Abstract
Reef sharks, like most shark species and other marine predators, are strongly impacted by human activities and are experiencing sharp population decline worldwide. Such decline poses great risk of causing a profound disruption of coral reef ecosystems where reef sharks play a key functional role. Effective and appropriate reef shark conservation measures are therefore urgently needed. Marine Protected Areas (MPAs) are one of the most widely used tools for the protection of marine ecosystems. Unfortunately, current MPAs are often ineffective for the protection of mobile species such as reef sharks. This PhD aims at studying the movements and space use of a common reef shark species in New Caledonia in order to improve the protection of this species. To achieve this goal, 147 grey reef sharks, Carcharhinus amblyrhynchos, were tagged with acoustic transmitters and their movements were monitored over three years within a network of 73 acoustic receivers deployed throughout the archipelago. The grey reef shark is a good model to evaluate how information on movement and space use may improve shark conservation. Indeed, it is one of the most common reef shark species in the Indo-Pacific and drastic population declines have been documented throughout their geographical range. In New Caledonia, grey reef shark abundance has dropped by over 90% in the most anthropized regions of the archipelago. In addition, local MPAs are not able to protect this species. The presence of a strong gradient of human proximity in New Caledonia, including highly impacted reefs near the capital Nouméa and wilderness reefs in the remote parts of the archipelago, made it possible to assess the impact of human activities on various aspects of grey reef sharks’ space use. Acoustic telemetry data revealed an increase in grey shark home range in the vicinity of human settlements. However, the causal links between population decline and changes in space use are difficult to establish. This increase in shark space use at human proximity can constitute a driver of population decline, via a reduction in fitness, as well as a consequence of it. A lower population density may indeed drive individuals to expand their movements in the pursuit of mates. The shark home range dataset was then used to inform MPAs’ ability to protect the species. Results are consistent with the reported inability of local MPAs to protect this species, emphasising MPA size are too small to cover shark home ranges, especially for adult males. However, these results validate the recent efforts of the Government of New Caledonia to establish very large MPAs within its Economic Exclusive Zone. Comparison of these results with the size of MPAs in the Indo-Pacific also confirms the recent efforts undertaken at this scale. This work provides concrete information on the minimum size of an MPA necessary for the protection of the grey reef shark. The analysis of large-scale movements also made it possible to document for the first time the existence of seasonal migrations in the grey reef shark. Seven adult males were observed to undertake a round-trip migration along the west coast of the main island, up to more than 300 km from their tagging area, and over several consecutive years. The fact that only adult males have been observed to migrate and the timing of these migrations suggest that this behaviour is related to reproduction. These results have important implications for the management of this species, particularly at the local scale for the identification of preferential breeding areas.