Drivers of abundance and spatial distribution of reef-associated sharks in an isolated atoll reef system
David Tickler, Tom Letessier, Jessica Meeuwig | May 31, 2017
Whole-of-ecosystem protection is essential to conserving reef shark populations.
Photo: Andrei Voinigescu
Tickler DM, Letessier TB, Koldewey HJ, Meeuwig, JJ. 2017. Drivers of abundance and spatial distribution of reef-associated sharks in an isolated atoll reef system. PloS ONE, 12(5): e0177374.
- Shark distribution across a large and diverse atoll system showed a strong relationship between shark and prey abundance.
- Habitat and depth were of secondary importance in predicting overall shark numbers, but were needed to explain the occurrence of different species and age groups of sharks.
- Taken together the results provide robust support for the view that large mobile reef predators with complex life-histories, such as sharks, require large-scale ‘ecosystem level’ protection.
- Large no-take marine reserves have been established in the Indian and Pacific Oceans and are proposed for Australian territorial waters as part of the Commonwealth Marine Reserves. Such protected areas of marine wilderness ensure that the broad suite of habitat zones required by different species at different life stages, and the food webs on which they depend, are all protected.
The study, published on 31st May 2017 in the journal PLOS ONE, was carried out by David Tickler, Dr Tom Letessier and Professor Jessica Meeuwig from Marine Futures group and Dr Heather Koldewey of ZSL (Zoological Society of London).
Baited video systems, called BRUVS, were used to survey shark and fish numbers across different habitats and depth zones of the reefs of the British Indian Ocean Territory Marine Reserve (BMR) in the central Indian Ocean.
Declared in April 2010, the BMR is one of the world’s largest no-take marine reserves, containing an isolated archipelago of reefs extending more than 300km north to south and 200km east to west, making it one of the best places in the world to study reef systems in their natural state.
The researchers found that reef shark abundance was most strongly linked to the amount of prey fish in an area rather than the quality of the reef habitat itself, suggesting that overall, sharks respond most strongly to food availability.
PREDATOR AND PREY
The study showed that to maintain both the abundance and diversity of reef sharks, their food sources needed to be protected and a range of habitats included. This is why large no-take marine reserves such as the BMR are important to shark conservation.
Lead author David Tickler, from UWA’s School of Biological Sciences, said individual shark species tended to vary in their habitat preferences.
“We found that shallow reef sites were important for common species such as white tip and grey reef sharks, whereas deep reefs on submerged seamounts were the haunt of hammerhead and silvertip sharks and isolated banks were important to juveniles,” Mr Tickler said.
“To maintain both abundance and diversity of reef sharks, we need to make sure that their food is protected and that a range of habitats are included. This is why a large no-take marine reserve, such as the BMR, is important to shark conservation.
“Reef systems are a complex interlocking web of predator and prey relationships; disturbing one element of that system has knock-on effects throughout the reef, and conservation goals are best achieved when all parts – predators, prey and habitat – are protected.”
Co-author Dr Tom Letessier from ZSL said no-take reserves such as the BMR, located far from humans, were home to a remarkable number and diversity of sharks.
“This study shows that at the local scale within the reserve, shark abundance is intimately linked to their prey (fish) and their habitats. Our work is evidence of the importance of protecting the entire ecosystem: effective shark conservation measures must include protection of shark prey and their habitats.”
Professor Meeuwig said the study had broad significance for marine conservation.
“Sharks are fundamental to healthy ecosystems and the threat they face from indiscriminate fishing is clear,” Professor Meeuwig said.
“Like the predators of the African plains, sharks dominate the marine ecosystems of the ‘Blue Serengeti’ and provide valuable ecosystem services as well as direct benefits to industries such as dive tourism. The demand for shark meat and fins has led to declines of up to 90 per cent in some species, and fully protected marine reserves are a vital tool to help rebuild their populations.”
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.
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