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Sperm whales communicate using codas (stereotyped click sequences). Females and juveniles live in long‐term social units, and units with similar coda repertoires share vocal clan membership. Vocal clans exhibit culturally defined differences in their multilevel social structure. Here, we aimed to identify different social units among sperm whales in Mauritius in the southwestern Indian Ocean, and to describe and compare their coda repertoires to investigate the presence of different vocal clans. We conducted six boat‐based surveys between 2008 and 2013, during which 101 different individuals were photo‐identified. Analysis of associations between 22 resighted individuals divided them into four candidate social units, but a lack of resightings impedes solid delineation of social units. Based on number and rhythm of clicks, at least 24 discrete coda types were detected among 4,767 analyzed codas using two different classification methods. Comparison of coda repertoires recorded from seven sperm whale groupings revealed the possible existence of two sympatric vocal clans, but the size differences of recorded repertoires warrant caution of interpretation. To further evaluate social structuring and the presence of different vocal clans in this region, future surveys should aim at obtaining long‐term sighting and acoustic data, and cover a more extensive area.

Spinner dolphins Stenella longirostris longirostris off the south-west coast of Mauritius are subject to ongoing anthropogenic disturbance in the form of daily dolphin tourism, which has intensified since 1998. Abundance of this species was estimated using photo-identification data and mark-recapture analysis. Between April 2008 and June 2010, identification photographs were collected from dolphins occurring along a 30 km length of the coast of south-west Mauritius. A total of 250 groups were encountered over 229 survey days. Mark-recapture analyses were performed on a photographic dataset of more than 8 000 good-and excellent-quality images and 83 animals were identified as distinctively marked individuals. The majority (85.5%) were seen more than once and resightings indicated a resident population. The compiled version of SOCPROG 2.4 was used to investigate the lagged identification rate. The fitted model supported a mostly resident population with additional animals moving in and out of the study area. The estimated abundance of the total population in the study area ranged between 138 and 399 individuals. Our results can be used for monitoring the population for fluctuations and for encouraging both the enforcement of laws regarding dolphin watching and the development of further means of management needed to ensure the long-term presence of this population.

There is limited information concerning the cetaceans inhabiting the coastal waters of Mauritius.  This document details the sightings of cetaceans encountered during April 2008 – September 2014. Boat-based surveys were conducted throughout the year, primarily focusing on spinner and bottlenose dolphin populations that are important for the local dolphin-watching industry.  More than 1246 hours over 749 days of surveys were spent searching for cetaceans.  All cetaceans encountered were recorded. During 2013 and 2014, dedicated diversity surveys were conducted and 2443.6km of search effort was covered around the islands. The encounter rate was 0.025 cetacean sightings/km. Over the total study period 13 species were recorded from 1459 sightings. Species included: Megaptera novaeangliae, Stenella longirostris, Tursiops aduncus, Tursiops truncatus, Globicephala macrorhynchus, Stenella attenuata, Physeter macrocephalus, Mesoplodon densirostris, Feresa attenuata, Peponocephala electra, Grampus griseus and Ziphius cavirostris. In addition, there were strandings of Kogia sima and Ziphius cavirostris. The most common species encountered were spinner dolphins, Indo-Pacific bottlenose dolphins, humpback whales and sperm whales. As expected only humpback whales showed any seasonality in monthly encounter rates (Kruskal-Wallis H = 42.39, Hc = 54.25, p < 0.001). The overall Shannon diversity index was 0.58 (95% CI 0.57–0.60) though this ranged between 0.77 in 2009 to 0.25 in 2014. The Shannon index value for the diversity study was 0.83 (95% CI 0.74–0.91). This study demonstrates the richness of the waters around Mauritius and the importance of the area for beaked whales, migrating humpback whales and the presence of resident sperm whales.