Olive Ridley and Leatherback Turtles Both Choose Indian Shores for Nesting, What Our Coastline Reveals

An adult Olive Ridley weighs around 45 kilograms. An adult Leatherback can weigh over 900 kilograms, roughly the mass of a small car. These are not variations on the same animal. They are, in almost every measurable way, opposites that happen to share an order.The Olive Ridley (Lepidochelys olivacea) has a hard, bony carapace with distinct scutes, a diet built around crustaceans, fish eggs, and jellyfish, and a body plan that has remained largely unchanged for millions of years. The Leatherback (Dermochelys coriacea) has no bony shell at all, its back is covered in a leathery, oil-saturated skin reinforced by thousands of tiny bone fragments called osteoderms. It is the only living member of its family, Dermochelyidae, a lineage that diverged from other sea turtles roughly 100 million years ago. Its diet is almost exclusively jellyfish, which means it can thrive in waters cold enough to kill most reptiles. Leatherbacks have been recorded diving to over 1,000 metres.One is a gregarious, warm-water nester that travels in enormous aggregations. The other is a solitary, wide-ranging deep-diver that tolerates near-polar temperatures. Both nest on Indian shores.

Gahirmatha Marine Sanctuary in Odisha is the world's largest known Olive Ridley rookery. Rushikulya, also in Odisha, is the second major mass-nesting site. Every year, between January and March, female Olive Ridleys congregate offshore in numbers that can exceed 500,000 before moving onto the beach in the mass synchronised event called an arribada.The Leatherback's Indian nesting grounds are quieter and less documented. Little Andaman and Great Nicobar Island in the Andaman and Nicobar archipelago hold the most significant Leatherback nesting populations in Indian territory. The beaches there, long, dark-sand stretches with direct access to deep water, suit an animal that cannot haul its enormous body far from the surf. Leatherbacks nest in smaller numbers, individually, between January and May.The two species do not share the same beaches. Their requirements are different enough that they have, effectively, divided the Indian coastline between them.

The arribada (Spanish for "arrival") is among the most dramatic biological events on the planet. Females that have been feeding hundreds of kilometres offshore synchronise their approach to the beach over a period of days, responding to cues that researchers believe include lunar cycles, wind direction, and chemical signals in the water. Once the arribada begins, the beach fills so completely with nesting turtles that later arrivals dig up the eggs of earlier nesters.Each female lays roughly 100 to 110 eggs per clutch, covers the nest, and returns to sea. The incubation period runs 45 to 60 days. Temperature inside the nest determines the sex of the hatchlings, warmer sand produces more females, a fact that makes rising beach temperatures a direct threat to population sex ratios.The Odisha coastline's gently sloping, wide beaches and the relatively low human settlement density near Gahirmatha are what make the arribada possible there. The Forest Department of Odisha, working with the Wildlife Institute of India, has documented and protected these beaches for decades, though fishing trawler activity in the offshore aggregation zone remains the single largest cause of Olive Ridley mortality in Indian waters.

The Andaman Sea is one of the most productive jellyfish zones in the Indo-Pacific. For an animal whose entire caloric intake depends on a prey item that is 95 percent water, productivity and density of jellyfish blooms matter enormously. A Leatherback must consume enormous quantities of jellyfish daily simply to maintain its body mass, estimates suggest upward of 73 percent of its body weight in jellyfish per day during active feeding periods.The deep water immediately adjacent to the Andaman nesting beaches is the other factor. Leatherbacks cannot rest comfortably on shallow continental shelves the way Olive Ridleys can. The Andaman and Nicobar Islands sit at the edge of the Andaman Sea's deep basin, giving nesting females direct access to foraging depth within a short swim of the beach. The 2004 Indian Ocean tsunami caused significant damage to Leatherback nesting habitat on Great Nicobar, and population recovery there has been slow. The Leatherback is listed as Vulnerable on the IUCN Red List globally, though some Pacific subpopulations are classified as Critically Endangered.Protecting Leatherback nesting in India is complicated by the remoteness of the sites and by the low annual nest counts, some years, researchers record fewer than 100 nests on all Indian Leatherback beaches combined. Low numbers make every nest count in a way that the Olive Ridley's mass nesting, for all its fragility, does not.

The Olive Ridley's preference for Odisha and the Leatherback's preference for the Andamans are not accidents of geography. They are the result of millions of years of natal homing, the behaviour by which female sea turtles return to the beach where they hatched to lay their own eggs. The beaches these turtles use today are beaches their ancestors used. The fact that both species still find Indian shores viable for nesting means the conditions those ancestors required have not collapsed entirely.That is a specific and limited piece of good news. Olive Ridley numbers at Gahirmatha have fluctuated sharply with trawler bycatch rates. Leatherback nests on Great Nicobar face pressure from the proposed development of a transshipment port and township on the island, a project that has drawn concern from wildlife biologists and the Bombay Natural History Society. The two turtles represent opposite ends of a spectrum, one species that nests in such numbers that the beach itself becomes the conservation unit, and one that nests so sparsely that the loss of a single female has measurable population consequences.The Olive Ridley and Leatherback have nothing obvious in common except the Indian Ocean and the instinct to return. What they share, without knowing it, is a coastline that still offers what they need, and a conservation record that has protected some of it, while threatening other parts with the same hand.