Nesting Sea Turtles on a Changing Caribbean Island

The Buck Island Sea Turtle Research Program (BISTRP) is a long-term sea turtle monitoring project that focuses on nesting sea turtles in the Caribbean. BISTRP was initiated by the National Park Service in 1988 after Buck Island was identified as an important nesting beach for sea turtles, in particular for the critically endangered Hawksbill sea turtle. Since 1988, the program has conducted annual monitoring of the nesting sea turtles on Buck Island with the goal of identifying each nesting female, collecting biological data, and tracking nest success on the island.

Buck Island provides nesting habitat for four species of sea turtles: Hawksbill (Eretmochelys imbricata), Green (Chelonia mydas), Leatherback (Dermochelys coriacea) and Loggerhead (Carretta carretta). Each of these species is included on the IUCN Red List as either threatened or endangered and all are protected under the Endangered Species Act of 1973. Research from BISTRP is used to inform local management practices and the recovery of Hawksbills in the Caribbean. Sea turtles face significant threats around the world. Threats to sea turtles include fishery interactions and bycatch, vessel strikes, entanglement in marine debris, direct harvest and human consumption, and the loss and degradation of both foraging and nesting habitat.

Sea turtles are a unique group of animals because while they spend the majority of their lives at sea, females remain tied to land in order to lay their eggs. They are long-lived and are estimated to reach sexual maturity when they are between 20-30 years old. Sea turtles exhibit a high degree of natal site fidelity. Every few years, females migrate back to beaches in the area where they hatched decades earlier. Since its inception in 1988, BISTRP has recorded 762 unique females that have come to Buck Island to nest. While Buck Island has been used as nesting habitat by four species of sea turtles, it is most regularly used by Hawksbills and Greens.

The Hawksbill sea turtle (Eretmochelys imbricata) gets its name for its hawklike beak which it uses to forage for sponges in hard-to-reach crevices. Hawksbills are estimated to live 50-60 years and reach maturity at around 20 to 35 years. Like other sea turtles, they make impressive reproductive migrations. One Hawksbill that was tagged on Buck Island was tracked 1,160 miles to a foraging site in Nicaragua! Hawksbills will nest every 1-5 years and will typically lay between 3-5 nests each season. The average clutch size for each of these nests is between 130-160 eggs.

 Hawksbills are a critically endangered species and face many threats globally.  One major threat to Hawksbills is the loss and degradation of nesting habitat due to coastal development, rising sea levels and an increased frequency in major storm events which leads to beach erosion and nest inundation. Coral reefs, which Hawksbills depend on for food and habitat, are also being threatened by climate change and widespread coral diseases.

Hawksbills are circumtropical: they exist in all tropical and subtropical waters around the world. The largest populations are found in the west Atlantic (Caribbean), Indian and Indo-Pacific Oceans. The most significant Hawksbill nesting sites in the United States are in Puerto Rico and the U.S. Virgin Islands.

The Green Sea Turtle (Chelonia mydas) is named for its herbivorous diet of seagrasses and algae which turns its fat a greenish color. They are the largest of the hard-shelled sea turtles, outsized only by the soft-shelled Leatherback. Greens can live to 70 years old or more and reach maturity between 25-35 years old. They undertake reproductive migrations every 2 to 5 years and will nest approximately every 10 days over the course of several months. The average clutch size of a Green sea turtle's nest is about 110 eggs.

 Green sea turtles also face a number of threats globally.  There are currently 11 distinct population segments (DPS) around the world that are listed under the Endangered Species Act. Similar to Hawksbills, one of these major threats includes the loss and degradation of nesting habitat. Another major threat to sea turtles is global warming, which is likely to lead to higher sand temperatures on nesting beaches. All sea turtle species exhibit temperature-dependent sex determination (TSD). This means that the sex of sea turtle hatchlings is determined by the temperature at which eggs are incubated. Nests incubated at warm temperatures, those above 87.8° F, result in all female hatchlings, while those incubated in cooler temperatures, those below 80.6° F, result in all male hatchlings. Rising sand temperatures on nesting beaches could lead to a female-biased skew in the sex ratio of many sea turtle populations. This in turn could lead to future difficulties with mate-searching and reproduction.

