Chagos 3,0 - Day 5

Day 5 finds us exploring the northern edge of the great Chagos Bank, terra (or rather aqua) incognita for us. With the important servicing work all but completed, we are focussing our efforts on getting almost 100 electronic tags out on sharks and manta rays, the focal species for this study.

We carry three types of tag on this trip. Our workhorses are the acoustic transmitter tags that communicate with our receiver network. We have acoustically tagged 92 sharks so far in this programme and will be adding another 75 animals to our group on this trip.

Whilst the acoustic tags can tell us about the fine scale movements of shark around our monitored reefs, satellite archival tags collect information on position, depth and temperature wherever the shark goes, and then detatch from the shark, float to the surface, and transmit the data back to the lab via a satellite. This allows us to see, amongst other things, the large scale movements and the diving behaviour of sharks which is vital to understanding how they use a large area like the Chagos Reserve.

Lastly, Dr Taylor Chapple has brought his camera tags with him again this year, and with three deployments already we are building up our shark’s eye view of Chagos.

But before we can deploy any of this technology, we must first catch a shark. This involves early starts and late finishes to take advantage of the fact that predators are often more active at dawn and dusk, and so our chances of an encounter are improved. We use barbless circle hooks that minimise the chance of the shark swallowing the hook, and make for simple removal of the hook after tagging. Baited with fish, the hooks are streamed off the back of the boat and tended by one of the team at all times so that as soon as a shark takes the bait we can bring it on board and tag. It’s all about ensuring the welfare of the animal, so before we lift a shark out of the water we make certain that everything is ready to making the tagging as quick a process as possible, and that the padded tagging mat and the saltwater hose to pump water over the shark’s gills are ready.

A hose is inserted in the shark's mouth to keep oxygen-rich
water flowing over its gills, while the scientists attach an
acoustic tag

Handling even a small shark has to be approached with care as they very strong for their size –both the head and tail are controlled while they are lifted, and once on the mat at least two of the team control the shark while the others operate. The Stanford team have years of tagging experience between them, and their guidance makes sure we stay safe on the swim step.

A small shark is returned to the sea

Acoustic tagging involves a quick incision in the abdominal wall through which the tag is inserted, then a suture to close it. While this is happening another scientist takes blood and biopsy samples for genetic and other tests. Lastly, a marker tag is placed just below the shark’s dorsal fin, identifying the shark as having been tagged internally so that we will know if we capture it again next year. Both the satellite and camera tags are attached externally, one with a titanium dart just below the dorsal fin and the other to the fin itself. Both are used in combination with the acoustic tags, so that multiple types of data can be collected from the movements of a single animal. This increases the amount we can learn from our sample of sharks and rays.

So far this trip we have tagged 20 sharks – with 5 days left in Chagos the pressure is high to keep up the work rate!

      -Dave Tickler