Density and Abundance of Green Turtles in the Saudi Arabian Red Sea

Effective management and conservation of sea turtles is often constrained by a lack of knowledge of at-sea distribution and abundance. While abundance estimates of nesting females are typically well-documented on nesting beaches, counting sea turtles at sea presents challenges due to their widespread distribution and cryptic habits. Given nesting beaches only document adult females, at-sea data are also more informative of greater population demographics. To estimate the abundance and density of green sea turtles (Chelonia mydas) in the Red Sea waters of Saudi Arabia we conducted strip transect aerial surveys in four survey zones that spanned ~66% of shallow water habitats (<200 m depth), within which we counted sea turtles, and also other species such as dugongs and other marine mammals, sharks, and rays. Corresponding abundance estimates were modelled to account for perception bias (whether a surveyor saw a turtle that was available) and detection bias (whether a turtle was available to be seen). Our results suggest an abundance of ~201,427 green sea turtles potentially present between the 200 m bathymetric contour and the Saudi Arabian shore. However, there was a statistically significant relationship between turtle location and proximity to coral reefs, with over 90% of turtles found within 3500 m of coral reef structures (whether coastal fringing reefs, barrier reefs or atolls), and therefore it would be inappropriate to use an estimate assuming equal distribution. Adjusting for this buffer area we estimated ~95,000 turtles (95% CI: 64,000–142,000) within the proximity of reef structures. These findings represent the first abundance estimates of green turtles in the Red Sea. Repeated over time, surveys such as these can identify changes in population structure, distribution and abundance, and inform conservation and management agencies.