3. RESULTS

3.1 Seabird sightings

In total, 15,063 seabirds were observed during the six expeditions between 2012 and 2017 (Figure 2) across all samples. Seven families were recorded: Laridae (noddies and terns), Sulidae (boobies), Procellariidae (shearwaters and petrels), Phaethontidae (tropicbirds), Fregatidae (frigatebird), Hydrobatidae (northern storm-petrels) and Oceanitidae (southern storm-petrels). Observations were dominated by the Laridae (8,817 individuals), Sulidae (5,057 individuals) and Procellariidae (872 individuals). These families were retained for further distribution modelling (Figure 2). For the Laridae, Sulidae, and Procellaridae, individuals per sample was highest in 2013 (33.1 ind. per sample), 2017 (24 per sample, and 2012 (16.3 ind. per samples), respectively and lowest in 2015 (16 ind. per sample), 2014 (3.2 ind. per sample) and 2013 (no ind.), respectively.

3.2 Predictive Modelling

3.2.1 Oceanic drivers of distribution

Total deviance explained for each GAM was 33.7% for Laridae, 46.3% for Sulidae and 21.2% for Procellariidae. Seabed depth explained 42.90% and 33.64% deviance in the distribution of Laridae and Sulidae respectively (Figure 3a and 3e). The only other geomorphometric variable that was important for Sulidae distribution was slope (28.68%, Figure 3f). In terms of oceanographic variables, sea level anomaly influenced the Procellariidae (Figure 3c, 3g and 3i), explaining 31.24% of the deviance. Sea surface temperature and chlorophyll-a were important variables for Laridae (13.11% and 42.60%) and for Procellariidae (24.31% and 23.97%, Figure 3b, 3d, 3h and 3j) but not the Sulidae.
Sulidae and Procellariidae showed high yearly variability (28.68 and 20.47% deviance respectively) consistently decreasing in abundance from 2012 to 2015, an increasing from 2015 to 2017 (Figure 4).

3.2.2 Spatial Predictions within the Chagos Archipelago

Spatial model predictions for Laridae and Sulidae distribution revealed a strong seabed depth signature (Figure 5a, b, d, and e), while Procellariidae distribution was more uniform with higher abundance levels near land and towards the northwest of the Archipelago (Figure 5c and f). Laridae abundance was pronounced over shallow seabeds (< 1000 m) in proximity to islands and atolls (Figure 5a, d). Sulidae abundance was more pronounced in pelagic and deeper areas (Figure 5b, e), and in areas with intermediate slope (c. 15º, Figure 5b). Both Laridae and Sulidae distribution was pronounced along the Lakshadweep-Maldives-Chagos ridge. Model prediction uncertainty was spatially and family specific (Appendix S2).

3.2.3 Response to rat presence

Rat-free BRT models explained more deviance (Sulidae ≈ 71%, Laridae ≈ 37%, and Procellariidae ≈ 20%) than rat-infested models (Sulidae ≈ 59%, Laridae ≈ 31%, and Procellariidae ≈ 12%). Distance to a rat-free or rat-infested island was important for Laridae and Sulidae (39.3 - 48.5%, 32.5 – 36.1% respectively) compared with the Procellariidae (0- 14.6 %). Deviance explained by Distance to a rat-free island was consistently higher than that by Distance to rat-infested island (Laridae ≈ 48%, Sulidae ≈ 36%, and Procellariidae ≈ 15%). Conversely, deviance explained by the area of the nearest island was higher in the rat-infested models (Laridae ≈ 8%, Sulidae ≈ 11%, and Procellariidae ≈ 5%; Figure 6a – 6g).
All seabird families were sensitive to the proximity to rat-free islands, as revealed by broken stick regressions (Figure 6g – 6i). Breaking points (BP) indicated the threshold to which the nearest island, whether infested or not, influenced the distribution of seabirds. Thresholds BP in the effect of islands were apparent for all families. The BP for Laridae was at 47.5 km for rat-free islands [CI 44.8, 50.2] and at 63.6 km for rat-infested islands [CI 59.1, 67.9]. The BP for Sulidae was at 16.51 km [CI 11.9, 21.1] for rat-free islands and 60.54 km for rat-infested islands [CI 54.0, 67.3]. No effect of rat presence on nearby islands was detected for the Procellaridae family, as the BP showed no significant difference in the CI between rat-free island (BP 82.9 km [CI 82.9, 96.4]) and rat-infested islands (BP 73.5 km [CI 68.6, 78.4]). Threshold breaking points are reported in Appendix S3 (Table S3).
The presence of rats on nearby islands reduced the abundance of all seabird families (Figure 7a – 7f). This pattern was more pronounced for Laridae (Figure 7b and 7e). Following rat eradication all seabird families increased in abundance (Figure 7). The gain was most pronounced near larger islands (Figure 7g – 7l). This was particularly pronounced for Laridae (Figure 7 k). Gains for Procellariidae were minor and uniformly distributed (Figure 7l).