Records from visual surveys, strandings and eDNA sampling reveal the regular use of Reunion waters by dwarf sperm whales.

Abstract

The genus Kogia includes two extant species, the dwarf sperm whale (Kogia sima) and the pygmy sperm whales (K. breviceps). Due to their elusive behavior at the surface, which limits opportunities for observation, they are amongst the least known species of cetaceans and knowledge of their ecology mostly comes from stranded individuals. Although they have overlapping ranges, dwarf sperm whales seem to be distributed preferentially in warmer tropical and subtropical waters, while pygmy sperm whales tend to be associated with more temperate waters. Both species have previously been recorded in the western Indian Ocean, but little is known about their distribution patterns. Data from different sources, including vessel-based and aerial surveys, environmental DNA and strandings were compiled to report on the occurrence of Kogia around the remote oceanic island of Reunion. The combination of sightings data, eDNA detections and stranding events indicated that the dwarf sperm whale was more common than the pygmy sperm whale and seems to use the territorial waters of Reunion on a regular basis. The northern part of the island in particular might provide suitable habitats for the species. Groups of 1-5 individuals were sighted and occurred mainly over the insular slope, in 1310 m deep waters and 8.2 km from the shore on average; no clear seasonality pattern could be determined. Stranding data were consistent with a calving period during the austral summer and highlighted the vulnerability of these species to human activities.