Population dynamics of flatback turtles nesting on Delambre Island, ascertained using a multi-faceted approach

Capture-mark-recapture (CMR) and count studies are often used to estimate population abundance and trends in threatened species to guide conservation and management actions. Despite the limitations of focussing on one life history stage, logistic and economic constraints often result in using adult population abundances and trends as a proxy for whole population health in many migratory, long-lived species. For flatback turtles, the remote and cryptic life history stages between hatching and returning to the beach as an adult has resulted in limited understanding of abundance, growth and distribution for these life history stages and a reliance on the adult female nesting population to understand population abundance. On Delambre Island, Western Australia, Australia, flatback turtles have been monitored for 16 yr using a multi-faceted approach involving track counts and turtle tagging. Both CMR estimates and track counts indicated an increase in population abundance between the 2008-2009 and 2017-2018 seasons, followed by a decrease from 2017-2018 until 2023-2024, showing an overall 20% reduction in population abundance. Although we can be cautiously optimistic that trends seen here are cyclic and not suggestive of a truly declining population, the low apparent survivorship of 0.85 (recaptures) and 0.65 (first captures) and the currently declining trend may be cause for concern and require further monitoring. The long-term duration of the monitoring program and relatively high within-season capture rates provide a good indication of population health for flatback turtles within the Pilbara region of Western Australia.