The dwindling numbers of leatherback turtles are signalling a threat to biodiversity in the oceans. A mathematical model based on our assessment of a once-large leatherback population predicts that unsustainable adult mortality, apparently due to human fishing activity, will soon drive this population to extinction. In 1982 there were 115,000 adult female leatherbacks in the world, but in 1996 there were only 34,500. However, many accounts of decline were based on anecdotal information and indirect measurement. Individuals could not be reliably identified over a period of many years, and the reproductive effort of individual turtles was seldom determined during a given year. Therefore, although leatherbacks had disappeared from India before 1930, declined to near zero in Sri Lanka by 1994, and fallen from thousands to two in Malaysia by 1994, we did not have enought data to predict the fate of other colonies. Since 1998 we have studied leatherbacks at Playa Grande, Costa Rica, the fourth largest nesting colony in the world. Since 1993 we have permanently identified female turtles by injecting them with passive integrated transponder tags. We encountered more than 95% of nesting turtles each night. This enabled us to determine how many nests a turtle laid, how many individual turtles nested in a given year, and how many turtles returned to nest in later years.
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