Environmental and anthropogenic effects impact species abundance, density and distribution

I will be investigating and comparing the spatial ecology of two coastal-pelagic shark species at Ascension Island: a remote UK Overseas Territory that is surrounded by one of the world's largest marine protected areas (MPA). Sharks have been legally protected at Ascension Island since 2017. Recently, however, increasingly regular, and unexplained movements of Galapagos sharks (Carcharhinus galapagensis) into shallow, coastal habitats have created significant conflicts with fishers and other recreational ocean users which threaten to undermine support for the MPA. Preliminary work by the supervisory team has identified several apparent hotspots around Ascension Island that frequently support mixed species 'super schools' of Galapagos and silky sharks (Carcharhinus falciformis). However, the significance of these sites and drivers of temporal variability in shark distributions - including aggregating behaviours and periodic inshore movements of Galapagos sharks - are currently unknown. Addressing this knowledge gap has been identified as a priority in the Ascension Island MPA Research Strategy.

The project will characterise the movements of galapagos and silky sharks at a range of spatial and temporal scales, and test hypotheses that interactions between bathymetry, oceanography, and prey availability drive their distributions. Specifically, I aim to:

1. Characterize broad-scale distribution dynamics of sharks at Ascension Island over multi-year timescales. Using passive acoustic telemetry, the project will track movements and depth use of tagged sharks on an existing island-wide hydrophone array, identify hotspots, and quantify inter- and intra- annual spatio-temporal variation in distributions linked to species, sex, and life stage. Social network analysis will characterise the fission-fusion dynamics of tagged sharks and explore how social structure contributes to observed distributions.
2. Characterize fine-scale distribution dynamics of a 'super school' at a known aggregation site. Using hydroacoustic surveys and a Vemco Positioning System the project will map the extent of an identified super school and reconstruct high-resolution, three-dimensional movements of acoustically tagged individuals within the school. The periodicity of school formation and behaviour (e.g. tidal/diel cycles), sex and size class structure, and intra- and inter-specific social interactions will be quantified.
3. Investigate the mechanisms that promote 'super school' formation. By integrating fine scale movement data with hydroacoustic biomass estimates, local oceanographic/hydrodynamic models, and accelerometer/sonar tags, activity patterns of sharks inside and outside the 'super school' will be quantified. The project will test hypotheses that prey convergences, bioenergetics, and social behaviours promote aggregations at specific sites.
4. Investigate the environmental and ecological drivers of nomadic movements into shallow, coastal habitats prone to human-shark conflict. By relating movements of acoustically tagged sharks to local oceanographic time series and proxies of diet/nutritional status (e.g. blood metabolites, stable isotopes, morphometrics) the project will test hypotheses that climate anomalies and/or fluctuating prey availability drive periodic fragmentation of aggregations and nomadic movements into inshore areas.
Collectively, the results will provide novel insights into the dynamic processes that affect marine predator distributions, as well as informing shark conflict management in one of the world's largest MPAs.