Tracking sharks at depth

by ZSL on

A recent expedition to the BIOT Marine Protected Area (MPA) was undertaken with collaborators from Stanford University to retrieve acoustic tracking equipment from the seabed.

For the project, which was funded by the Bertarelli Programme in Marine Science, receivers that have been monitoring shark movements in the deep channels between islands were recovered from depths of more than 500m. These will provide a wealth of new data on ecological connectivity within the reserve, as ZSL's David jacoby explains. 

David Jacoby with an acoustic release receiver used for tracking sharks
The relief is evident as the first acoustic release (AR) receiver surfaces from 14 months at 550m and is brought on board the boat over Sandes Seamount

The British Indian Ocean Territory (BIOT), otherwise known as the Chagos Archipelago, is one of the world’s largest no-take marine protected areas and it’s a difficult place to get to. 

Having spent the last two years poring over the Admiralty Charts of the reserve trying to figure out how sharks move within the MPA, I jumped at the chance to get a first-hand perspective of this unique set of islands. And so via a commercial flight to Bahrain, followed by a heavily delayed military flight to Diego Garcia, myself and Dr Taylor Chapple (Stanford University) under the cover of darkness boarded the BPV Grampian Frontier at 02:30, ready to head out into this remote Indian Ocean archipelago. 

Our mission was to retrieve 16 acoustic release (AR) receivers that had been sat on the seabed in the deep waters between islands and over seamounts, continuously listening out for over 400 fish carrying long-term acoustic pinger tags. 

Deployed over 14 months prior to our expedition, these receivers took the total of acoustic monitoring receivers in the reserve to an impressive 93 devices. By communicating with the AR receivers via a surface modem dangled over the side of the boat, we sent a message to the device to unscrew from its mooring and in doing so allowing the attached buoys to bring the whole receiver to the surface.

David Jacoby with a satellite transmitting receiver
Switching batteries on one of our satellite transmitting receivers that provide real-time detections of tagged sharks

An anxious wait ensued before the first receiver popped up over Sandes Seamount. Once back aboard the BPV we then downloaded the data gathered from each receiver which will ultimately help us shed some light on how far some of our tagged sharks actually venture. 

During the 500 mile round trip we were able to retrieve 12 out of 16 devices in addition to switching the battery on two of our satellite-linkage receivers that transmit tag data in real-time. Of the four receivers not recovered, due to being in water much deeper than anticipated, one was found to contain no detections. These four receivers aside however, the total number of detections on the recovered devices was more than 500,000 detections, making this expedition a huge success. 

My research focuses on how sharks connect habitats through their movements, which I like to think of as a network of linkages. These data from some of the deep water ‘blind spots’ around the reserve will be critical for informing this picture and it is hoped that with an improved knowledge of the structure of these networks, we can begin to predict where and when large reef predators are most vulnerable to illegal fishing in BIOT.

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