Green sea turtles are also circumtropical. They nest in over 80 countries around the world but primary nesting sites in the United States are in Hawaii, Pacific Island territories, Florida, Puerto Rico, and the U.S. Virgin Islands.

Buck Island Reef National Monument (BIRNM) is located approximately one mile northeast of mainland Saint Croix. It is a fully protected marine area that encompasses 19,015 acres and includes Buck Island, a 176-acre dry tropical forest island. The island consists of approximately 1.5 km of suitable shoreline nesting habitat and 2.58 km of rocky shoreline that is unsuitable for nesting.

BISTRP divides the suitable shoreline habitat into four beach sectors: North Shore (375 m), West Beach (510 m), South Shore (260 m) and Turtle Bay (255 m). These are the four beach sectors they monitor for sea turtle nesting activities.

The four monitored beach sectors of Buck Island are marked off approximately every 15 meters with semi-permanent beach markers. These beach markers are PVC stakes numbered from 001-100 and help BISTRP researchers mark the locations of observed sea turtle nesting activities.

North Shore (markers 001--0124) consists of a very slender sandy beach leading into a dense beach forest. The beach forest is composed of woody plants and trees such as Sea Grape (Cocoloba uvifera), Casha (Acacia tortuosa), and Manchineel (Hippomane mancinella). There are frequent physical obstructions on the shoreline of North Shore due to fallen trees and eroded roots.

West Beach (markers 025--058) consists of a wide, open sandy beach transitioning into short, sprawling vegetation like Sea Purslane (Sesuvium portulacastrum), Inkberry (Scaevola plumieri) and Sand Bur (Cenchrus echinatus). Unlike the other beach sectors, there is no immediate barrier reef to shield West Beach from wave action. Subsequently, West Beach is subject to significant sand displacement and seasonal shifts in beach layout.

South Shore (markers 059-082) is slender beach consisting of coral cobble and sand which leads into an area of low-lying shrubs and seaward vegetation before becoming a dense beach forest. South Shore is relatively protected from prevailing winds and waves and the beach is bordered by fringes of reef.

Turtle Bay (markers 083-100) is the eastern most nesting beach on Buck Island. It is a crescent-shaped sandy beach with plentiful low-lying vegetation like Sea Purslane, Inkberry, and native grasses. The seaward vegetation leads into a low Sea Grape forest.

The Buck Island Sea Turtle Research Program engages in nightly beach patrols throughout the peak nesting season. The peak sea turtle nesting season is from July through October. Nocturnal patrols are conducted by research technicians on foot every hour, with the objective of observing and recording every individual sea turtle who crawls up the beach. Each female is either identified by existing tags or new tags are applied.

Each sea turtle receives a full work-up once during the season, although components of the work-up may be spread out among encounters. A work-up consists of carapace measurements, photographs, tissue and carapace samples, and adding or replacing missing tags. It can take a female anywhere from 45 minutes to 2 hours to begin laying her nest. This is because she will first try out several places before settling on a suitable nest site and digging a nest chamber. Once a turtle begins physically laying her eggs, she will enter into a trance-like state. This is when the research permit allows BISTRP researchers to conduct work-ups.

Other important data that are collected for each successful and non-successful nesting activity include GPS coordinates, distance and bearing measurements to the nearest beach marker, paces to the high water mark, and a hand-drawn diagram of the nest's location.

