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SEA TURTLE NESTING SITE PREFERENCES CEA-OPERATION WALLACEA 2013 Studies and conservation efforts that focus on nesting habitats are needed to improve management and ensure the continued funding of conservation initiatives for sea turtles. The identification of variables that influence the nesting beach selection and the specific nesting site location within the beach by marine turtles has been a focus of ecological research for many years. Despite efforts to understand how their nesting behavior and preferences have evolved, several studies suggest different findings, making it difficult to develop and implement adequate protection strategies for critical nesting habitats and nesting populations (Cuevas et al. 2010). Nest Site Preferences Sea turtles provide no neonatal care once the eggs have hatched, thus the importance of nest placement is essential (Kamel & Mrosovsky 2005). The selection of the nest habitat is a key determinant of reproductive success because it can affect embryo survival, body size and sex ratio of hatchlings, and the orientation of offspring as they crawl from the sand and make their way to the water (Hays et a., 1995). Nests must be easily accessible from the ocean, be high enough to avoid being inundated frequently by high tides, have enough sand cohesion to allow construction, and the sand must facilitate gas diffusion and have temperatures conducive to egg development (Mortimer 1990). Therefore, selection of a nest site is an adaptive trade-off between the costs and benefits. Costs include searching for a site, both in terms of energy expenditure and predation risk. Benefits include the increase in reproductive success due to selecting a site suitable for a successful incubation (Wood & Bjorndal 2000). Nest site selection in sea turtles can be divided into three phases: beach selection, emergence of the female, and nest placement. Beach selection most likely depends on offshore cues and beach characteristics. It is not well understood how the female sea turtle decides where to emerge at a particular location along the beach (Wood & Bjorndal 2000). Even after a turtle has crawled out of the ocean onto a particular beach, lights (especially moving lights), noises, obstacles, hard compacted sand, or other impediments may cause the turtle to retreat to the sea without nesting. Sometimes a female turtle makes one or more attempts at excavating a nest before abandoning the effort. A turtle that makes a false crawl will usually emerge at a nearby location later that night or the next and lay her eggs (Bjorndal & Cousteau 2003). Once a female has selected a beach on which to lay her eggs, she must choose where on the beach to excavate the nest. Placement of the nests close to the sea increases the likelihood of inundation and egg loss due to erosion. Conversely, placement of nests farther inland increases the likelihood of desiccation, hatchling disorientation, and predation on the females, eggs and hatchlings (Wood & Bjorndal 2000). Hatchling Sex Ratios Sea turtles as exothermic reptiles depend on the environment to regulate body temperature and to determine the sex of the offspring during incubation on the beach. This happens in the first trimester of incubation period; embryos develop into males when the temperature is approximately 28°C (82°F), whereas the female embryo develops at approximately 31°C (88°F). Temperatures between these two extremes produce clutches with a mix of both males and females. To complicate matters further, the metabolic heat produced by the eggs themselves 1 affects the sex of the embryos. Eggs in the center of the nest are warmer, and more likely to become females, while eggs on the outer edge are more likely to become males (Spotila 2004). In the long-term, increases in sand temperature due to global warming may alter hatchling sex ratios and survival. Adaptive management may be necessary to mitigate the predicted impacts of climate change and to ensure that sea turtles have a more realistic opportunity to adapt (Hawkes et al., 2007; 2009). Managers may choose to protect important male-producing regions to promote future population viability or use manipulative methods such as modifying the sand temperature through artificial shading, vegetation cover, or sprinkling cool water into the sand on nesting beaches to maintain temperatures within the thermal tolerance for the species’ incubation. Relocating nests to more suitable incubating environments may also be an option (Fuentes et al. 2010). Research Objectives: 1) To determine what physical characteristics of the beach female sea turtles prefer for nesting at Akumal in order to protect them 2) To determine how ambient temperature inside the nests determines the sex ratio of hatchlings in Akumal to determine if adaptive management of nests should be used in the future Methods Data will be collected each night during turtle nesting season (May to October) in Akumal Bay and Half Moon Bay. Each night the weather conditions, tide level, and moon phase will be