About 95 silvertip and grey reef sharks have been acoustically tagged to measure how effective the British Indian Ocean Territory (BIOT) Marine Protected Area (MPA) is at protecting them.
A recent study shows that the synchronous loss of 15 acoustic tags in BIOT concurs with two illegal fishing ships being arrested for having 359 sharks on deck. Although its primary use was to map sharks’ movements around the reef, the acoustic tag’s data can now be used to inform on the presence of illegal fishing vessels. In spring of 2015, scientists returned to BIOT to service their acoustic receivers surrounding the archipelago, and to download the tag data from their 95 tagged sharks. To their surprise, in ten days 15 of these 95 sharks disappeared, presumably lost to illegal fishing.
Marine conservationists are increasingly using acoustic telemetry as a way to collect data from species. Scientists from Stanford University Hopkins Marine Station, ZSL and University of Western Australia have collaborated using such technology to track reef shark movement around the MPA since 2013. Acoustic tags emit a unique sound received by hydrophones which then enables their localisation. Accuracy is not optimal, locations pinpointed can still be hundreds of metres away which is why such technology (acoustic telemetry) is mostly used on more resident species than highly mobile ones.
During 2013 and 2014 and after a lengthy developmental phase and ethical review, the team tagged 47 grey reef sharks (Carcharhinus amblyrhynchos) and 48 silvertip sharks (Carcharhinus albimarginatus). These sharks were tagged and released in the BIOT MPA. The aim of this release is to track shark movements. The transmitters are usually placed on the shark’s dorsal fin. They allow a fine scale mapping of their habitat as they remain highly loyal to their reef and therefore, remain within range of the receivers. The sudden disappearance of 15 sharks is not common and most likely represents a successful illegal fishing operation.
Sharks have been particularly targeted in the last decade in BIOT and around the world. 90% of illegal fishing ship encountered in BIOT had sharks on board, making up most of the catch. This coincides with a global trend where sharks have been targeted worldwide to fuel the demand in fin soup which has now gown in popularity over Asia and is becoming cheaper and more accessible.
The team studying sharks in BIOT during spring of 2015 decided to investigate the possible relationship between illegal fishing events and the disappearance of their tags. They found that, statistically, less than 50 tags were enough to detect such illegal events and could therefore be used to detect poaching events. Nonetheless, improvements in technology are still needed, there is a need for the data from tags to be collected quicker for a more effective intervention from the BIOT Patrol vessel. In the meantime, the team on location is teaching enforcement officers where and when sharks aggregate using the data that already exists combined with historical fisheries.
“Using animals as sentinels, both to detect illegal activity and to monitor environmental conditions, extends our capabilities and reach, making us more effective stewards of our oceans.”
D. M. Tickler