Brightness of the night sky affects loggerhead (Caretta caretta) sea turtle hatchling misorientation but not nest site selection

Sea turtles in the Gulf of Mexico, which are listed as either threatened or endangered under the US Endangered Species Act, face numerous threats from many sources but are particularly susceptible to the effects of light pollution on nesting beaches. Light pollution affects the distribution, density, and placement of nests on beaches, and disrupts seafinding in hatchlings emerging from nests – often leading to their death. Rapid urban growth near Gulf Islands National Seashore (GUIS) over the last century has contributed to increased light pollution on its beaches, and there is concern that light pollution is causing females to lay nests in at-risk locations subject to erosion and flooding and is causing the observed high rates of hatchling misorientation. In this study our objectives were to measure brightness of the nighttime sky (and other variables) at GUIS at loggerhead (Caretta caretta) nests at the time of laying and hatching, and to assess the effects of brightness on the laying of at-risk nests and hatchling misorientation. In addition, we quantified the effects of relocating at-risk nests on nesting success. We found that the contrast in brightness between the landward and seaward directions at GUIS is at least partially responsible for high rates of hatchling misorientation, and we found a strong moderating influence of lunar fraction and lunar altitude on hatchling misorientation (larger lunar fractions and lower lunar altitudes reduced misorientation). We were unable to document any effects of artificial light, lunar fraction, or horizon altitude on the propensity of loggerheads to lay nests in at risk locations; and we found no evidence that relocating nests at GUIS reduced loggerhead nesting success. Rather, we found nesting success and hatchling misorientation rates were improved for relocated loggerhead nests.