Going far, deep and cool!

Tagging a whale shark is easy compared with other species such as, for example, bull sharks. Whale sharks spend a lot of their time very close or even at the surface and are relatively slow moving animals. So all it takes to be successful is finding a whale shark at the water surface and a snorkeller with a speargun. That’s what we did back in February 2006 off the coast of southern Mozambique. We attached marker tags and two pop-up satellite archival tags to whale sharks and at the same time took tissue samples. These samples helped to reveal the genetic structure of the Earth’s largest fish.

One of the two pop-up tags popped up prematurely after only seven days. The other one popped up prematurely as well (that happens to a lot of pop-up tags!), but only after being attached to the female whale shark for 87 days. The first transmission was received from the southeast coast of Madagascar making this the first whale shark that could be reliably tracked into Malagassy waters. The point-to-point horizontal movement of 1200 km was not that impressive given the whale sharks probable ability to show large-scale movements (although there are only very few reliable tracks that really confirm this), but it meant that the animal had to cross the very deep waters (5000+ m) of the southern Mozambique Channel. Looking at the depth data from this animal, one can easily see that it showed some remarkable diving behaviour. Several dive profiles during the crossing of the Mozambique Channel and then again in the Madagascar Basin southeast of Madagascar include dives to well below 1000 m. The deepest recorded depth was 1286 m, which is the deepest ever directly recorded (published) depth for any elasmobranch. The animal did probably even dive deeper, but this was the maximum depth this tag type could record. In the meanwhile, whale sharks in other geographic locations were recorded diving to depths well below 1500 m! So these are truly specatcular divers! The temperature that the tag recorded at 1286 m was 3.4 °C – chili! If you’re interested in the full description of the horizontal and vertical movement of this individual in the western Indian Ocean, have a look at the paper we just published.

Why do whale sharks perform these deep dives in bathymetrically non-constraining habitats? Well, we can’t be sure about this at this stage, but looking at individual dive profiles and analysing them with appropriate statistical tools, we believe that whale sharks (as many other ocean predators) apply different search strategies depending on the habitat they are moving in. There is more to come from this whale shark. Watch this space!