between 756 and 1389 m but the encounter rate was higher between depths of 1389 and 2021 m. The sightings were more frequent (34%) where the sea floor slope was between 31 and 51 m/km but the encounter rate was higher where the sea floor slope was between 11 and 31 m/km. The encounter rate for Cuvier's beaked whales was higher where the depth anomaly was positive with values between 342 and 586 m.