Posted by Laurent Dagorn
27 April 2012
Over the past three weeks, the researchers aboard the Torre Giulia visited 25 additional FADs – collecting data from a total of 40 so far during this cruise alone. JD and Fabien dove on each one, conducting an Underwater Visual Census that has provided us with new information on how fish aggregate around FADs when there are no tuna present. This is one of the first times anyone has been able to collect data this way – to date, the only data that scientists have on these kinds of aggregations are from observers that are aboard vessels, not in the water.
Two weeks ago, they tagged 24 individuals of different species: silky sharks, yellowfin tuna, oceanic triggerfish, rainbow runners. At this FAD, we were also able to actively track a shark for three hours with an acoustic tag before it returned to the FAD. With this sort of information, we can view and begin to understand the distances these incredible animals travel when they leave the FAD. Everyday since the tagging of those fish, I am receiving data from satellite from this FAD, indicating if tagged fish are still around the FAD or if they left it. Some tagged sharks and yellowfin tuna left the FAD, some of them never returned and some came back after a few days. Today, two weeks after tagging, we still have 17 out of our 24 tagged fish around this FAD. It is quite unique to be able to remotely follow the behavior of these tagged fish around their FAD in the middle of the ocean every day.
Yesterday, the team has just tagged 17 fish on another FAD: skipjack, yellowfin and bigeye tuna, as well as oceanic triggerfish and rainbow runners. Our taggings are providing some incredibly useful and interesting information that will not only improve our knowledge on the behavior of fish around FADs, but will help us examine if there are some particular times of the day when some species leave the FAD while other stay associated.
The team onboard has also tried on 4 occasions to attract sharks and bycatch out of the net. The idea was to tow the FAD with or without chum and pass through a gap between the boat and the net. Most of fish seemed to be scared when approaching the vessel. The Torre Giulia’s skipper suggested the way we were proposing to release sharks inadvertently caught in the vessel’s nets, could be improved. He believes that rather than releasing sharks through the gap in the front of the vessel, we should try to attract sharks to another part of the net and design a new piece of equipment – a 15 meter long square with monofilament and large mesh, so that fish can escape. Scientists can really learn from working closely with experienced crew.
I’ve uploaded some images from the team on the Torre Giulia below.