Largest Sea Turtle Population changing due to Environmental Warming

Sea Turtles have temperature-dependent sex determination. Meaning that the sex of the hatchlings are determined by the incubation temperature during embryonic development. The critical point of development, the thermosensitive period, is during the middle of the third period in incubation. The pivotal temperature is the point at which half of each gender is hatched; while colder temperatures produce more males, warmer temperatures will produce more females.

The transitional range is based around the pivotal temperature, to where all males or females will be hatched. The pivotal temperature only differs by a few degrees ether way to produce all of one gender in the hatch. With global warming causing temperatures to rise above the pivotal point more females are being hatched. The climate change is also causing the mortality rate of the hatchlings to rise.

In Australia there are two breeding populations of turtles the Southern Great Barrier Reef stock (sGBR) as well as the northern Great Barrier Reef stock (nGBR). Turtles mate along the area of their nesting beaches. Females can lay multiple clutches of eggs each with around 100 eggs. Each clutch that is laid is incubated around 55 days. These hatchlings make their way into the open ocean and stay there till they reach the age of sexual maturity, around 25. At this time in their life they return to the same beach they were hatched at to mate.

Figure 1: Nesting Areas

The northern Great Barrier Reef population is one of the largest green sea turtle populations in the world. The figure above shows the area of Australia where the two populations nesting sites. The population is estimated to have around 200,000 females nesting. The population has shown that there is an increase of nesting females despite the fact that there was a signs of declining population in the 1990's. Research has been completed on this population of turtles to better understand the way the climate changes affect the gender and growth of the population.

To test this effect the best approach would be to estimate the exact number of males and females that are being hatched during nesting season. The issue with recording this data is you are unable to see the sex of a turtle upon hatching. Researchers are able to determine the sex of the hatchlings by histological examining the gonads. However, this would mean sacrificing the hatchling's life which would go against the point of the research of studying the population’s growth, not to mention the ethical issue as well. There are tests though that can be run on the dead hatchlings found left over in the nest.

There is a foraging ground that is also labeled in figured one, Howick group of island. Genetic testing and tagging that have been completed show if the turtles are from the nGBR or the sGBR, they age and gender. At this area researchers tested turtles laparoscopic to determine the gender and genetics of the sexually immature turtles at the feeding ground. Also data was taken from three age ranges. The results show that all ages of the turtles were found to be mostly female.  

Figure 2: Data of Howick Islands

Another point of the analysis was to estimate the temperature of the sand over the years that these turtles would have been incubated in. There was no recorded numbers for the sand over the years but there was recorded temperatures of the air and sea surface. These numbers were used to find around what temperature the sand would have been at the time. Figure 3 shows the estimated temperatures based around 0 the pivotal temperature. The majority of the graph shows that the temperature would have been above the pivotal point. The information of the estimated sand temperature correlates to the data in figure 2, showing that mostly would have been hatched in previous years. 

Figure 3: estimated sand temperatures

The researchers are noticing the number of females increasing every year with the temperatures being above the pivotal point. The large number of females is concerning for the future of the turtle population. There is no way to tell for sure how many males are needed to sustain the population and/or to keep the same large numbers present. With the females continuing to outnumber the males in the population there will be less sperm to repopulate. The lacking number of males has already decreased the population’s number but the concern is if this trend continues the population will reach extinction over time. The sea turtles may have the ability to adapt to the climate change but these adaptations will take time to occur. While the rate of global warming is happening much faster than the progression of the adaptation. The idea for solving this problem is to slow down global warming and help the preserve the turtle population as best as possible.

Jensen, M., Allen, C., Eguchi, T., Bell, I., LasCasella, E., Hilton, W., Hof, C., & Dutton. P. (2018).
            Environmental Warming and Feminizarion. Retrieved from
            https://www.sciencedirect.com/science/article/pii/S0960982217315397?via%3Dihub

NOAA. (2017). What Causes Sea Turtles to be Male or Female? Retrieved from
            https://oceanservice.noaa.gov/facts/temperature-dependent.html