Once the nocturnal field season ends, BISTRP researchers continue to monitor all four beach sectors in order to record any new nocturnal nesting activities by assessing fresh tracks in the sand. The diurnal season is also when BISTRP begins to collect data about nest success on Buck Island. Each of the nests that was observed during the nocturnal season is excavated and its contents are recorded. Nest excavations typically occur 60 days after the nest was laid or after hatchling tracks are observed. During excavations, hatched shells and hatchlings are counted and any intact eggs are opened to evaluate causes of mortality. Any live hatchlings that are still trapped within the nests are released from the beach.

Sometimes nests are extremely difficult to find, even if researchers take thorough notes and precise measurements at the time of nesting. The ID tags that BISTRP researchers place on nearby vegetation during the nocturnal season can rust and fall off. ID tags can also be buried by sea turtles in the process of camouflaging their nests. Nesting sea turtles will create giant craters in the beach during their process of selecting a suitable nest site. New nesting activities that occur in the same location as older activities can can change the overall look of the area. Tropical storms and seasonal erosion can also change the layout of the beach in many areas and landmarks may move or disappear.

Sea turtles exhibit a high degree of site fidelity and natal homing. This means that adult female sea turtles will return to beaches in the area where they hatched decades earlier, sometimes the very same beach, in order to lay their nests. Since its inception in 1988, BISTRP has recorded 762 unique females that have used the beaches of Buck Island to lay their nests. During the 2022 nesting season, we observed a total of 57 individuals: 29 Hawksbills, 27 Greens and 1 Loggerhead. I was immediately curious about the degree of site fidelity shown by Buck Island's turtles. I wanted to know if they show a preference for specific beaches on Buck Island each time they return or whether they make use of all four beaches indiscriminately.

To answer this question, I chose one individual from each species that has returned to Buck Island consistently over the last 20 years. Because BISTRP did not begin taking GPS points for nest locations until 2019, I had to calculate coordinates for earlier years using recorded measurements of distance and bearing to the beach markers. I then plotted each of these points on a map and time-enabled the layers to display nest locations chronologically.

Hawksbill XXB328 regularly uses Buck Island's beaches as nesting habitat and shows a clear preference for nesting on North Shore. 86% of her nests from 2002-2022 were laid on North Shore. This may be because Hawksbills prefer to nest under vegetation and the beach forest on North Shore provides ample cover. However, North Shore is not the only beach sector with plentiful vegetation. This female's high fidelity to North Shore could also indicate that North Shore was her natal beach. Her high fidelity to one beach sector on Buck Island could also imply a lack of adaptability that could be concerning in light of the changing climate.

Green XXB809 also regularly uses Buck Island as nesting habitat and shows high fidelity to one of the four nesting beaches: South Shore. 74.3% of her nests between 2004 and 2022 were laid on South Shore. XXB809 also occasionally makes use of Turtle Bay: 20% of her nests in the last 18 years were laid there. Similar to Hawksbill XXB328, this Green female shows clear preference for certain beaches on Buck Island. This could indicate that she hatched out from one of these two southward facing beaches (South Shore, Turtle Bay) decades ago and is navigating back to this site each breeding season. Similarly, her high site fidelity may indicate a lack of adaptability and could be problematic in the face of climate change.

Each year, the beaches of Buck Island are subject to seasonal wind and wave patterns that direct the movement of sand and sediment. This can lead to a complete reconfiguration of the open, sandy beach on West Beach. These seasonal wind and wave patterns transport sediment from onshore to offshore as well as between West Beach and South Shore. Buck Island Reef National Monument is also subject to many major storm events in the form of tropical storms and hurricanes. High storm surge from such events can cause much more dramatic and problematic beach erosion. Storm surge can cut deep into Buck Island's beaches and erode out adult trees, blocking the approach crawls of sea turtles attempting to nest. This is especially prevalent on North Shore, where the beach forest leads right up to the sea without much of a sandy beach buffer.

