The whale shark (Rhincodon typus) is the largest fish in the ocean, yet the specifics of its life cycle remain a scientific puzzle. This highly migratory species travels vast distances across tropical and temperate waters worldwide. Because their movements are extensive and sightings of their young are incredibly rare, the location where the whale shark gives birth is one of the deep ocean’s most enduring mysteries. The challenge of studying an animal that spends significant time in remote, deep-water environments contributes to the difficulty researchers face in understanding its reproductive habits.
The Reproductive Strategy
Whale sharks employ a reproductive method known as ovoviviparity, which involves internal development. The female retains the fertilized eggs inside her body, where the pups hatch and continue to develop before being born live. This strategy protects the developing embryos but means there are no easily traceable egg cases left in the environment to pinpoint a birthing location.
This reproductive secret was confirmed by the capture of a single pregnant female off the coast of Taiwan in 1995. Researchers discovered she was carrying over 300 embryos, a number far exceeding that of any other known shark species. The embryos were found at various stages of development, suggesting the female may stagger the births of her young over a long period. The pups, measuring between 40 and 60 centimeters at birth, are immediately independent.
The Global Search for Birth Sites
Despite decades of research, the precise location where female whale sharks give birth—known as the parturition ground—remains elusive. Scientists hypothesize that birth occurs in the vast, remote pelagic environment of the open ocean, far from coastal shelf waters. This offshore birthing strategy would offer protection for the vulnerable newborns from coastal predators.
Tracking pregnant females with satellite tags is a primary method used to locate these sites, but the limitations of technology and the animals’ behavior present hurdles. Tagging data shows that large females often undertake deep dives, frequently descending beyond 2,000 meters, which can exceed the operational depth limits of tracking devices.
Recent studies suggest a potential link between the rare sightings of neonate whale sharks and areas of the ocean known as Oxygen Minimum Zones (OMZs). These are regions where dissolved oxygen levels are naturally low at depth. Researchers propose two possibilities for this connection:
OMZ Hypotheses
One hypothesis is that females intentionally give birth near these zones because they offer a refuge, as many large predators tend to avoid the low-oxygen water. The other theory is that the neonates are born randomly but are then restricted by their physiology to remain in the more oxygenated surface waters above the OMZs, making them more likely to be encountered by chance.
Known Juvenile Nursery Habitats
While the birthing grounds are unknown, scientists have identified several nursery areas where juvenile whale sharks congregate to feed and grow. These habitats are characterized by abundant plankton and small fish, providing the food source for the young sharks to thrive. Juvenile sharks typically first appear at these feeding aggregations when they reach a length of approximately 2.5 to 4 meters.
One notable example is the Bird’s Head Seascape in West Papua, Indonesia, where aggregations consist predominantly of juvenile males. The Gulf of Tadjoura in Djibouti also hosts a significant number of small juvenile individuals. In contrast, the waters around Darwin Island in the Galapagos Archipelago see a seasonal influx of very large, likely pregnant, females, suggesting the area serves as a stopover point before the females move into the open ocean for parturition.
These aggregation sites are feeding and developmental habitats, not the birthing grounds themselves. This highlights the “lost years” period, during which newborn pups, measuring less than 1.5 meters, disappear from observable waters until they are large enough to join the coastal feeding groups.
Conservation and Population Health
The lack of knowledge regarding the whale shark’s birthing grounds significantly complicates efforts to ensure the species’ survival. Whale shark populations have declined globally, leading to their classification as an endangered species. This population decrease is driven by threats such as accidental capture in fishing gear, ship strikes in coastal feeding areas, and habitat degradation.
Protecting a species that is highly migratory requires international cooperation, which is difficult when the place of birth cannot be identified. If the specific areas where the young are born and spend their first months remain unknown, conservationists cannot implement targeted protections. Understanding the role of deep-ocean features, like Oxygen Minimum Zones, in the reproductive cycle is an important step toward safeguarding future generations of these ocean giants.