Nature is full of mysteries and arribada is one of the more spectacular ones. Arribada (a Spanish word meaning arrival) is when hundreds of thousands of olive ridley turtles come ashore to nest simultaneously. The sea turtles travel thousands of kilometres to lay their eggs ashore on long stretches of sandy beaches.
The mainland coast of India as well as its two island groups — the Andaman and Nicobar Islands, and the Lakshadweep Islands — serve as nesting grounds for four species of marine turtles including olive ridley (Lepidochelys olivacea), green (Chelonia mydas), leatherback (Dermochelys coriacea) and hawksbill (Eretmochelys imbricata) turtles.
But it is the olive ridleys (with their olive green hue shells) that are the most abundant species in India, with Gahirmatha and Rushikulya in Odisha serving as major mass nesting or arribada rookeries outside Central America. The rookeries are believed to be the ancestral source of contemporary global populations.
But, there is trouble in this paradise of the smallest sea turtle species, which is categorised as ‘vulnerable’ in the IUCN Red List.
“Over the last 15 years, we have been monitoring nest temperatures and hatchling sex ratios at the Rushikulya rookery… The hatchling sex ratio from arribadas at Rushikulya was found to be about 71% female on average,” says a recent report titled Monitoring Sea Turtles in India 2008-2024. The report is by Bengaluru-based Dakshin Foundation, a non-profit that works on conservation and natural resource management.
According to the report, which is the only long-term study on sea turtles in India, sex ratios at Rushikulya are skewed towards females but not to the extent seen in certain global sea turtle populations. “While some years had extremely female-biased sex ratios due to high nest temperatures, a few years produced male-biased sex ratios.”
Rising Temperatures Affecting Sex Ratio
Sea turtles exhibit temperature-dependent sex determination. In most species, sex is determined during fertilisation. However, the sex of most turtles, alligators, and crocodiles is determined after fertilisation. The temperature of the developing eggs is what decides whether the offspring will be male or female. This is called temperature-dependent sex determination, or TSD.
Research shows that if a turtle's eggs incubate below 27.7° Celsius (81.86° Fahrenheit), the turtle hatchlings will be male. If the eggs incubate above 31° Celsius (88.8° Fahrenheit), however, the hatchlings will be female, notes the National Oceanic and Atmospheric Administration (NOAA), a science agency under the US Department of Commerce.
According to NOAA, researchers have also noted that the warmer the sand, the higher the ratio of female turtles. Climate change and the increased temperatures could result in skewed and even lethal incubation conditions, which could impact turtle species and other reptiles.
Similar concerns have been raised by the World Wildlife Fund (WWF), which warns that unusually warm temperatures caused by climate change could be disrupting normal sex ratios, resulting in fewer male baby turtles. Warmer sea surface temperatures can also lead to the loss of important foraging grounds for marine turtles, while increasingly severe storms and sea level rise can destroy critical nesting beaches and damage nests.
With rising global temperatures and climate change, sea turtle populations might become increasingly feminised which raises questions about the long-term viability of the population, warns the long-term Dakshin Foundation study in India.
Rushikulya, being one of the largest mass nesting rookeries in India, contributes significantly towards hatchling recruitment. Estimating the sex ratios of hatchlings produced at this rookery is, therefore, useful for understanding the future impacts of climate warming.
Monitoring Olive Ridley Turtles in Odisha
Dakshin Foundation initiated a long-term monitoring programme at Rushikulya in 2008, along with the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru, and in collaboration with the Odisha State Forest Department. It is currently the longest-running study on olive ridley turtles in India.
The objective of the programme is to monitor their nearshore and nesting populations of the olive ridleys at the index site, Rushikulya, where the impact of climate change on the sex ratios of hatchlings has also been assessed. These findings are based on data compiled between 2008 and 2024.
