Carcharhinus amblyrhynchos and Carcharhinus albimarginatus underwater acoustic telemetry data - Chagos Archipelago 2014-2018

A wide array of technologies are available for gaining insight into the
movement of wild aquatic animals. Although acoustic telemetry may lack the
fine-scale resolution of some satellite tracking technologies, the
substantially longer battery life can yield important long-term data on
behaviour and movement for low per-unit cost. Typically, however, receiver
arrays are designed to maximise spatial coverage at the cost of positional
accuracy leading to potentially longer detection gaps as individuals move
out of range between monitored locations. This is particularly true when
these technologies are deployed to monitor species in hard-to-access
locations. We develop a novel approach to analysing acoustic telemetry
data, using the timing and duration of gaps between animal detections to
classify movement behaviours into ‘restricted’ or potential wider ‘out of
range’ movements synonymous with longer distance dispersal. We apply this
method to investigate spatial and temporal segregation of inferred
movement patterns in two sympatric species of reef shark within a large,
remote Marine Protected Area. Drivers of these movements were identified
using generalised linear mixed models and multi-model inference. Species,
diel period and season were significant predictors of ‘out of range’
movements. Silvertip sharks were overall more likely to undertake ‘out of
range’ movements, compared to grey reef sharks, indicating spatial
segregation, and corroborating previous stable isotope work between these
two species. High individual variability in ‘out of range’ movements in
both species was also identified. We present a novel gap analysis of
telemetry data to help infer differential movement and space use patterns
where acoustic coverage is imperfect and other tracking methods
impractical. In remote locations, inference may be the best available tool
and this approach shows that acoustic telemetry gap analysis can be used
for comparative studies in fish ecology, or combined with other research
techniques to better understand functional mechanisms driving behaviour.