By Alison Green, senior marine scientist
Recently I had one of the most exhilarating experiences of my life – swimming with whale sharks.
These magnificent and awe-inspiring gentle giants are the largest fish in the world, and for a fish biologist like me, this was on the top of my bucket list.
Imagine this. One minute we were hanging around in downtown La Paz on the coast of Baja California in Mexico. Then we got into a small boat and headed out into the bay; just 10 minutes later we were surrounded by about 10 of the world’s largest fish casually swimming around us!
We worked out what direction they were swimming in, slipped into the water in front of them and before we knew it: there they were, right in front of us. The huge, spectacular fish slowly cruised by. Wow!
We swam like crazy to try and keep up. We eventually saw the head, the body and then the tail cruise by and out of sight. Unbelievable: it was easily one of the best experiences of my life.
Meet the Whale Shark
Whale sharks are distributed in tropical and warm oceans around the world), mostly between the latitudes of 30o N and 35o C. They are typically found in the open ocean where the sea surface temperatures range between 28 and 32 degrees Celsius (82 and 90 degrees Fahrenheit), although they do range to depths of 240 meters or more where temperatures can go below 10 degrees Celsius (50 degrees Fahrenheit).
Whale sharks are migratory, and their movement patterns appear to be related to physical and biological oceanographic features, such as seamounts and boundary currents, where primary productivity (and therefore their food) may be enhanced.
Satellite tracking has shown that they range over huge distances, with one individual tagged in the Gulf of California moving to the Federated States of Micronesia in the western Pacific Ocean, covering approximately 13,000 kilometers of ocean over a period of three years. (Eckert and Stewart 2001).
While they are generally slow powerful swimmers (moving around 24 kilometers a day), they occasionally move up to 96 kilometers a day.
At certain times of the year, whale sharks form predictable feeding aggregations at several coastal sites around the world.
For about six months of the year (peak season is from October to March), they return to the coastal waters off La Paz in the Gulf of California. This is one of the few places that people can reliably have the exhilarating, awe-inspiring experience of a lifetime swimming with these majestic animals in their natural environment.
Whale sharks are the largest known species of fish, with the largest recorded individuals growing to 16 to 20 meters long and weighing as much as 34 metric tons.
The ones that we swam with in La Paz were juveniles, probably ranging from 7 to 9 meters in length. But it’s hard to tell when they are so big!
Whale sharks have a lifespan of about 70 years, and their name comes from them being as large as some species of whale and the fact that they are filter feeders — also like some whales.
Whale sharks hold many records for sheer size in the animal kingdom, most notably being by far the largest living non-mammalian vertebrate, rivalling many dinosaurs in weight. The species is also quite old, originating approximately 60 million years ago.
Unlike other big sharks, swimming with whale sharks is relatively safe because they are filter feeders and have a docile nature.
These gentle giants have very large mouths that can be 1.5 meters wide. They also have 300 to 350 rows of tiny teeth and 10 filter pads that they use to filter feed, feeding on some of the smallest creatures in the ocean (mostly crustaceans, although they also eat algae, small fish and occasionally squid).
Whale sharks have two types of feeding behavior depending on the type and density of their prey.
The ones that we saw were moving slowly through the water with their mouths open, sucking in thousands of liters of water and harvesting food from the nutrient-rich waters of the Gulf of California.
Sometimes, they remain stationary and vertical in the water column, and feed by pumping prey-laden water in through their mouths and out over their gills.
Whale sharks tend to segregate by sex and size. Scientists believe that the juvenile whale sharks (mostly males, less than 9 meters total length) that we saw in La Paz may be aggregating inshore to take advantage of the seasonal abundance of a preferred prey (microscopic crustaceans called copepods) needed for their fast growth rates.
Adults (mostly females, greater than 9 meters total length) tend to be found in small aggregations in oceanic waters offshore, where they feed on diffuse patches of other crustaceans (euphausids).
Can Tourism Help the World’s Largest Fish?
Once plentiful in the world’s oceans, whale sharks are now vulnerable to extinction and protected by international law.
However they are still targeted by fishers in some areas, such as in Southeast Asia.
Fortunately, tourists are willing to pay for the privilege of swimming with these impressive creatures, and this has become an important ecotourism activity in areas where whale sharks aggregate to feed.
Hopefully their interactions with people will help save these magnificent animals by protecting them from overfishing — provided the industry is managed in a way that ensures the safety of both the whale sharks and the people who swim with them.
The Nature Conservancy is contributing to the protection of these gentle giants, since La Paz is one of our priority areas in the Baja Marine Initiative, where we are working with local stakeholders to develop and apply innovative conservation and fisheries management tools.