There have been numerous studies conducted on the impact of beach factors on sea turtle nesting behavior. Sea turtles spend the majority of the year underwater, emerging only in the summer to lay their sand-nested eggs. The temperature at the beach and other environmental factors have a big effect on turtle reproduction. For instance, according to Hays (2001), sea turtles have a sex determination that is influenced by the ambient temperature. The environmental conditions at the beaches are not consistent because, among other things, sand conductivity and albedo have an impact on temperature. Since the sex is temperature-dependent it is possible that at a constant or temperature, a nest will contain all male or all female hatchings. Cooler temperatures are for instance associated with male hatchings while warmer temperatures are known to lead to more females. Based on the temperatures it is thus possible to predict the adult turtle sex ratios. The present project investigates the role of the beach characteristics on the nesting behavior of the sea turtles as well as the influence of beach conditions on the development and ratio of actively reproductive male and females sea turtles around Heron Island.
Problem Statement
The increasing mean temperature due to climate change has impacted negatively on the turtle sex ratios. According to Laloë et al (2016) only 2.4% of all the sea turtles will be males by 2030 as the increasing temperatures are seen to be female biased. Given the changes in the environment, the sea turtles are expected to adapt by altering their nesting behavior so as to attain the desirable hatching ratios. It is thus important to understand the thermal environment so as to enable the conservancy efforts that are aimed at producing the hatchings in which both the sexes are available.
Research Questions
Some of the questions that were sought to be answered by the answers of the research include:
How do the sand physical characteristics vary around the island?
How does the thermal incubation environment change with sand depth and time?
Are turtle nests evenly distributed around the island?
Does nesting behaviour change with sand type?
How many male and female turtles are present in the surrounding reef?
Methods
The methodology used in the research is as described by Booth & Astill (2001). The study was carried out on Heron Island of the Capricorn group of islands. Sand samples were taken from five different sites and tested to compare the size of the grain, pH, moisture content and salinity. The three locations from which the sand samples were collected included the Shark bay, North Beach and South Beach. The temperature was tested at different times of the day at different nest depths so as to investigate the effect should the eggs be buried at different depths of the nest. Further studies were undertaken to investigate if the nests were evenly distributed evenly on the island. To investigate if the nesting behavior was influenced by the sand type, the results of various physical components of sand with the depth of sand and the time spent digging the nest. Finally the number of males and females turtles available on the island was determined.
The report consists of literature review in which primary data about temperature and nesting behavior were analyzed to provide a theoretical background as well as for explaining the results obtained. The results were tabulated and analyzed appropriately after which interpretation of the data done followed by discussion and a conclusion. Analysis and processing of the results were done through the though the use of various statistical measures.
Results and Discussion
Variation of Physical Characteristics around the Island
The main physical characteristics of the sand that were studied in the island include sand weight, grain size and average sand moisture from the three locations of Shark bay, North Beach and South Beach. The averages of the results of the physical parameters are presented in the following table. One-way ANOVA was used to evaluate whether there was significant variation in the physical across the island.
Shark Bay
North Beach
South Beach
Sand Moisture (%)
2.95066667
3.836667
2.9926667
Temperature (°C)
26.87878733
28.258454
26.74106114
North Beach can be seen to have the highest sand moisture average of the three locations. At the same time the location is observed to have the highest temperatures. Booth & Astill (2001) explains that the north facing beach is warmer than the other locations since it receives more solar radiation hence more heat is transferred to the sand. The moisture content of the sand has been proven to have no effect on the temperature of the sand. Moisture content is one of the important sand characteristics that influence the nesting behavior of the female turtles. The turtles are more likely to nest in areas with moist sands so as to avoid desiccation of the eggs. The moisture also enables the ease of digging the nests as the water content softens the sand.
