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Abstract
Sharks have declined worldwide and remote sanctuaries are becoming crucial for shark conservation. The southwest Indian Ocean is a hotspot of both terrestrial and marine biodiversity mostly impacted by anthropogenic damage. Sharks were observed during surveys performed from April to June 2013 in the virtually pristine coral reefs around Europa Island, a remote Marine Protected Area located in the southern Mozambique Channel. Observation events comprised 67 1-hour scientific dives between 5 – 35m depth and 7 snorkeling inspections, as well as 4 dinghy-based observations in the shallow lagoon. In a period of 24 days, 475 sharks were tallied. Carcharhinus galapagensis was most encountered and contributed 20% of the abundance during diving, followed by C. albimarginatus (10%). Both species were more abundant between 11-14h, and on the exposed sides of the island. Numbers of Sphyrna lewini were highest with 370 individuals windward and leeward, mostly schooling. S. lewini aggregations in the area are hypothesized to be attracted to the seamount archipelago offering favorable conditions for deep incursions and of which Europa Island forms part. C. amblyrhynchos, Galeocerdo cuvier and S. mokarran were uncommon, while there was an additional observation of Rhincodon typus. The lagoon of Europa was a nursery ground for C. melanopterus where it was the only species present. A total of 8 species was recorded, contributing to the shark diversity of 15 species reported from Europa since 1952 in the scientific and gray literature. Overall, with the occurrence of several species of apex predators in addition to that of R. typus, large schools of S. lewini, fair numbers of reef sharks and a nursery of C. melanopterus, Europa’s sharks constitute a significant reservoir of biodiversity, which contributes to preserve the functioning of the ecosystem. Our observations highlight the relevance of Europa Island for shark conservation and the need for shark-targeted management in the EEZ of both Europa and Bassas da India.
Highlights
Sharks as predators are a major component of marine food webs [1, 2]
Our observations highlight the relevance of Europa Island for shark conservation and the need for sharktargeted management in the EEZ of both Europa and Bassas da India
One school, extending from the sea surface to ~15 m depth, was estimated at 150 individuals (S1 File) and smaller schools numbering 50, 30 and 20 individuals respectively contributed to this high abundance
Summary
The larger species, such as the great and the scalloped hammerhead sharks Sphyrna mokarran and S. lewini and the tiger shark, Galeocerdo cuvier, are transient apex predators on coral reefs [2, 3] They are capable of structuring coral reef food webs both directly through predation and indirectly by modifying prey behavior [3]. Most shark species, among which the gray reef shark, Carcharhinus amblyrhynchos, the Galapagos shark, C. galapagensis, the blacktip reef shark, C. melanopterus, are reef-associated meso-predators [2, 3] They share functional redundancy among each other and with large piscivorous fishes such as groupers or snappers; all are assigned to the trophic group of high-level meso-predators and are considered to exert a limited influence on the community structure [2,3,4]. In the Indian Ocean, three decades of poaching by illegal vessels induced collapse by greater than 90% of shark populations in the remote expanses of the Chagos Archipelago [6]
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