Download YA291: Sea Turtle behaviour and nesting site preferences in Mexico

YA291: Sea Turtle behaviour and nesting site preferences in Mexico
Dr Kathy Slater, Operation Wallacea
All sea turtles in the Caribbean are listed by the IUCN (2012) as endangered (green turtle, Chelonia mydas
and loggerhead turtle, Caretta caretta) or critically endangered (hawksbill turtle, Eretmochelys imbricate,
and leatherback turtle, Dermochelys coriacea). Sea turtles are increasingly popular as a form of wildlife
tourism, but little is known about the effect of tourism on turtle behaviour (Wilson & Tisdell, 2001; Tisdell &
Wilson, 2002). Data from other wild species suggests that if tourism is not correctly managed, interactions
with tourists can lead to behavioural changes and increased anxiety in the target animals (Constantine,
2001). Immature turtles are known to aggregate around areas of sea grasses in relatively shallow water
(Arthur et al., 2008), making them accessible to swim and snorkel based tourism. However, these turtles can
also migrate from these feeding zones once their preferred food sources have become depleted. Successful
management of tourism involving immature turtles therefore requires careful management of both tourists
and sea grasses.
Sea turtles spend over 90% of their time far out to sea where they are largely inaccessible. Consequently,
little is known about their behaviour, diet and habitat preferences. Green turtles (Chelonia mydas) are
generally herbivorous with sea grass and algae as the main components of their diet (Lopez-Mendilaharsu et
al., 2005) although data from animal-borne video cameras suggests that ctenophores and jellyfish are also
important components of their diet (Heithaus et al., 2002). The diet and behaviour of green turtles are
reported to change from the juvenile to adult life stages, with a more varied diet in adult turtles, whereas
juveniles and sub-adults feed exclusively on sea grasses (Arthur et al., 2008).
Sea turtles repeatedly return to the same beach to nest, but population monitoring is complicated by large
inter-annual variations in nesting numbers caused by individual variation in re-migration intervals (Solow et
al., 2002). Evidence suggests that this variation arises from environmental conditions such as sea surface
temperature that affect the feeding conditions at sea (Solow et al., 2002). Nest site selection of turtles can
affect the survival of their offspring because embryo survivorship, hatchling quality, and sex ratio are all
affected by environmental factors (Wood & Bjorndal, 2000). Placement of nests farther inland increases the
likelihood of desiccation, hatchling misorientation, and predation on nesting females, eggs, and hatchlings.
Conversely, nests close to the sea increases the likelihood of inundation and egg loss to erosion. Studies of
nest placement in sea turtles have revealed that green turtles tend to nest in vegetated areas behind open
sand whereas loggerhead turtles tend to clump their nests in specific locations on the beaches. Investigation
of loggerhead nesting preferences indicated that of the four environmental factors evaluated, (slope,
temperature, moisture, and salinity), slope appeared to have the greatest influence on nest site selection
(Wood & Bjorndal, 2000).
Two species of turtle nest on the beaches of Akumal in the Yucatan Peninsula of Mexico: the green turtle,
(Chelonia mydas) and loggerhead turtle, (Caretta caretta). The increasing popularity of the Riviera Maya as a
tourist destination, and beach erosion (man-made and natural) has drastically reduced the number of
suitable nesting sites for the turtles in the Yucatan Peninsula. Moreover, light pollution along the coastline
has affected the number of turtles that come in to nest. Although Akumal is also a popular tourist
destination, the beaches are managed by a local NGO called Centro Ecologico Akumal. Daily patrols locate
turtle nests and place protective barriers around them, and tourists are not allowed on the beaches at night.
Local residents have agreed to minimise light pollution by closing all shops, bars and restaurants before
11pm and local fishermen and tour boats abide by “no go” areas in which areas of sea grasses are roped off
so that feeding turtles will not be disturbed by boats.
As a result of these efforts, the number of turtle nests and hatchling survival are starting to increase over
time. In addition, the protection of sea grasses has resulted in a permanent presence of green turtles in the
bays. However, climate change that affects temperature of nest sites is still a major concern as sex ratios of
hatchlings are increasingly biased towards females. The first aim of this project is therefore to investigate
green turtle and loggerhead turtle nest site preferences in relation to hatchling viability and sex ratios. As
the permanent presence of turtles in Akumal Bay has caused a significant increase in tourists, the second
aim of this project is to investigate the effect of tourist numbers on turtle behaviour, diet and habitat
preferences to provide information on maximum tourist numbers that should be allowed to visit the turtles
at any given time.
Methods
Akumal is a small coastal town located approximately a 1.5 hour drive south from the major tourist
destination of Cancun. The name Akumal literally means “home of the turtles” in Maya. It earned this name
due to the numerous turtle nesting sites along the bays located in the Akumal area and due to the
permanent presence of juvenile turtles in the sea grasses just off shore. Three species can regularly be found
around Akumal. Local beaches are nesting ground for two of these species: the Loggerhead Turtle (Caretta
caretta) and the Green Turtle (Chelonia mydas). The Hawksbill Turtle (Eretmochelys imbricate) can be found
feeding around the reef and year-round you can find juvenile green turtles feeding on sea grasses in the
bays.
Centro Ecologico Akumal (CEA) is a Mexican NGO that is responsible for managing 50km of coastline that
covers some of the most important turtle nesting sites in the Caribbean. There are 4 bays in Akumal
(aventuras bay, akumal bay, jade bay and half moon bay: Figure 1) that are important turtle nesting sites.