Sand Grain Sizes
The grain sizes were measured using coarse, medium and fine to describe the individual grain sizes. Consequently, 1mm was used to describe coarse sand; size of greater than 500Mic describes medium sized grain and less than 500Mic representing fine grain sand when sampled from a nest depth of up to 50cm. The sampled grains were of sizes are presented in the table.
% Sand grain size
North Beach
Shark bay
South Beach
50cm
Average grain size percentage
Coarse >1mm
4.26930631
7.094388686
3.042485436
4.802060144
Medium>500Mic
32.81058434
55.073458
37.83745342
41.90716525
Fine<500
62.50195055
37.70021484
59.14121243
53.11445927
The average grain sizes percentages for fine, medium and coarse grains across the island are seen as 53.11445927, 41.90716525 and 4.802060144 respectively. It can thus be concluded that the island is largely made of fine and medium grain sand sizes. The impact of sand grain size is mainly noted in the thermal conductivity of the sands and subsequently the effect temperature. According to Speakman, Hays & Lindblad (1998) the grain structure is highly related to the thermal conductivity of the sand. Since the variation of the grain size percentages in each of the three locations did not differ significantly, the temperature variations can be attributed to solar radiation as compared to the effects of thermal conductivity. Larger grain sands are associated with relatively lower temperatures. The Shark bay is noted have more medium size grains (55.073458%) and relatively high coarse grain percentage (7.094388686%) when compared to the other locations has an average temperature of 26.87878733°C that is relatively lower as compared to North Beach that has a high percentage of fine grain sands.
Thermal Environment at Different Locations and Times
The incubation thermal environment was investigated at 50 cm depth in different locations at different times. The depth investigations were carried out on the right and left of the transects conducted on different days. Variation of the subsurface temperatures up to the depth of 50 cm is seen to be fairly dependent on the surface temperature, grain size and to a small extent the moisture content of the sand (Booth & Astill, 2001). The differences in the vertical temperature gradient differences therefore means that the nest placement have a significant influence on the resulting sexes of the hatchings.
The surface environment at the North Beach was observed to comprise of lots of vegetation with the areas close to the trees being cooler when compared to the other areas in the same vicinity. Next to the coastline with high tide presented cooler temperatures as well as windy conditions.
The present study established that the depth temperatures varied daily at different times of the day. It is observed that there are variations in the temperatures in the probes conducted on Friday and Saturday at different times. The findings are supported by Speakman, Hays & Lindblad (1998)’s study that the nests of the turtles are usually placed at depths of between 25-150 cm where the main form of heat transfer is via conductance. Possibilities of forced convection and radiation are eliminated and thus the thermal conductivity depends on factors such as sand grain structure (Spotila et al., 1987). Conductivity is therefore noted to as a major determinant of the temperature variation with depths at different times and locations and therefore has a significant influence on the sex ratio.
Distribution of the Nests across the Island
The distribution of the nests across the island was measured by obtaining nest frequencies in the three sites at an area of density of (12x50m). The table below is representation of the distribution of nests in the land.
Nest density (12x50m)
North Beach (sign/rock )
Shark bay (palm/sign)
South Beach (path/57 pole)
Frequency
45
44
52
The results indicate that most nests are constructed on the South Beach (52) as compared to the North beach that has 45 nest frequencies. The findings contradict the outcome by Booth & Astill (2001) that more nests are expected in northern section of the island during the warmest months. The Shark bay recorded a total nest frequency of 44. While it was expected that the distribution to follow the postulations by Booth & Astill (2001), it should be noted that there has been significant changes in climatic conditions from the time the study was conducted in 2001. Due to the rising temperatures it is expected that more hatchings will lead to female turtles (Hawkes et al., 2007). As an adaptive behavior to avoid imbalances of sexes it can be seen that the turtles are constructing their nests in cooler locations. The present study was able to establish that the South Beach has the lowest average surface temperature (26.74106114) as compared to the other two sites of the study. The research was able to further establish a correlation between temperature and the nest distribution. From the study it can be noted that the nest frequencies decrease with the decreasing temperatures. North Beach with the highest temperatures (28.258454) has the lowest number of nest. The Shark bay that has a temperature that is lower than the highest but higher than the lowest recorded temperatures can also be noted to follow the temperature nest frequency pattern.
