Feeding behavior and visual field differences in loggerhead and leatherback sea turtles may explain differences in longline fisheries interactions

Loggerhead Caretta caretta and leatherback Dermochelys coriacea sea turtles are caught frequently as longline bycatch. Loggerhead turtles are often hooked in the mouth, while leatherbacks often are hooked in the shoulder or flippers. Comparisons of feeding behavior and accuracy in biting surrogate ‘prey’ targets, in the presence of waterborne food odors, identified species-specific differences that may predispose the turtles to be hooked differently. Additionally, the visual fields of post-hatchling loggerhead and leatherback sea turtles were measured and compared to determine if field of view could further contribute to feeding behavior differences and refining the explanation as to why the 2 species are hooked differently. We found that loggerhead turtles, which have a larger visual field and a small area of binocular vision, were more accurate when biting a target than leatherbacks. These traits may explain their greater incidence of mouth, throat, and stomach hookings. Leatherback turtles have a smaller field of view and no overlap of visual fields, and they were less accurate in apprehending targets. Leatherbacks lack binocular vision, which might explain why they are prone to entanglement or hooking external to the mouth more often than loggerheads. Together, visual field and biting accuracy may help explain why leatherbacks and loggerheads differ in how they interact with fishing lines and the locations of their hookings. Such understanding of the species’ behavior and limits of visual fields is essential for formulating further bycatch reduction approaches.