Trends in the abundance of sea turtles resident in southern Great Barrier Reef waters Original Research Article

A mark-recapture study was used to estimate trends in annual abundance of green and loggerhead turtles resident in southern Great Barrier Reef (sGBR) waters between 1985–1992. Abundance was derived using a Horvitz–Thompson type estimator based on sex- and ageclass-specific recapture probabilities conditioned on annual sampling effort. The resident green turtle population increased over the 8 years by 11% pa and comprised 1300 individuals in 1992. The female nesting population also increased but more slowly at 3% pa and has continued to do so. The increase may be due to favourable environmental conditions affecting breeding behaviour. On the other hand, the resident loggerhead population declined at 3% pa and comprised < 40 adults by 1992. The female loggerhead nesting population also declined over the same period at 8% pa and has continued to do so. Yet loggerhead survival was high and constant so the decline may be due to recruitment failures resulting from (1) fox predation of eggs at mainland rookeries during the 1960s and (2) pelagic juvenile mortality from incidental capture in longline fisheries since the 1970s. The sGBR loggerhead stock is clearly endangered.