Download sea turtle nesting site preferences

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