4.1 High-use area in Mauritius waters
Sperm whales disperse widely in all ocean basins and their global
abundance estimate is in the hundreds of thousands (Whitehead 2002).
Results from our limited sample size from a localized population in the
South-West Indian Ocean may not be representative of the behaviour of
all sperm whales but fill a critical gap in our understanding of this
cryptic species. The satellite tracked individuals highlighted two
critical hotspots close to Mauritius as well as a migratory route
between Mauritius and Rodrigues. Among the 21 sperm whales satellite
tracked, 14 remained in close proximity to Mauritius up to a maximum of
107 days. The Mascarene islands (Reunion and Mauritius islands) have
previously been identified as a suitable habitat for this species
(Mannocci et al. 2014b), using aerial survey data. Here, satellite
tracking data have allowed both resident and migratory movements of
individually tracked whales to be described in this area and quantified.
Although the time spent west of Mauritius varied across individuals, the
kernel densities showed two clear hotspots located west and south-west
of Mauritius. These two core areas might correspond to a breeding and a
nursery ground during the wet and dry season, respectively. Despite
mature males being observed from September to June in Mauritius waters,
a larger proportion of mature males is seen between October and December
in one of the highlighted core areas while more calves are mostly
observed between March and April in the second one (M. Vely, unpublished
data). The 16 months gestation period of this species (Ohsumi 1965) and
a previous study showing that conception takes place in austral summer
south-east of South Africa (Findlay & Best 2016) together with
observations of sperm whales giving birth in April (Gambell 1966)
reinforce the importance of these potential breeding and nursery
habitats in Mauritius waters.