The effects of light intensity and wavelength on in-water orientation of olive ridley sea turtle hatchlings (Lepidochelys olivacea)

The first hours of a sea turtle’s emergence are a time of high vulnerability due to shoreline predation and exposure to the harsh, high temperature beach environment. Within these hours, hatchlings emerge from the nest at night, crawl to the shore and swim through the surf into the open ocean. Photopollution that accompanies coastal habitat development poses an increasing, additional threat within coastal habitats of sea turtles. Studies have examined the detrimental effects of artificial light during the hatchlings’ seaward crawl along the beach, yet little is known regarding the impacts of artificial light on hatchlings after leaving the beach. In this study, we examined the effects of different light intensities (5 lumens to 2,000 lumens) at variable wavelengths (red, 720nm; green, 520nm; yellow, 660nm) on olive ridley sea turtle hatchlings (Lepidochelys olivacea). Our laboratory consisted of a circular pool with a camera positioned above to capture behavior and orientation headings. This footage was used to analyze the mean angle of orientation. Additionally, we examined the change of orientation in relation to possible threshold intensities of light at different wavelengths. Negative effects were exhibited by observed disorientation (lack of set direction) and deviation from natural headings known as misorientation. Olive ridley hatchlings oriented to a heading of 180° under 0 lux. Under red light, hatchlings on average oriented towards a similar heading to the orientation under 0 lux. The minimum light intensities that attracted hatchlings under: 1) red light was 39 lux, 2) yellow light was 10 lux, and 3) green light was 5 lux. Green light had the lowest minimum light intensity inferring an attraction to green light at low intensities. Hatchlings were more closely attracted to the source of yellow light than the source of red and green light. Our minimum intensity results suggest that a threshold amount of light can be used to examine potential impacts of illumination around nesting beaches (e.g. Playa Grande, Costa Rica) upon initial neonate dispersal. This information can be translated into artificial illumination thresholds, enabling managers and planners to promote more turtle-friendly coastal development policies and practices. Our findings contribute to the field of knowledge concerning hatchling orientation and dispersal and illuminate how artificial light can cause a shift from reliance on geomagnetic cues to a reliance on light cues at low intensities