Turtle-unfriendly fishing practices, development and exploitation of nesting beaches for ports, and tourist centres have disturbed the breeding of the olive-ridley species across the planet Dominic Tilley

The largest breeding colony in the Atlantic of the vulnerable olive ridley turtle has been uncovered in the central African country of Gabon.

A study from the University of Exeter combined existing monitoring data with a back-to-basics coastal survey of the area to find the colony which could have around 9,800 nests per year compared with the 3,300 in French Guiana and 3,000 in Brazil.

Dr Kristian Metcalfe, lead author from the Centre for Ecology and Conservation (CEC) at the University of Exeter who undertook the coastal survey with colleagues, said: "Conservation efforts for sea turtles can be hampered by their migratory life cycles, which carry them across jurisdictional boundaries and international waters. That makes this first population assessment which covered extensive areas of Gabon's coast outside of monitored regions all the more valuable and worthwhile, and demonstrates the importance of focusing beyond intensively monitored beaches."

Considered one of the most abundant of the marine turtles, the olive ridley is in trouble. Their numbers have been declining over the past few years, and the species is recognised as vulnerable by the IUCN Red list.

Turtle unfriendly fishing practices, development and exploitation of nesting beaches for ports, and tourist centres have disturbed the breeding of the species.

Rough estimates put the worldwide population of nesting females at about 800,000, but in the western Atlantic in particular the numbers have declined precipitously.

Eggs are taken and nesting females are slaughtered for meat and skin. Fishing nets also take a large toll, frequently snagging and drowning these turtles.

Growing to about two feet in length, and 50kg in weight, the olive ridley gets its name from its olive coloured carapace, which is heart-shaped and rounded.

The turtles are solitary, preferring the open ocean. They migrate hundreds or even thousands of miles every year, and come together as a group only once a year for the arribada, when females return by the thousands to nest on the same beaches where they hatched.

Olive ridleys have nesting sites all over the world, on tropical and subtropical beaches. Females lay about a hundred eggs, but may nest up to three times a year. The nesting season is from June to December.

It is estimated that approximately only one hatchling survives to reach adulthood for every 1,000 hatchlings that enter the sea waters. This may be the reason why arribadas happen and a single female lays 80 to 120 eggs and sometimes even twice in a season, says WWF.

The coast of Orissa in India is the largest mass nesting site for the olive-ridley, followed by the coasts of Mexico and Costa Rica. The number of nesting sites in India however decreased from 637,000 in 2011 to 172,800 in 2012.

The present study is hoped to help the Gabon government in managing and conserving the species.