Whale Sharks, Belize's Banquet Of Behemoths
by: >Rachel Graham, Research Association, University Of York for the Placencia Breeze
Whale sharks are the true behemoths of the fish world: they can grow longer than a bus (up to a purported 18 meters long) yet eat little animals no bigger than your finger - known as zooplankton- by filtering them out from the surrounding water, similar to a whale's feeding strategy. However, finding these big fish is not always that easy. Only 9 places in the world, all located in tropical waters, are known so far to have predictable whale shark visits, and Belize is one of the few chosen countries that the whale sharks predictably visit.
Known locally as "Sapodilla Tom" for the fisherman who first saw them near the Sapodilla Cayes on the southern end of the Belize Barrier Reef, whale sharks congregate at Gladden Spit every March to June. Shaped like an elbow pointing out to the Caribbean Sea, Gladden Spit is also a very important place for over 25 species of reef fish that come together at specific times of the year, usually right around the full moon, to reproduce. Although some reef fish species reproduce in pairs or in small groups, dog and cubera snappers aggregate in the hundreds or even thousands to reproduce, known as spawning aggregations. During a spawning event, female and male fish rise in the water as groups, releasing millions of eggs and sperm. The fertilized eggs float and are carried by currents until the larvae hatch. A healthy aggregation can produce hundreds of billions of eggs. And like the eggs on our breakfast table, they contain a lot of energy which makes this caviar very attractive to whale sharks. However, the eggs need to be in dense clumps for them to be attractive to the whale sharks and spawned eggs disperse very quickly. So the whale shark and many other species of fish that feed on the eggs are only able to capture some of the fertilized eggs before it is no longer worth their while to target this food.
Local fishermen and tour-guides know the fish's ways and means better than anyone else. Yet they too have questions about the why's, how's, when's and wherefore's of fish biology that we as marine biologists can help answer. This makes for a good collaboration! They were curious to know where the whale sharks came from and where they went to after the peak spawning time of April and May. Did the sharks stay around Gladden but remained deep of did they traveled far away, and if so how far? We had our work cut out for us. We knew from other research results in the Pacific that whale sharks travel across the Pacific Ocean from Baja, Mexico covering almost 13,000km. So it was possible that the sharks in the Caribbean were doing the same thing. But were they stopping off at other places along the way, and were the same sharks coming back to Gladden every year? Also we had no idea how many whale sharks visited Belize or even exist in the world. It is important to study whale sharks since we know so little about their life history and they are becoming an important tourism resource, and primarily because predictable sightings have dramatically declined at certain sites in the Indo-Pacific and greater Pacific Oceans, regions where they are fished for their meat and fins. The consensus among shark biologists is that worldwide populations are low.
So we set out to try and solve the mysteries and find the pieces to this big puzzle. Not an easy task! Imagine having to work out in the sea 40km from the mainland, often in seas 2-3m high with large waves - all in small open 7m boats. How do you keep an eye on the different sharks, count them and make sure you're not counting the same individuals over again? Also how do you know if, where and when they leave Gladden Spit? To get around all of these problems we use a range of different research techniques that we've been able to try-out thanks to the financial support from the UK Darwin Initiative and the Natural Environmental Research Council.
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 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 catalogued 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 (www.sharktrust.org). 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 80Celcius 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. This is particularly important as 2002 is the year in which the Convention on the International Trade of Endangered Species (CITES) will table a proposal later this year to list the whale shark on the Appendix II list, regulating its trade and supporting its conservation. We are thankful to work with local fishermen, tour-guides, local conservation organizations such as Placencia's own Friends of Nature, the Belize 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.
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