Effect of light intensity and wavelength on the in-water orientation of olive ridley turtle hatchlings

Light pollution, associated with coastal development, poses a growing threat to sea turtles. Hatchlings are particularly affected during their crawl to the ocean since they exhibit phototaxis and may move towards or be disoriented by artificial lights. Although much is known about how hatchlings respond to artificial light while crawling to the ocean, far less is known about their response after reaching the water. Here, we investigate how hatchling olive ridley turtles (Lepidochelys olivacea) held in artificial pools responded to light of different wavelengths (red, 720 nm; yellow, 660 nm and green, 520 nm) and intensities (0.1-3.3 lx, mean 0.87 lx, SD = 0.85, 10.3-45.9 lx, mean 15.75 lx,SD = 7.12; 47.5-84.2 lx; mean 52.02 lx, SD = 9.11; 91.3-140.8 lx, mean 105 lx, SD = 13.24; 150.1-623 lx, mean 172.18 lx, SD = 73.42). When no light or red light below 39 lx was present, hatchlings oriented at a mean angle of 180° from true north and did not orient towards any discernable feature. However, hatchlings swam towards the light at intensities of red light above 39 lx, yellow light above 10 lx and green light above 5 lx. Our findings indicate that sea turtles will swim towards artificial lights even after reaching the water. Thus, we recommend light mitigation efforts should extend beyond nesting beaches and into the associated oceanic habitats.