Determining Critical Internesting and Foraging Habitats for the Conservation of Sea Turtles in Gabon, Africa Using Satellite Tracking and Stable Isotope Analysis

Understanding the horizontal and vertical habitat of olive ridley sea turtles (Lepidochelys olivacea), a threatened species, is critical for determining regions of protection that may effectively reduce bycatch, the largest threat to this species. Satellite transmitters were used to determine the movement and dive behavior of 21 female olive ridley turtles tagged in Pongara National Park, Gabon during the 2012, 2013, and 2015 nesting seasons. A switching state space model was used to filter the tracking data and categorize the internesting and post-nesting movements. Gridded utilization distribution (UD) home range analysis of tracking data revealed that the entire core habitat occurred in the Komo Estuary and overlaps with an important area for artisanal fishing (79% overlap) and commercial shipping traffic (58% overlap). Dive data revealed turtles spent a majority of their time resting on the estuary seabed (20% of all dive time), indicating that bottom set gear may pose the greatest potential for fisheries interactions. The results from this study provide data that can inform the design and implementation of marine protected areas and fishery zones, currently underway. Additionally, post-nesting foraging sites are not well defined for this population. δ13C and δ15N stable isotope analysis of 149 epidermis samples from female nesting olive ridley turtles in Gabon, 38 of which also had satellite transmitters, was used to determine if distinct foraging locations exist for this population. Tracked turtles allow for visualization of foraging areas, which can then be correlated to their distinct isotopic signatures and further matched to untracked turtle signatures. Cluster analysis of the isotope ratios from all turtles produced two significant clusters with a 95% predictability rate. Track visualization of each cluster did not show any foraging patterns and a generalized linear model determined that nesting beach, sampling year, and average straight-line distance from foraging location were not indicators for cluster assignment. Due to olive ridley turtles being opportunistic feeders and following a “looping” pattern to find new food sources, it is difficult to use stable isotope analysis to identify foraging habitat for non-tracked turtles in this population