CEA is located within a 4 hectare compound in the northern part of Akumal bay. In 1993, Centro Ecológico
Akumal (CEA) began its conservation activities and operates its Sea Turtle Protection program through
permission from the Wildlife Agency, focusing on protecting the nesting populations of Loggerhead and
Green turtles. At the end of 2006, CEA began monitoring juvenile green turtles in Akumal Bay. This is a key
foraging area due to the abundant sea grasses. The local population in the bay fluctuates, with as many as 50
individual turtles present at a time. The increase in numbers of juvenile Green turtles in Akumal has resulted
in an increase in tourists coming to see them.
Figure 1: Map of Akumal showing location of the bays and Yal Ku lagoon. Centro Ecologico Akumal is located
in Akumal bay.
Effect of tourists on turtle behaviour
Behavioural observations will be conducted on the immature green turtles in Akumal Bay each day for
roughly 2 hour periods, divided into different time block (e.g. 7-9am, 9-11am). An equal number of
observations for each time block will be conducted throughout the duration of the study. Each student will
select one individual turtle to follow for the duration of the observation period. The age class and sex of this
individual will be recorded and where relevant, the ID tag number of the individual (located on the fin of the
turtle). Activity budget data will be collected on individual turtles using focal animal continuous samples
(Altmann 1974) during which the behaviour of the turtle (see Table 1) is recorded in real time for a 20
minute period. Throughout the 20-minute samples, the number of tourists present within a 5 metre radius
of the turtle and the behaviour of each tourist will be recorded (see Table 2). The habitat type will also be
noted and when the turtle is feeding, the food type will also be recorded.
Table 1: Ethogram of turtle behaviour used for the study
Behaviour
Description
Dive
Turtle swims quickly in a roughly vertical, downward position
Evasive
Turtle changes behaviour and moves rapidly in an attempt to escape from an
approaching individual or other entity. Individual could be another turtle or a tourist
and other entities include kayaks and boats
Feed
Turtle masticates food while food is in its mouth or is foraging on prey items
Rest
Turtle is stationary and is not engaged in any other behaviour
Surface
Turtle swims in a roughly vertical position towards the water surface to breathe
Swim
Turtle moves through the water in a relatively horizontal position
Table 2: Ethogram of tourist behaviour
Behaviour
Acceptable
Description
Tourists maintain the 5m distance from the turtle, do not crowd around the turtle to
prevent escape routes and do not attempt to interact with the turtle in any way
Approach
Tourist moves within 5m of turtle, but does not attempt to interact
Photograph
Tourist moves within 5m of turtle and attempts to photograph the turtle at close range
Touch
Tourist physically touches the turtle
Crowd
Two or more tourists move around the turtles in such a way that they block its travel
path and potential escape routes. This behaviour can occur even if the 5m distance
barrier is maintained
Turtle nest site preferences and viability
Beaches will be patrolled each night to monitor existing nests and emergence of females from the water as
they select a new nesting site. Each night the weather conditions, tide level, and moon phase will be
recorded. When an nesting female in encountered, the species of turtle, 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 a
measuring rule into the bottom of the nest, without disturbing any of the eggs. Then, to gain an
understanding of what light level she perceives, cloud cover and artificial light will be graded on a five point
scale (0-4). 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 2). 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
used to predict 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.
Suggested Reading
Altmann, J. 1974. Observational study of behaviour: sampling methods. Behaviour 49: 227-267.
Arthur, K.E., Boyle, M.C. & Limpus, C.J. 2008. Ontogenetic changes in diet and habitat use in green sea turtle
(Chelonia mydas) life history. Marine Ecology Process Series 362: 303-311.
Constantine, R. 2001. Increased avoidance of swimmers by wild bottlenose dolphins (Tursiops truncatus) due
to long-term exposure to swim-with-dolphin tourism. Marine Mammal Science 17: 689-702
Heithaus, M.R., McLash, J.J., Frid, A. & Dill, L.M. 2002. Novel insights into green sea turtle behaviour using
animal-borne video cameras. Journal of the Marine Biology Association U.K. 82: 1049-1050.
IUCN (2012). The IUCN Red List of Threatened Species. http://www.iucnredlist.org/ (accessed 20.12.12).
Lopez-Mendilaharsu, M., Gardner, S.C., Seminoff, J.A. & Riosmena-Rodriguez, R. 2005. Identifying critical
foraging habitats of the green turtle (Chelonia mydas) along the Pacific coast of the Baja California peninsula,
Mexico. Aquatic Conservation: Marine & Freshwater Ecosystems 15: 259–269.
Solow, A.R., Bjorndal, K.A. & Bolten, A.B. 2002. Annual variation in nesting numbers of marine turtles: the
effect of sea surface temperature on re-migration intervals. Ecology Letters 5: 742–746
Tisdell, C. & Wilson, C., 2002. Ecotourism for the survival of sea turtles and other wildlife. Biodiversity and
Conservation 11: 1521-1538.
Wilson, C. & Tisdell, C., 2001. Sea turtles as a non-consumptive tourism resource especially in Australia.
Tourism Management 22: 279-288.
Wood, D.W. & Bjorndal, K.A. 2000. Relation of temperature, moisture, salinity, and slope to nest site
selection in loggerhead sea turtles. Copeia 1: 119-128.