As the largest fish in the sea, the magnificent and gentle whale shark can reach lengths of 41.5 feet or more and weigh up to 20 tons. Whale sharks are the biggest living fish, but their elusive nature makes it almost impossible for scientists to learn more about their migration patterns and reproductive habits.
When it comes to whale shark migration, scientists have more questions than answers. Due to a few successful tagging efforts, researchers agree that the sharks are highly nomadic, but they’re yet to understand the motivation behind the nomadic behaviors. The usually solitary creatures, come together for months in large aggregations to feed in plankton-dense waters. But after feeding, they drift off in random directions, completely disappearing during winter and spring. Based on a few inconclusive observations, scientists assert that the sharks move to areas in the southwestern Gulf in search of a safe place to ride out icy temperatures. Only data collected from successful tagging could substantiate this cherished hypothesis.
Researchers have conclusive answers to questions about the age-based and gender-based travels of other sharks. The same is not true for whale sharks. A careful review of large aggregations reveals a predominance of smaller sharks, but there’s insufficient evidence to conclude that larger sharks don’t migrate. Technological advances will, hopefully, provide greater insight into the migration patterns of the species. While marine enthusiasts await answers to a growing list of migration questions, previous attempts at satellite tracking, confirm that the sharks travel thousands of miles in search of food and a haven for birthing their young ones.
Whale sharks move intentionally slow when they’re feeding at the surface. With a swim speed of 2.3 mph when surface feeding, whale sharks are vulnerable to commercial fishers’ harpoons and nets. When threatened, a whale shark can accelerate their movements to 3 to 4 mph, which is still slow by a shark’s standard. The gigantic fish appears surprisingly graceful, moving its entire body from side to side as it navigates the vast seas. A study of whale sharks in the coastal areas of China found that the surface dwellers spent approximately 45% of its time above the 10m water depth. This tendency to dwell on the surface when foraging for food poses serious complications for conservationists.
How Diet Influences Migration
With formidably sized jaws that open up to 4.6 feet wide, whale sharks can prey on almost any underwater delicacy. Fortunately, for sea-dwellers and deep sea divers, the colossal creatures prefer to feast on the smallest fishes, crustaceans, and tiny planktons. They gulp mouthfuls of water in their huge mouths, forcing it past their gills, which filters over 1,500 gallons of water an hour. The whale shark’s denticles and pharynx (a rake-like structure) trap prey as the water filters in and out.
Whale sharks are usually solitary and nomadic, spending most of their lives wandering the shadowy ocean’s depths in search of plankton – their food of choice. Their preference for plankton contributes, in part, to their nomadic habits as they drift in water temperatures of 21°C to 25°C surrounding atolls and reef regions. During the summer, the gentle giants migrate to the nutrient-rich waters near the Yucatan Peninsula of Mexico. Other hot spots include Belize, Japan, Honduras, South Africa, Australia, the Galapagos, and the Seychelles. The predominantly male population (almost 70% based on several reports), gather by the hundreds to feed on plankton. No one knows exactly how the sharks discern when and where to locate the nutrient-rich waters. However, scientists use the opportunity to analyze the creatures, ranging in sizes from 8.2 feet to 32 feet, and learn more about their elusive habits. Using satellite tags, photo-identification, and conventional tagging systems, scientists have, so far, been unable to determine where the creatures come from and where they go after they fill up on plankton.
After years of tagging and tracking, researchers are in agreement on one important discovery: whale sharks will return to nutrient-rich feeding areas in subsequent years. Based on the path traveled by one female shark from the coast of Quintana Roo, scientists hypothesized that female whale sharks travel to the open waters of the Mid-Atlantic where they give birth in relative seclusion.
Reproduction and Migration
Scientists’ knowledge of whale sharks’ reproduction hinges solely on the observation of a pregnant female caught in Taiwan in 1995. The discovery of 304 embryos in various stages of development confirmed that whale sharks are viviparous, meaning they give birth to live young. Beyond that fact, there are huge gaps in scientific knowledge regarding reproduction and mating behaviors.
Female sharks increase the chances of their young ones’ survival by sheltering developing embryo is inside their bodies. Females are not as visible in the aggregates during the summer as their male counterparts. Scientists conclude that they stay out of the feeding frenzy in order to protect their young ones.
Attempts to track the progress of one female shark, Rio Lady, confirm that pregnant females migrate long distances to give birth near remote islands where the baby sharks will be out of the reach of common predators.
Researchers desire to learn more about the correlation between migration and reproduction to safeguard females and their young ones from predators. They hope that repeated attempts to determine the shark’s travels, feeding cycles and reproduction will one day prove successful and shed some light on well-concealed habits.
Climate Change and Migration
After a 16-year study of whale sharks’ migration behaviors, researchers discussed the effects of climate change on the shark’s habitat in a Plos One report. The increasing global sea temperatures drove hundreds of sharks miles away to the ocean surrounding the volcanic Azore Islands – an area in Portugal that boasts nine volcanic islands in the North Atlantic. The report concludes that whale sharks prefer cooler temperatures even though they can thrive in 26 to 30-degree Celsius water. Researchers call the water around the Azore islands the “Goldilocks” temperature as it is not too cold or too hot, but just right.
By analyzing the results of various studies, researchers agree that the patterns of whale sharks’ migration depend largely on their habitat and features, such as food availability, sea surface temperatures and the instinctive need to protect their young. Their response to external stimuli proves that whale sharks, for all their mystery, have much in common with the rest of the animal kingdom.