Sea Turtles Use Multiple Cues for Seafinding, Offshore Migration, and Nest Site Selection: Urban Development Disrupts Cue Recognition
M.S. Marine Biology
Loggerhead (Caretta caretta) sea turtle hatchlings emerge from an egg chamber after an average incubation of 60 days. Hatchlings sense a temperature decrease in order to emerge at night, and thus enabling them to avoid desiccation by the sun and predation in the daylight. This is the beginning of a journey that depends on a very specialized set of cues. Sea turtle hatchlings initially rely on visual cues for seafinding after emergence from the nest. There are two prominent visual cues: visible light and horizon elevation. On a natural, undeveloped beach, hatchlings move away from dark, solid silhouettes, dunes and vegetation, and toward greater illumination, such as moonlight reflecting from the sea. Artificial lighting from urban development distracts hatchlings emerging from their nests. It also negatively influences nest site selection by adult females. Beachfront lighting is an obvious visual cue that is commonly more intense than the glow of the moon. Such point source light pollution attracts hatchlings landward, often in a direct path toward the source. Development inshore of the beach may also affect seafinding through a phenomenon called urban glow. Urban glow results from the reflection of numerous city lights on the condensation in low clouds, producing a glow that is visible from the beaches. Without a distinct path, hatchlings become disoriented, defined as random wandering, or misoriented, defined as intent on the "wrong" direction. Response to a misdirected cue can be fatal for an entire clutch.
Sensing visual cues on land involves a very limited field of vision with an emphasis on the horizon. Experiments conducted on freshly deceased hatchlings yield information on retinal morphology and visual acuity in loggerhead (Caretta caretta), green (Chelonia mydas), and leatherback (Dermochelys coriacea) species.
Hatchlings rely more strongly on wave action versus visual cues when the surf is reached. Waves refract parallel to shore as they near the beach. Hatchlings use rotational cues to direct their path seaward. It is believed that magnetic cues enable loggerhead hatchlings departing Florida's coastline to stay within the North Atlantic Gyre. Magnetic cues are also believed to facilitate natal beach homing, because sexually mature female sea turtles return to the beaches on which they were born to lay their eggs.
In Broward County, instinctual cues and specialized retinal development are no longer sufficient considering the overwhelming degree of urban development adjacent to nesting beaches. To compensate, most nests are relocated to gated and open beach hatcheries. Hatchling disorientations documented by the Broward County sea turtle conservation program for the nesting season 2003 were analyzed to attempt to quantify the significance of direct beachfront lighting versus urban glow with regard to the occurrence of hatchling misorientation. Using aerial views of Broward County beaches, nests resulting in disoriented clutches were indicated corresponding to site descriptions recorded by students in incident reports. A substantial majority of disoriented hatchlings were observed for the open beach hatchery in Pompano Beach, Florida. For each dated incident of hatchling misorientation, daily weather conditions were assessed to indicate the possible effects of urban glow. The data were used to evaluate the null hypothesis that urban glow does not substantially influence the misorientation of Broward County hatchlings in open beach hatcheries. Correlation analysis showed a weak direct relationship (p = .049) between the frequency of hatchling disorientations and a cloud cover index that ranged between 0 (clear) and 10 (heavy clouds), however sky clarity accounted for only 4 percent of the variance in disorientation frequency. Direct beachfront lighting was the primary cause of hatchling misorientation. Lunar phases were considered to asses the natural illumination contributing to light intensity. Lunar contributions were not significant due to overwhelming intensity of artificial beachfront light pollution.
Amber C. Hester. 2004. Sea Turtles Use Multiple Cues for Seafinding, Offshore Migration, and Nest Site Selection: Urban Development Disrupts Cue Recognition. Capstone. Nova Southeastern University. Retrieved from NSUWorks, . (147)