3.1 General tracking data
Across the 22 individuals equipped in Mauritius waters, three were males (#3963a, #20166b and #50681b) and all remaining whales were females. The number of locations recorded per sperm whale ranged from 7 to 176 (#50678 vs. #93106, respectively) – Table 1. The tracking duration was on average 34±20 days (range: 4-109 days). The total distance travelled varied between 154 (#6337a) and 3112 km (#24642), and the average horizontal speed was 2.7±0.3 km.h-1.
3.2 Seasonal movements
A seasonal pattern in the movements of the whales was observed between the wet and dry seasons. Sixteen individuals were observed during the wet season from December to March (Fig. 2a) and the tracks of 14 whales were available during the dry season from April to November (Fig. 2b). The male tracked in 2018 (#50681b) was the only one migrating southward (Fig. 2a). The kernel densities showed a core of activity in shallow waters near Mauritius for both seasons. During the wet season, the core home range (50% kernel contour) was located south-west of Mauritius (Fig. 2c). During the dry season, the resident whales exhibited strong site fidelity by inhabiting shallow waters along the west coast of the island (Fig. 2d). The core home ranges were also located on variable topography for both seasons, e.g. high slopes (Fig. 2e, f).
3.3 Migratory patterns
Among the 16 individuals tracked during the wet season, eight whales (#20158a, #24642, #26712, #27261, #50678b, #50682b, #7618 and #7926) left Mauritius waters in December heading towards Rodrigues Island (Figs. 2a and 3). Except one, these individuals were all mature females (based on Photo ID, morphology, size and mother-calf association) and showed a synchronized departure from Mauritius. They made a loop eastward of Mauritius before returning at different times. In 2014, only one individual was considered migrant (departure on the 2014-12-25) while in 2018, seven whales initiated their short migration on the exact same date (2018-12-15). The increased distance from Mauritius matched the decrease in SST (extracted at the whales’ positions) for all migrant whales (Fig. 3c,d). Unlike the decrease in SST values at the whale’s locations between December and January, the SST extracted inside the 50% kernel increased for both years from December to January (Fig. 3e,f).
3.2 Algorithms comparison
The variation of the performance metrics across the ten simulation runs is illustrated by the box plots in Figure 4. Indeed, the small range of each boxplot indicated little sensitivity to pseudo-absence generation across the ten runs for both seasons. The mean values of the six performance metrics (accuracy, kappa, sensibility, sensitivity, specificity, F1 score and TSS) calculated from the 10-fold cross-validation were high (mean range: 0.81-0.99) for the 14 algorithms for both seasons (Fig. 4), showing good predictive performance. Based on the six performance metrics, the best model was the Random Forest (RF) for both seasons, with values ranging from 0.93 to 0.99.
When comparing the tuned RF and the stacking method during the dry season, the tuned RF approach had slightly but significantly higher performance metrics compared to the stacking for all metrics: accuracy (mean: 0.978 vs. 0.970), specificity (mean: 0.980 vs. 0.970), F1 score (mean: 0.978 vs. 0.970) and TSS (mean: 0.957 vs. 0.941) (Kruskal-Wallis test, p<0.05, Fig. 5a,d,e,f). However, no significant difference was observed between both methods for the wet season (Kruskal-Wallis test, p>0.05, Fig. 5). Given such low differences, both methods were used to generate predictions of the whales’ potential distribution for the wet and dry season separately.
3.4 Predicted distributions
The maps of the predicted distributions of the sperm whales reflected a pronounced seasonal pattern (Fig. 6). Small differences were observed between both approaches, with similar spatial patterns and globally higher probabilities for the stacking method. During the wet season, the potential distribution was widely spread around Mauritius and between 59-62°E, which coincides with the migration of the eight individuals that left the coastal areas of the island (Fig. 6a,b). In contrast, the favourable habitats during the dry season were mostly confined to areas close to Mauritius (mostly west and south-west) and also north of Reunion Island. Some high probabilities of sperm whale presence were also identified on steep sloping habitats during both seasons, i.e. south of Mauritius (Fig. 6c,d). The most important covariates were the SSH and bottom temperature for the wet and dry season (for the tuned RF), respectively (SI Fig. 1). The coefficients of variation were globally low (<3.2%), confirming the low variability between the ten simulations for both approaches (SI Fig. 2).
3.5 Diving behaviour
A total of 500 maximum dive depths and 529 maximum dive durations were recorded from the eight whales equipped with SPLASH10 tags. The distributions of the dive depths were bimodal with mainly shallow (<500 m during the day and <400 m at night) and deep dives, i.e. between 600-1400 m during the day and 400-1400 m at night (Fig 7a). When looking at the deep dives (>200 m), a diel pattern was observed for the maximum dive depth, with significantly deeper dives during the day (mean: 1146 m) compared to night-time dives (mean: 816 m, Kruskal-Wallis test, p<0.001) – See Fig. 7a, b.
Similarly, the distributions of the dive durations were also bimodal, with short (<30 min during the day and <25 min at night) and long dives (between 30-70 min during the day and 25-70 min at night, Fig 7b). Dive durations lasted on average 34 min (Fig. 7b). Twenty percent of the dives were short and lasted less than 10 min, and 45% were long (between 40 and 60 min). However, there was no significant difference in terms of dive duration between day (mean=31 min) and night (mean=35 min, Kruskal-Wallis test, p=0.9322).