Drivers of abundance and spatial distribution of reef-associated sharks in an isolated atoll reef system

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.

Fig 1. Maps of BIOT showing areas sampled with stereo-BRUVS.

Panels a) to d) show locations of individual samples on reefs (black triangles) and lagoons (grey circles).

Fig 2. Mean abundance (MaxN hr^-1), and per cent of samples where present, for reef-associated sharks by species across the BMR.

Error bars show 95% confidence interval.

Fig 3. Mean abundance of reef-associated sharks at individual atolls; error bars show confidence interval of means.

'Other’ species are tawny nurse, blacktip reef, tiger, scalloped hammerhead and great hammerhead.

Fig 4. Mean depth (± 95% CI) of shark species observed on BRUVS samples in the BMR; species are ranked in order of increasing depth.

Fig 5

a) Mean values (± 95% CI) and b) density plots of fork lengths for grey reef sharks at Victory Bank, Peros Banhos, North Brother and Salomon reefs. Shaded region in b) indicates estimated size range of 46–73 cm FL for young of year.

Fig 6

Observed vs fitted values for generalised linear models of a) log transformed total shark abundance b) log transformed abundance of grey reef sharks only.