The first techniques involve diving a lot with the sharks and getting to know them up close and personal. This means learning to differentiate between individuals by the spot patterns on their dorsal- and tail fins, estimating size with a diver next to it, and “checking under the hood” for the sex. If it’s a male it will have two large pencils known as claspers under the pelvic fins. Most of the sharks that we have recorded at Gladden Spit near Placencia Belize are juvenile males. Unlike bony fish, sharks reproduce using internal fertilization as opposed to spawning. We do not yet know if Gladden spit is also a mating area for whale sharks, although one large male showed evidence of having recently mated.
We take underwater pictures and video, and all of this information is recorded on a sheet for each individual and attached to photos for easy identification. Information on the whale shark individuals is currently being cataloged and will be deposited with the local conservation organization the Friends of Nature and with the Department of Fisheries and included in the global whale shark database, an initiative spearheaded by the UK-based Shark Trust . To rapidly recognize individual sharks, establish counts and promote participation in the research we tag the whale sharks with numbered marker tags.
The marker tags also help us determine movement: two dive groups saw a whale shark with our tag north of Cancún, Mexico, over 600km away from Gladden Spit! What if the sharks are present and you don’t see them? Or what if it’s night or the weather is bad and we can’t be on the sea for stretches of time? To help us get around these difficulties, we also tag some of the whale sharks with acoustic or “pinger” tags.
Each pinger emits a different pulse of sound that helps us differentiate between the sharks. The sound is then picked up by underwater receivers stationed at different points along the reef which record shark presence and absence, day and night, month after month, good weather or bad. These tags and receivers have told us that the same whale sharks are coming back to Gladden every year and arrive when the fish start to spawn. The receivers also tell us that the sharks often leave after the two-week spawning moon and return for the next moon with the fish, only to leave for the rest of the year. So where are whale sharks going to between spawning moons and where are they the rest of the year? To try and elucidate this mystery we are using the latest technology: satellite pop-off tags.
The satellite tag is an amazing piece of technology. A mini-computer surrounded by dense floating foam with an antenna at one end, the tag records depth, temperature and light levels that the animal is diving to – every minute of the day for as long as it stays on the shark. At a pre-programmed date, the tag detaches itself from the shark and floats to the surface. The information is transmitted to a satellite overhead, and sent to us as an email. The tags have revealed that whale sharks can dive beyond 700m, and withstand temperatures below 80 Celcius for short periods of time. They appear to make most of the deep dives between the full moons when the snappers are taking a rest from spawning, and are therefore possibly looking for food at depth.
We only have part of the whale shark puzzle. We still need to know how do they know to arrive when they do and what other sites are they visiting, what is their fine-scale movement behavior between and beyond the fish spawning moons, and how much energy are they using to find food? The only way to get the necessary information in time to protect this phenomenally charismatic creature is by working as a team locally, regionally and globally. We are thankful to work with local fishermen, tour-guides, local conservation organizations such as Placencia’s own Friends of Nature, the government and other researchers – and as a team, we are pooling our pieces of the puzzle and finally beginning to see the larger picture of the life and ways of our giant ambassador of the seas.
Article Contributed by Dr. Rachael Graham