Abstract
We investigated drivers of reef shark demography across a large and isolated marine protected area, the British Indian Ocean Territory Marine Reserve, using stereo baited remote underwater video systems. We modelled shark abundance against biotic and abiotic variables at 35 sites across the reserve and found that the biomass of low trophic order fish (specifically planktivores) had the greatest effect on shark abundance, although models also included habitat variables (depth, coral cover and site type). There was significant variation in the composition of the shark assemblage at different atolls within the reserve. In particular, the deepest habitat sampled (a seamount at 70-80m visited for the first time in this study) recorded large numbers of scalloped hammerhead sharks (Sphyrna lewini) not observed elsewhere. Size structure of the most abundant and common species, grey reef sharks (Carcharhinus amblyrhynchos), varied with location. Individuals at an isolated bank were 30% smaller than those at the main atolls, with size structure significantly biased towards the size range for young of year (YOY). The 18 individuals judged to be YOY represented the offspring of between four and six females, so, whilst inconclusive, these data suggest the possible use of a common pupping site by grey reef sharks. The importance of low trophic order fish biomass (i.e. potential prey) in predicting spatial variation in shark abundance is consistent with other studies both in marine and terrestrial systems which suggest that prey availability may be a more important predictor of predator distribution than habitat suitability. This result supports the need for ecosystem level rather than species-specific conservation measures to support shark recovery. The observed spatial partitioning amongst sites for species and life-stages also implies the need to include a diversity of habitats and reef types within a protected area for adequate protection of reef-associated shark assemblages.
Highlights
Top predators, occupying the highest position in food webs, are thought to exert significant influence on the structure and function of ecosystems
It is hypothesised that top marine predators influence community structure in their host ecosystems both directly, through the mortality they inflict on their immediate prey, and through indirect effects resulting from behavioural changes in prey and competitors
The aim of our study was to model the regional drivers of spatial variation in shark abundance at the scale of an entire reef system, using data from a large and relatively undisturbed reef system
Summary
Top predators, occupying the highest position in food webs, are thought to exert significant influence on the structure and function of ecosystems. It is hypothesised that top marine predators influence community structure in their host ecosystems both directly, through the mortality they inflict on their immediate prey, and through indirect effects resulting from behavioural changes in prey and competitors (so called ‘fear effects’). This in turn may impact the abundance and ecological function of species at the lowest levels in the food web [7,8]. Such interactions may lead to important changes to marine ecosystem function, productivity and socioeconomic value [9,10]
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