In addition to physical obstructions from eroded trees and root tangles, storm erosion can heighten the beach berm by up to 3 feet (KellerLynn, K. 2011). This means that the berm can become too steep for a sea turtle to climb and effectively blocks all nesting in that area. Some sea turtles that become frustrated by repeated unsuccessful attempts to nest in their preferred habitat may end up laying halfway in the berm or much too close to the high water mark and their nest could be washed out by waves. With Hawksbills, another big problem that arises when they cannot access their preferred forest habitat is that Hawksbill nests laid in open beach habitats are typically too shallow to escape rainwater inundation and solar radiation. Additionally, nests that are laid too close to the high water mark or nests that gradually become closer to the high water mark due to seasonal erosion patterns can then be subject to washover and water inundation from storm surge.

Barrier reefs around Buck Island help to mitigate seasonal and storm erosion by providing a buffer from waves, but these reefs are also facing degradation from warming temperatures, major storms and infectious coral diseases. If these reefs continue to degrade, the beaches of Buck Island will be left unprotected and the impacts of storm surge on Buck Island could become even more dramatic.

The map below shows a time-enabled animation of the 2022 nesting season on Buck Island. Each line represents the shoreline of Buck Island on a given date during the 2022 nesting season. The triangles represent Hawksbill nests while the circles represent Green nests. Each nest is colored on a diverging scale from red to green according to its level of hatch success. Red shapes indicate low hatch success and greener shapes indicate higher hatch success. Play the animation to watch how the shoreline of Buck Island changed during the peak sea turtle nesting season. Click on each shape to learn more about each nest that was laid and its success.

Two major storm events, Tropical Storm Earl and Tropical Storm Fiona, hit Buck Island in the month of September. The resulting storm surge washed out one Hawksbill nest on North Shore that was laid too close to the high water mark. The entire nest was lost. On the south-facing beaches, one area of Turtle Bay at beach marker 100 was subject to water inundation from storm surge. This can be clearly seen on the map by looking at the far end of Turtle Bay and the five Green sea turtle nests that had a hatch success rate of approximately zero percent (symbolized by red circles). These nests were completely inundated with seawater and completely unsuccessful.

The sea turtles that regularly use Buck Island as nesting habitat have clear habitat preferences and consistently depend on specific stretches of beach to lay their nests. The high levels of site fidelity shown by Buck Island's sea turtles could indicate a lack of adaptability which could be problematic in the face of climate change and the worsening erosion of Buck Island's nesting beaches. Seasonal and storm-caused beach erosion can have dramatic effects on the availability of suitable sea turtle nesting habitat on Buck Island. Trees that are eroded out of the shoreline on North Shore create physical obstructions for Hawksbill sea turtles attempting to access their preferred nesting habitat. Erosion caused by high storm surge can also create a beach berm that is too steep for nesting sea turtles to climb. Frustrated sea turtles may end up laying in nesting habitat that is unsuitable for their species. Hawksbills in particular really rely on dense vegetation cover to shade their nests from the sun and protect their nests from rainwater inundation. On Buck Island, the National Park Service has successfully managed for many threats to nesting sea turtles, including the removal of mammalian predators and the prohibition of poaching and harvest of adults, eggs and hatchlings. However even on this federally protected and carefully managed monument, there are still present and significant threats to the conservation of these threatened and endangered species. Going forward, an important management focus on Buck Island will be addressing the erosion and degradation of these critically important nesting habitats.

I would like to thank Kristen Ewen from the National Park Service unit in Saint Croix for her mentorship throughout the 2022 field season. Thank you to the wonderful 2022 BISTRP research team for all of their hard work and good company, and for enduring the long nights, spiny plants, and perpetually wet and sandy feet in order to collect this data. Thank you to The Buccaneer Hotel staff for housing me and providing me with delicious meals during my time in Saint Croix. Special thank you to Sophie Mirotznik and Skyler Roberts, without whom I would have surely lost my sanity on Buck Island. And the biggest thank you of all goes to the sea turtles who made this project possible, their charming lives were a most delightful thing to fixate on for a final project.