Sea turtles are long-lived, late-maturing, and a highly migratory species. Changes in their populations occur over years or decades, making long-term monitoring essential for understanding population trends and environmental impacts.
Olive ridley turtles forage in pelagic waters (open, free waters away from the shore) but migrate to nearshore waters during the breeding season. Male and female olive ridley turtles migrate to Rushikulya around November, with males arriving before females. Mating takes place in the nearshore waters after which the males return to their foraging grounds. The females remain in the nearshore waters until they complete nesting.
Pregnant females return to the same beach where they were born to lay their eggs. This means that a female hatched in Rushikulya will not nest elsewhere.
Two months after the eggs are laid, hatchlings emerge — each about the size of a matchbox — and make their way to the ocean, where they spend the rest of their lives swimming, eating, and travelling great distances. When the turtles attain sexual maturity, thirteen years later, they make the journey back to their natal home, often travelling hundreds of kilometres.
The arribada or mass nesting of olive ridley turtles usually occurs once or, rarely, twice each nesting season at both Gahirmatha and Rushikulya. At Rushikulya, mass nesting occurs on a four kilometres beach stretch extending north of the Rushikulya river mouth. These mass nesting events in Odisha last anywhere between two and 10 days.
But Sea Turtle Population Is Also Growing
The monitoring of the turtles at Rushikulya by Dakshin Foundation has shown a steady or growing population from 25,000–50,000 nests a season in the early 2000s to over 150,000 in the last decade; over 400,000 nesting turtles have been estimated in some years.
“Since 2008, mass nesting events have occurred almost every year at Rushikulya with >100,000 - 200,000 turtles being recorded in many years. This suggests that the ridley population in this region is either stable or increasing,” infers the monitoring report.
Since 2008, a standardised census protocol has been used to monitor the mass nesting population. The number of turtles nesting in arribadas is estimated using a strip transect method. Estimates of the number of nesting turtles at arribadas over the last decade indicate an increasing trend at this rookery, says Dakshin Foundation.
“This long-term study would not have been possible without the support of the Odisha Forest Department. The overall trend from nearly two decades of monitoring is that the ridley population is stable or increasing,” said Kartik Shanker, a Professor with Centre for Ecological Sciences, Indian Institute of Science, Bengaluru.
“The fact that arribadas don’t occur in some years is puzzling, as our offshore monitoring indicates that there are large numbers of turtles in the water, but it may not be a cause for immediate alarm. Nevertheless, we must remain wary of threats to both coastal and oceanic habitats,” he said.
Future Directions
The Indian coastline, offshore waters, and reef ecosystems are important habitats for marine turtles in the region. But, threats from coastal development projects, offshore oil and gas exploration, mining, construction of new ports, vessel traffic, and interactions with fisheries continue to impact sea turtle populations and their habitat.
Over the last two decades, the sea turtle population has shown signs of recovery, attributed to a combination of conservation measures and the resilience of the population. However, the turtles continue to face threats from fishing, coastal development, and climate change. It is, therefore, crucial to monitor the population, both on land and at sea, to ensure their continued recovery and long-term survival.
Long-term monitoring of olive ridley turtles in Odisha is essential to understand population status, assess the impacts of climate change, and observe changes to nesting habitats. Since climate change impacts take time to manifest, it is crucial to conduct longitudinal studies to assess population change over time.
Conservation initiatives for the protection of the olive ridleys in Odisha need careful consideration of the social consequences for local fishing communities. In India, foraging and breeding sites of various sea turtle species overlap with important commercial fishing grounds of coastal communities. As a result, the consequences of protection measures inevitably impact the livelihoods of these communities.
It is particularly important to engage with and ensure the well-being of local communities that live alongside and share space with these species. This calls for devising measures that prioritise the protection of high-density areas rather than large-scale spatial restrictions or fishing bans. It is important to promote sustainable practices in coastal and fishing communities, and explore alternative livelihood opportunities, recommends Dakshin Foundation’s report.
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