Sand Type and Nesting Behavior
The sand type is also noted to influence the nesting behavior of the sea turtles. In the present study it can be noted that the sand characteristics do not have any impact on the nesting behavior of the turtles. Instead it is the temperature that is seen to be major a driver of the nesting behavior. Furthermore it has been established that the island is made predominantly of fine to medium-sized grains. The sand grains are almost uniform with the average percentages of the fine, medium and coarse sand types in all the study sites showing no significant differences.
Various sand types and grains can be seen to affect the nesting behavior (Booth & Evans, 2011). In coarse dry sand the female turtles find it difficult to dig the chambers. The female may make several nests in multiple nights before finally depositing the eggs (Ischer, Ireland & Booth, 2009). On the other hand it can be noted that fine sand enable easier digging of the chambers and thus encouraging more nests to be constructed in areas with fine sand.
Timing of Egg Chamber Digging
The time taken by the female to dig an egg chamber for depositing the eggs depends on various factors such as moisture content, sand grain size and among others (Fadini, Silva & Ferreira-Júnior, 2011). The time for digging the egg chamber is particularly noted to influence the nesting behavior in an area as the females will is likely to stop digging with the varying levels of difficulties that is encountered in the digging process. The time taken to dig egg chambers was recorded by observing the behaviors of five different turtles in the three locations. In North Beach, only one turtle dug a chamber and deposited eggs taking a time of 39.49 minutes. Four other turtles crawled back without digging or depositing (false crawl). On the observation and timing Thursday night at Shark bay, it can be noted that turtles 1, 2, 3 and 4 successfully and deposited eggs in the chambers taking 24.34 minutes, 1 hour, 1 hour and 13 minutes and 20 minutes respectively. At South Beach only one turtle dug the egg chamber (36) with the rest having false crawls. There was no observation made on the observation made on the timing Sunday.
The false crawls were confirmed by the absence of disturbed sand just the tracks and in case of the disturbed sand, the tracks can be seen to leading away from the area before returning to the ocean. It can also be noted that there were back stop with sand not thrown over the emerging crawls. False crawls are evidence of abandoned nesting attempts (Booth & Evans, 2011). The false crawls recorded in the study could have resulted to factors such as activities by people around the vicinity or predation threat. It could also mean that the intended sites were not ideal for the nesting activities.
Timing of egg chamber digging
North Beach
Shark bay
South Beach
Time turtle 1
39.49 (sat)
26.34 min (thurs)
36 mins (sat)
Time turtle 2
False crawl(sat)
1hr(thurs)
False crawl(sat)
Time turtle 3
1hr 13mins(thurs)
Time turtle 4
20mins(thurs)
Time turtle 5
False crawl(thurs)
4 failed (sun)
6 failed (sun)
3 (sun)
Turtle Sex Ratio
Sex identification was performed according to the method described by Booth & Astill (2001). Full grown males were noted to have longer tails than the females of the same species. Another key criterion for sex identification was the location of cloaca. The males were also identified as having long nails in their front feet as compared to females that have shorter nails in general.
Turtle sex ratio
Location
Blue pools
Out from dock
Male
1
0
female
2
4
Boat Counts
Boat counts
Female
Male
Start October
6
4
4
6
1
6
8
11
2
2
1
3
4
8
5
4
0
4
Sum
32
48
End October
Start December
8
1
1
0
13
0
5
3
Sum
27
4
From the Blue pools, it can be noted that the number of the male counted was only one while the females were two. There was no male turtle from out of the dock. From the boat counts it can be noted the population of the male and female turtles vary. From October to December more males (48) are counted as compared to 27 for females. The total turtles observed in the period from October to December are noted as 75. From the start of December a total of 31sea turtles are counted with 27 females and 4 males. It should however be observed that the population of the turtles are significantly reduced in the period from the start of December. The period from the start of December is associated with winter and lower temperatures (Matsuzawa et al. 2002). The population during winter is noted to be low as most turtles do not return to shore during the period. Again the observations indicate that more males are hatched during colder periods.
