Eye-to-eye with a krill swarm
Yesterday morning before breakfast we started to see dispersed layers of krill at depths of about 50-60 metres. We decided to target trawl because the pattern of this layer looked interesting to our esteemed acoustician, Gavin. Great team work led to a successful trawl with krill hunters, Jess and Maddie, directing the crew to submerge the net to the correct depth, while Abbie used the SH90 (an omni-directional sonar) to provide information to the bridge on the direction to the krill layer. The trawl was a success and we caught about 2000 live krill in good condition.
We were in split minds as to whether we should conduct the first ever swarm study of its kind on this krill layer. The krill we caught with the trawl were from a thin layer, only four metres thick, but extending horizontally for more than one kilometre. It was not ideal as our criteria for a suitable aggregation to pursue a successful swarm study is:
- a dense discrete swarm;
- a centre depth between 30-55m (it cannot be deeper than this because the video cameras rely on natural light to avoid any effects on behaviour caused by artificial lighting);
- a horizontal length of 200-300 metres.
However, given the superb weather with almost no swell, we thought it would be an excellent opportunity for a first go, and to run through the whole swarm study procedure to get our processes right for when we encounter a denser swarm in the future. We reminded ourselves not to have any high expectations. Any krill captured in the footage would be a bonus!
We first mapped out this huge thin layer of krill over a couple of hours using the ME70 (a multibeam echosounder) to get a grasp of the layer and find the best spot for a swarm study deployment.
While the mapping was being conducted, preparation for the deployment of the system by the krill and acoustic teams progressed under the direction of Rob who designed this unique system. The main frame of the system is covered with multiple cameras facing various directions to record individual krill’s behaviour in a school in 3D. Additionally, an autonomous echosounder WBAT (we call it Wombat!) is mounted in the centre of the frame to record the location of individual krill and their behaviour in 3D.
The significance of this system is that (if successful) we are able to analyse the collective behaviour of krill in the natural environment for the first time, and directly relate the behaviour of individual krill to echosound signals.
Alicia (who will also be undertaking the video analysis of krill behaviour) has been responsible for setting up the cameras for the deployment. A generous last minute loan of the WBAT by CSIRO, to replace AAD’s unit which became faulty just before sailing from Hobart, completed the novel frame. Now all we had to do was deploy it and then get it back.
Gavin, the acoustic guru, called out the centre depth of the layer reading out from the echosounder. The deck team quickly inserted the colour-coded rope of the correct length to adjust the depth of the frame to the nearest 2.5 metres within a matter of 40 seconds. As the thickness of the layer was only four metres, it was absolutely essential to get this length right.
The deployment used the stern A-frame. At the time when the ship’s stern passed above the spot the yellow main buoy was released, followed immediately afterwards by the main frame.
The deployment went smoothly. The ship moved away from the camera and Dale and Gavin began tracking the position of the camera for the next hour using the ship’s SH90 sonar and via a GPS tracker affixed to the buoy, before making the decision to approach the system and retrieve it.
Rob fired the pneumatic grapple gun to catch the lead rope extending from the system in order to pull the system close to the ship so it could be retrieved to deck. Straight out of grapple gun training school, he hit the target first shot! He looked visibly relieved. The system was then retrieved using the A-frame; a carefully choreographed operation involving the crew and science team.
The entire process included lots of excitement, with nice weather and lots of photo opportunities due to the backdrop of icebergs. However, the real excitement came after viewing the footage.
KRILL! All lined up neatly and heading in the same direction. The school seemed not affected by the camera system. Then another excitement. The Wombat successfully recorded echoes from the krill that matched with those filmed by the cameras.
We know that it is too early to get too excited, but we still have enough ship time to attempt another 8-10 deployments for the swarm study. We just need to find the right swarm to target. The crew and the science team eagerly await the next opportunity to come face to face with our quarry and in so doing, unlock the patterns of individual distribution and behaviour within the superorganism that is a school of krill!
So, Rob, and Gavin
Follow the ship’s track at: https://www.marine.csiro.au/data/underway/