Estimating carrying capacity at the green turtle nesting beach of East Island, French Frigate Shoals

Abstract

Many sea turtle populations are at a fraction of their historical abundance, and under- standing ecological processes, under current and climate change scenarios, is critical for establishing recovery goals. In the Hawaiian Islands, the nesting population of the green turtle Chelonia mydas on East Island, French Frigate Shoals, has been recovering at a rate of 5.7% per year. Climate change models, however, predict a loss in nesting habitat on East Island of up to 30% due to sea level rise by 2100. Therefore, the objective of the present study was to determine the carrying capacity of East Island for hatchlings and nesting females under current conditions and predictions of sea level rise. In the simulation model, density-dependent nest destruction was the primary factor regulating population size. Carrying capacity was reached between 1.9 and 2.1 million hatchlings at current conditions; carrying capacity was approached when 80 000 to 120 000 nests were laid on the beach, representing 20 000 to 30 000 nesting females. With a rise in sea level, carrying capacity was reached when 60 000 to 100 000 nests were laid on the beach. The current mean estimate of 390 nesting females per year, over the past 10 yr, at East Island represents 1.3 to 2% of the females that would nest at carrying capacity. The beach at East Island is well below carrying capacity and is capable of supporting a larger nesting population. However, the availability of suitable coastal habitats may play a bigger role in regulating the Hawaiian green turtle population than available nesting habitat.