recorded. When an nesting female in encountered, the time and date when she begins to lay eggs, nest number, curved carapaced length and width, whether the female it a neophyte or not and the tag number located in the front flippers (where applicable) will be recorded. Once the turtle chooses her nesting spot, digs the chamber and begins laying eggs, nest depth will be measured by carefully inserting thermometer into the bottom of the nest, without disturbing any of the eggs. Then, to gain an understanding of what light level she perceives, the luminance level will be recorded by placing the light meter close to the turtle’s eye. For each nest, a pen thermometer will be inserted into the nest measuring to the nearest 0.1 °C 2 cm into the sand at the cloaca level as soon as the first eggs are laid, to get the most accurate reading of what the female perceives as the ideal temperature. Nest elevation will be calculated by multiplying the sine of overall slope (using a clinometer and tape measure) from the mid-tide line to the nest and the straight-line distance from the mid-tide line to the nest. Straight-line distance from the mid-tide line to the nest and from the nest to the back of the beach will be measured using meter tape. In a selection of nests, ambient temperature will be recorded using HOBO data loggers (Figure 1). Data loggers will be placed at nest depth after turtle starts laying, to record temperature and humidity every 2 hours for the duration of the incubation period (50-65 days). These data can then be matched to the sex ratio of hatchlings from each nest. For hatchling success rate, nests will be excavated and inventoried within 24 hours after hatchlings emerge from their nest. Figure 1: HOBO data logger Variables to be measured for each nest site are: 2 a. b. c. d. e. f. g. h. i. j. k. l. m. n. o. Distance to waterline Slope Elevation Nest depth Sand Temperature at 2 cm Sand Temperature at nest depth Luminance levels Distance to back of beach Species of vegetation if applicable Height of vegetation if applicable Sand Moisture at 2 cm Sand Moisture at nest depth Air temp at time of nesting on beach Air humidity at time of nesting on beach Human disturbance (none, man-made obstacle, human obstacle, human voices, human presence) p. Obstacles in tide zone (Rocks, tree trunks) q. Neophyte or not r. Species of turtle 2. Variables to be measured at randomly selected sites within the zones where historically there are never high densities of nests are: a. b. c. d. e. f. g. h. i. Distance to waterline (average of observed nests) Slope Elevation Sand Temperature at 2 cm Luminance levels Distance to back of beach Species of vegetation if applicable Height of vegetation if applicable Sand Moisture at 2 cm 3. Variables to be measured at rejected nest sites along the track are: a. b. c. d. e. f. g. Type of attempt-test, bed, chamber Distance to waterline Slope Elevation Sand Temperature at 2 cm Luminance levels Sand Moisture at 2 cm Suggested Reading 1. Bjorndal, K. A., and J.-M. Cousteau. 2003. Mating behavior and reproduction. Page 43 Sea turtles of the world. Voyaguer Press Inc., Stillwater, MN. 2. Cuevas, E., M. de los Angeles Liceaga-Correa, and I. Marino-Tapia. 2010. Influence of beach slope and width on hawksbill (Eretmochelys imbricata) and green turtle (Chelonia mydas) nesting activity in El Cuyo, Yucatan, Mexico. Chelonian Conservation and Biology 9:262–267. 3. Fuentes, M. M. P. B., M. Hamann, and C. J. Limpus. 2010. Past, current and future thermal changes of green turtle nesting grounds: Implications from climate change. Journal of Experimental Marine Biology and Ecology 383:56–64. 3 4. Hawkes, L. A., A. C. Broderick, M. H. Godfrey, and B. J. Godley. 2007. Investigating the potential impacts of climate change on a marine turtle population. Global Change Biology 13:1–10. 5. Hawkes, L. A., A. C. Broderick, M. H. Godfrey, and B. J. Godley. 2009. Climate change and sea turtles. Endangered Species Research 7:137–154. 6. Hays, G. C., A. MacKay, C. R. Adams, J. A. Mortimer, J. R. Speakerman, and M. Boerema. 1995. Nest site selection by sea turtles. Journal of the Marine Biological Association of the United Kingdom 75:667–674. 7. Kamel, S. J., and N. Mrosovsky. 2005. Repeatability of nesting preferences in the hawksbill sea turtle, Eretmochelys imbricata. Animal Behavior 70:819–828. 8. Mortimer, J. A. 1990. The influence of beach sand characteristics on the nesting behavior and clutch survival of green turtles (Chelonia mydas). Copeia:802–817. 9. Spotila, J. 2004. Life cycles: From sand to sea. Page 15 Sea turtles: A complete guide to their biology, behavior, and conservation. The John Hopkins University Press, Baltimore, MD. 10. Wood, D. W., and K. A. Bjorndal. 2000. Relation of temperature, moisture, salinity, and slope to nest site selection in loggerhead sea turtles. Copeia 1:119–128. Methods adapted from Marielle’s Livesey study titled “Hatchling Success Rates as a Function of Environmental Nesting Preferences for Green Sea Turtles and Potential Impacts of Climate Change at Akumal Nesting Beaches” 4