The Effect of Artificial Light on Orientation of Hatchling Loggerhead Sea Turtles (Caretta caretta)
Sea turtle hatchlings primarily use sight to detect differences in the angle of elevation in relation to the beach as well as light intensity present along the horizon to navigate from the nest to the water’s edge. Artificial lights can cause visual disorientation (aimlessly wandering in circular paths around the nest) or misorientation (moving in distinct paths away from ocean). Several studies have been conducted on the effects high levels of light pollution associated with major cities can have on hatchling sea turtle orientation, whereas few studies have been done on the effects of comparatively lower levels of light pollution associated with rural areas, which are often considered ideal nesting habitats. This study examined lower light intensity found in areas surrounding large cities to identify if there is a threshold of light pollution above which hatchling orientation is negatively affected. During the 2016 nesting season, a Geovision GV-FER5303 non-illuminating infrared camera recorded hatchling trajectory at 21 Loggerhead Sea Turtle nests from areas varying in light intensity along the Grand Strand region of South Carolina. Fan spread, orientation deviation, and average crawl speeds were measured from each nest to determine if parameters associated with sea-finding behavior were significantly affected by radiance conditions present at the time of emergence. Average crawl speeds were significantly influenced by radiance conditions, with faster average crawl speeds recorded for nests emerging under conditions when moonlight was present, and lower average crawl speeds recorded under low and high levels of light pollution. Studies that examine how sea-finding behavior could be altered, even in areas of relatively low radiance, can help inform management practices to enhance hatchling survival.
Schematic diagram of camera setup for detecting hatchling turtle nest emergence and migration across the beach. Camera is suspended in center of PVC structure standing 3 m tall and spanning a distance of 1.8 m wide. Camera is attached to external marine battery and router to display live video footage.
Tracking image produced from MATLAB algorithm. Discrete hatchling tracks are represented by green lines within a 7.6 m radius camera field of view.
Sea-finding behaviors: (A) average crawl speed, (B) fan spread, (C) deviation from seaward direction, and (D) deviation from brightest light source measured under varying lighting conditions. Average crawl speed recorded under conditions when moonlight is present is significantly faster than crawl speeds recorded under low and high levels of light pollution at the P < 0.05 level. Varying levels of significance denoted by a, b, and ab. Circles indicate outliers, and stars indicate extreme outlines.
Hatchling orientation circular histograms for (A) low light pollution (Huntington Beach State Park 12), (B) moderate light pollution (Pawley’s Island 16), (C) high light pollution (Garden City Beach 10), and (D) radiance conditions when moonlight is present (North Litchfield 4). Red line: mean angle of orientation, relative abundance measured in radians.
Hatchling orientation diagrams for (A) low light pollution (Huntington Beach State Park 12), (B) moderate light pollution (Pawley’s Island 16), (C) high light pollution (Garden City Beach 10), and (D) radiance conditions when moonlight is present (North Litchfield 4). All are significantly oriented at the P < 0.05 level by a Rayleigh test. N: number of hatchlings; r: r-vector; a, ▶: mean angle.
Hatchling orientation under high light pollution when no moonlight is present and brightest light source is from varying directions. Nest emergence (A) circular histogram and (C) orientation diagram for Waties Island nest 9 with brightest light source 90°. Nest emergence (B) circular histogram and (D) orientation diagram for Garden City nest 14 with brightest light source 270°. For circular histograms, red line: mean angle of orientation, relative abundance measured in radians. For orientation diagrams, all are significantly oriented at the P < 0.05 level by a Rayleigh test. N: number of hatchlings; r: r-vector; a, ▶: mean angle.
Contributor Notes
Associate Editor: J. D. Litzgus.