Conclusion
The fact that sex determination is determined by environmental conditions forms an intriguing research discipline given the various changes that are associated with the beach areas (Burgess, Booth & Lanyon, 2006). In investigating the factors that affect the nesting behavior of the sea turtles, it has been established that temperature, sand characteristics and factors such as predations to significantly influence the choice locations. The environmental conditions in the beaches are not uniform since temperature is affected by factors such as sand albedo and conductivity among other factors. Dependence on temperature by the sea turtles is noted to be greatly impacted by the current climatic changes leading to temperature elevations. Warmer temperatures are associated with hatching of more female turtles while cooler temperatures are related to more males. The sea turtles are thus noted to have started to adapt by choosing their nesting sites carefully so as to make a balance of the resulting sex ratios. The study has established that sand characteristics affect the nesting behavior significantly.
References
Booth, D. T., & Astill, K. (2001). Temperature variation within and between nests of the green sea turtle, Chelonia mydas (Chelonia: Cheloniidae) on Heron Island, Great Barrier Reef. Australian Journal of Zoology, 49(1), 71-84.
Booth, D. T., & Evans, A. (2011). Warm water and cool nests are best. How global warming might influence hatchling green turtle swimming performance. Plos One, 6(8), e23162.
Burgess, E. A., Booth, D. T., & Lanyon, J. M. (2006). Swimming performance of hatchling green turtles is affected by incubation temperature. Coral Reefs, 25(3), 341-349.
Fadini, L. S., Silva, A. G., & Ferreira-Júnior, P. D. (2011). Sedimentary characteristics and their effects on hatching success and incubation duration of Caretta caretta (Testudines: Cheloniidae) in Espirito Santo, Brazil. Zoologia (Curitiba), 28(3), 312-320.
Hawkes, L. A., Broderick, A. C., Godfrey, M. H., & Godley, B. J. (2007). Investigating the potential impacts of climate change on a marine turtle population. Global Change Biology, 13(5), 923-932.
Hays, G. C., Ashworth, J. S., Barnsley, M. J., Broderick, A. C., Emery, D. R., Godley, B. J., ... & Jones, E. L. (2001). The importance of sand albedo for the thermal conditions on sea turtle nesting beaches. Oikos, 93(1), 87-94.
Ischer, T., Ireland, K., & Booth, D. T. (2009). Locomotion performance of green turtle hatchlings from the Heron Island Rookery, Great Barrier Reef. Marine Biology, 156(7), 1399-1409.
Laloë, J. O., Esteban, N., Berkel, J., & Hays, G. C. (2016). Sand temperatures for nesting sea turtles in the Caribbean: Implications for hatchling sex ratios in the face of climate change. Journal of Experimental Marine Biology and Ecology, 474, 92-99.
Matsuzawa, Y., Sato, K., Sakamoto, W., & Bjorndal, K. (2002). Seasonal fluctuations in sand temperature: effects on the incubation period and mortality of loggerhead sea turtle (Caretta caretta) pre-emergent hatchlings in Minabe, Japan. Marine Biology, 140(3), 639-646.
Speakman, J. R., Hays, G. C., & Lindblad, E. (1998). Thermal conductivity of sand and its effect on the temperature of loggerhead sea turtle (Caretta caretta) nests. Journal of the Marine Biological Association of the United Kingdom, 78(04), 1337-1352.
Spotila, J. R., Standora, E. A., Morreale, S. J., & Ruiz, G. J. (1987). Temperature dependent sex determination in the green turtle (Chelonia mydas): effects on the sex ratio on a natural nesting beach. Herpetologica, 74-81.
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