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International audienceAlthough the striped dolphin Stenella coeruleoalba has a world status of Least Concern, a recent IUCN Red List assessment has proposed that the Mediterranean population be listed as Vulnerable and stresses the need for an estimate of abundance and distribution. While substantial efforts have been made to study cetaceans in regions of the western Mediterranean Sea (WM), we have little knowledge of their large-scale distribution and interaction with oceanographic features at various scales. We conducted 18 basin-wide surveys from ferry platforms within the WM from early September 2006 to late July 2007 and used spatial modelling to investigate the distribution, abundance and habitat use of striped dolphins. Most striped dolphins were sighted north of the Balearic Islands, where they are closely associated with the counter-clockwise circulation defined by negative absolute dynamic topography. In the Algerian basin, dolphins were mostly found in recent Atlantic water mass. Densities of striped dolphins were also related to high gradients of sea surface temperature and high chlorophyll concentrations. Moreover, we found similar results (1) when the northern and southern data sets were pooled and (2) in the northern area only, where the majority of sightings occurred. These relationships suggest that dolphins were found in dynamic and productive areas generated by mesoscale processes which could create favourable foraging conditions. Despite our relatively small data set, dolphins were found year-round. We estimated the number of striped dolphins in the area between 3° and 6° E to have been 38 600 (95% CI: 25 900 to 53 900) before the resurgence of the morbillivirus epizootic in summer 2007. This is considerably more than the estimate from the 1991 WM survey, which reported a 43% lower abundance, probably affected by the 1990 epizootic. Platforms of opportunity are therefore relevant not only to assess animal ecology but also to monitor population in a given area. Such interanual monitoring is especially important to detect and quantify the response of animal populations, in terms of distribution and density, to environmental stresses

The spinner dolphin (Stenella longirostris) is the most common among 16 species in the Society Islands (French Polynesia). They are observed yearround during daytime in sheltered bays or within lagoons. From 1995 to 2002, we studied spinner dolphins from a shore site in Baie des Pêcheurs, a bay on the west coast of Tahiti, performing 1,033 sighting sessions with binoculars. Presence, position, and school size were noted, as well as various behavioral and environmental variables. Human presence also was recorded. Dolphins were present on average 73.3% of the days, with a higher presence rate from May to November (81.0%) than from December to April (66.7%). Dolphins stayed within the bay from early morning until 1200 to 1500 h and had school sizes ranging from as small as 15 to 30 to as large as 100 to 140 individuals. Dolphins began to move slowly offshore after 1100 h. On average, they stayed 400 m from shore, although they approached as close as 100 to 150 m. Dolphin presence and residence time seemed to be negatively affected by surface water turbidity (river flow) and lagoon current strength. Recreational dolphin watching was low from Monday to Thursday (0.20 to 0.35 boat per sighting session) and high on Sunday, with an average of 1.67 boats per session. There was a lower dolphin presence rate from Monday to Thursday (69%) than from Friday to Sunday (78%). Presence patterns were similar to those found in Hawaii, accounting for differences in environmental characteristics.

Oceanic odontocetes rely on echolocation to forage on pelagic or benthic prey, but their feeding ecology is difficult to study. We studied sperm whale foraging dives during summer in the north-western Mediterranean, using visual and passive acoustic observations. Clicking and creaking activities were recorded during dives of focal whales, at distances ,3000 m using a towed hydrophone and DAT recorder. A total of 52 sperm whales were recorded over at least one full dive cycle. Data were obtained for 156 complete dives in total, including sequences of up to nine consecutive dives. Various dive and environmental variables were entered in multiple linear regression and principal components analysis, as well as estimated mass of whales. Creak rate was 0.80 creak/minute on average, with moderate variance. Bigger whales tended to dive longer at greater depths (as suggested by ascent durations), and emitted more creaks during a dive: 20.2 creaks/dive on average for individuals ,24 tons, compared to 25.6 creaks/dive for animals .24 tons of estimated mass. For individual whales, creak rates did not vary significantly with size (range 0.78 -0.80 creak/minute), but decreased with time of the day, and increased for shorter foraging phases. For different dives, higher creak rates were also observed earlier in the day, and linked to shorter foraging phases and surface durations. Although the exact significance of creak emissions (i.e. foraging attempt or prey capture) is not precisely determined, creak rates may be reliably used to quantify sperm whale foraging when single animal dives can be followed acoustically.