After the success of GOMBESSA II Expedition in 2014 (results published in Current Biology; Mourier et al. 2016) which focused on the role and functioning of grouper spawning aggregations in providing subsidies to maintain an inverted trophic pyramid led by the biggest grey reef shark aggregation, here we come again!!!
The same team (including Laurent Ballesta and colleagues from Andromede Oceanologie) joined by other international researchers (including Yannis Papastamatiou, Charlie Huveneers, Damien Sonny or Eric Parmentier) went back to Fakarava for a new expedition GOMBESSA IV from June to July 2017. This expedition of course will lead to a nice documentary but one of the main aim is to conduct a cutting-edge shark science project! And trust me, all ingredients were involved to produce amazing science and better understand the behaviour of the biggest grey reef shark aggregation in the world!
In June 7th, I arrived in Fakarava with Charlie Huveneers (Flinders University) to help me install an array of Vemco VR2W acoustic receivers in the famous South pass of Fakarava. The idea was to cover the entire channel to better monitor shark movements in the channel. We conducted detection range tests to evaluate at which distance receivers can detect an acoustic tag. Our tests confirmed what I found previously (Mourier et al. 2016) that detection rate clearly dropped from 50 m. So we decided to install receivers 100 m apart. As such, 25 receivers were installed in the pass.
Later, in mid-June, another shark expert Yannis Papastamatiou (Florida International University) joined us to start catching and tagging grey reef sharks. We tested a new capture and tagging technique this year. Indeed, instead of hooking sharks, Laurent and his team was catching sharks underwater, putting them into tonic immobility, attaching a rope around the caudal fin linked to the boat so that we could bring the shark back to us. The shark was then placed into a stretcher alongside the boat to be measured, given a biopsy for isotope analysis and fin clipped for genetic analysis. A transmitter was inserted into its peritoneal cavity and sutured using a surgical stapler. These transmitters not only give the position of the shark within the array of receivers but also important additional data: depth and acceleration! With depth, we will be able to model the 3D movement patterns of sharks in the pass and with accelerometry we will be able to investigate and map activity of sharks and how they monitor energy expenditure especially during hunting phases!
Charlie left us after 2 weeks and Yannis also brought a great gadget to play with: a CatCam which is an animal-borne camera, i.e. a camera we attached to the dorsal fin of a shark and which detached after 2 or 3 days. During the deployment, the logger not only film what the shark see during the day but also record important fine scale behavioural data such as fine accelerometry in the 3 axes. In combination with the acoustic tags, we will better understand how shark hunt and how the manage their energy balance. We succeeded to deploy 5 CatCams and recovered all of them! While some detached earlier than expected, others revealed long-range excursions in open ocean.
CATS cam deployed on a grey reef shark showing how it socialises in the shark aggregation
After Yannis left our team on June 1st, I finished the tagging program to reach an impressive total of 38 grey reef sharks which will provide us with great data for a year. At the end of the trip, the team retrieved the receivers to download preliminary data and redeployed them the day after. During a day, I worked on analyzing the data and build a small animation showing all tagged sharks moving simultaneously in the pass (note that I used 32 individuals here because some sharks were tagged at the end of the trip so they did not provide enough data yet).
Of course this year, I also conducted several shark count surveys like in 2014 in which I drifted as fast as possible in the school of sharks to count them and avoid them to come back in frame (i.e. avoiding repeated count of same individuals).
During this expedition, we also used other tools to study sharks. For example, we used the 3 CCTV cameras from the previous expedition to monitor in real-time from our dry office what sharks were doing but also which potential preys are present.
But the year, Damien Sonny (ProFish) brought us a new interesting tool to monitor sharks not only during the day but also at night; a BlueView acoustic camera! It provide images like an echography but have the advantages to provide the same image at night, so ideal to compare the behaviour and activity of sharks and their prey between day and night.
And of course we conducted many night dives in the channel in order to record hours of shark hunting and better understand predator-prey interactions. These drifting dives into >500 sharks were conducted in all hours of the night and will inform us about the periods and locations of maximum hunting activities. Analyzing the slow-motion videos of predation will allow us to identify prey species and in combination with stable isotope analyses will allow us to better understand the trophic position of grey reef sharks in the pass of Fakarava.
Finally, with Eric Parmentier (University of Liege) we will investigate the sound produced by fishes at night and how it influences their probability of being predated.
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