Reassessment of the olive ridley sea turtle Lepidochelys olivacea nesting population at Nancite Beach, Costa Rica

Nancite is a fully protected beach within the Guanacaste Conservation Area, Costa Rica where olive ridley (Lepidochelys olivacea) sea turtles nest in synchronous mass-nesting events known as arribadas. Arribadas decreased in magnitude at this beach by approximately 90% during the period 1971–2007 due to unknown causes. The total count estimate of females nesting in a year also decreased over these decades. In the present study, from August 2009 to February 2021, the trend of arribada nesting female abundance and estimated annual production of hatchlings were assessed and compared with previous trend analyses. A total of 62 mass-nesting events were quantified in the period 2009–2021, with an estimated annual average of 64,694 nesting females. Trend analysis indicated that during our recent study period, the number of females per arribada event increased by an estimated 14% (8%, 20% CI_95%). During this period, a mean hatching rate of 33.4% was estimated, corresponding to an overall estimated production average of 2,165,597 per season, which represented an increase of 82.7% compared with a previous estimate for the period 1980–1984, when the arribadas were larger. We suggest that the growth in hatchling production over the past four decades is at least in part responsible for the slight increase in the estimated size of arribadas since 2009/2010. However, because maximal hatching success did not increase above 60%, the beach may be close to reaching carrying capacity, and it seems that high hatchling production may soon level off or begin to decrease. Thus, it is not clear whether current hatchling production will lead to the recovery of the Nancite population to the historical levels documented in the 1970s and early 1980s.