Hawksbills: The Most Beautiful of Sea Turtles
By Jeanne A. Mortimer, Katia Ballorain, Carlos E. Diez, Nicole Esteban, Nancy FitzSimmons, Alexander R. Gaos, Graeme Hays, Christine A. Madden Hof, Michael P. Jensen, Michael Liles, Roderic Mast, Anne Meylan, Nicolas Pilcher, Jeffrey A. Seminoff, and Scott Whiting
A hawksbill turtle surfaces at sunset on the Great Barrier Reef as a group of Black Noddies flies overhead. © Jordan Robins Photography
Remarkable, Tropical, and Hunted for Centuries
Noted for the thick, overlapping, dappled cream-and-brown scutes that cover its carapace and plastron and provide excellent camouflage against a coral reef backdrop, the hawksbill sea turtle is considered by many to be the most beautiful of all turtle species. Its pointed, beak-like head, from which its common name derives, enables it to forage in crevices and feed on prey with leathery or heavily armored outer surfaces. It is the most tropical of sea turtles, nesting on the coasts and islands of some 70 countries whose shores are located primarily between the Tropic of Cancer and the Tropic of Capricorn. The hawksbill demonstrates many surprising biological and ecological traits that make it remarkable among sea turtles.
Hawksbills are pantropical and found in every major ocean basin worldwide. As can be seen on the maps below, the largest population centers for nesting hawksbills include the warm waters of the Gulf of Mexico and the Caribbean Sea, the southwest and northwest Indian Ocean (including the Red Sea and the Persian/Arabian Gulf), and tropical Australasia. Smaller numbers of hawksbills nest in Oceania and the Hawaiian Islands, the eastern Atlantic (West Africa), the western Atlantic (Brazil), and in the recently rediscovered eastern Pacific populations. The IUCN-SSC Marine Turtle Specialist Group recognizes 13 regional management units (RMUs) for hawksbills. Each RMU comprises multiple genetic stocks or management units (MUs) that are mostly defined by significant differences in mitochondrial DNA haplotypes. Each MU may comprise several neighboring rookeries facing similar threats. To date, 30 MUs for hawksbills have been identified globally, and many more are expected to be identified in the near future. Most of the detailed genetic studies undertaken thus far have focused on the western Atlantic and eastern Pacific regions. These have shown genetic differences at unexpectedly fine scales, including differences between rookeries found on opposite sides of the same island. In the Indo-West Pacific, only eight MUs have been defined to date, and the majority of hawksbill populations in the western Pacific have not yet been assessed for their genetic population structure. Significant gaps remain in our understanding of the genetics of hawksbill populations, as evidenced by studies of foraging grounds that have shown variants not previously recorded at rookeries. For more on hawksbill genetics, see “Genetic Tools for Sea Turtle Conservation.”
Decades ago, biologists thought that hawksbill sea turtles were by nature solitary nesters, but today it is commonly believed that their characteristic low-density nesting is likely an artifact of the long-term overexploitation that has persecuted the species for centuries, mostly for its prized shell (often called tortoiseshell). Tortoiseshell can be fashioned into valuable items ranging from jewelry and trinkets to elaborately carved combs, eyeglass frames, sculptures, and even spurs for fighting roosters. Trade statistics are key to understanding the enormous impact that this commerce has wrought on hawksbills over time. Millions of hawksbills have died in the past century alone, yet the trade can be traced back millennia. Tortoiseshell objects have been found in the graves of the Nubian rulers of predynastic Egypt, the ruins of China’s Han Empire, and the middens of pre-Columbian cultures in the Caribbean. Throughout much of human history, hawksbills have paid a lethal price for their beauty.
A hawksbill turtle hovers above the reef. Larger hawksbills are seen on the coral reefs outside of the lagoon in Europa National Natural Reserve, Eparses Islands, France, Indian Ocean. © H. Sauvignet
Distinctive among Sea Turtles
Among the six species of hard-shelled sea turtles, adult female hawksbills tend to be smaller than green turtles, loggerheads, and flatbacks, but larger than olive or Kemp’s ridleys. Throughout most of their range, hawksbills nest primarily at night, as do the larger species of sea turtle, but in much of the western Indian Ocean they more typically nest during the day. What adaptive value diurnal nesting may offer these animals remains a mystery. Unlike other sea turtles, hawksbills tend to dig relatively shallow nests that are often placed under vegetation, possibly to help optimize incubation temperatures and humidity in their shallow nests. In several areas of the eastern Pacific, hawksbills are distinctly associated with mangrove ecosystems, where they even nest on sandy banks hidden amid mangrove roots and shoots.
The diet of hawksbills is peculiar among sea turtles. During their posthatchling pelagic phase, western Atlantic hawksbills are closely associated with floating rafts of the brown algae Sargassum and appear to share the omnivorous and opportunistic diet of posthatchling loggerheads, green turtles, and Kemp’s ridleys, feeding on Sargassum, fish eggs, tunicates, goose barnacles, and more. When older, hawksbills transition to benthic feeding habitats, where they dine predominantly on sponges, and on items that can include corallimorphs (coral-like anemones), zoanthids, tunicates, and algae. At some sites, the hawksbill diet shows variability, particularly where sponges are scarce or absent.
Spongivory is especially rare among other marine creatures given the array of toxic chemical compounds that can be found in sponges, not to mention the gut-piercing glass spicules found in some. Yet hawksbills take these dietary challenges in stride and consume specific sponge species in large quantities. The tendency of hawksbills to consume prey items that other species do not is a strategy that may limit interspecific competition. Moreover, whether by coincidence or evolutionary design, a diet rich in sponges may be the reason hawksbill meat is sometimes toxic or even fatal to humans who consume it. For this reason, hawksbill meat is frequently off the human menu in some localities in the Indo-Pacific region and elsewhere, where other sea turtle species are preferentially consumed.
Foraging resident hawksbills have a close association with coral reefs and rocky reefs, but they also thrive in a wide variety of other habitats, including seagrass, algal beds, mangrove bays, creeks, and even mud flats. In the eastern Pacific, juvenile and adult hawksbills can spend virtually all of their lives in mangrove-lined estuaries, foraging among aboveground mangrove roots, and even feeding directly on mangrove fruits and seeds.
High density aggregations of juvenile hawksbills have been documented in coastal waters near cities as well. Those “urban hawksbills” inhabit degraded reefs that provide them with food, shelter, and resting sites. In Puerto Rico, the growth rates and weight-to-length relationships of these city-dwelling hawksbills are similar to those of animals that live in more natural habitats, suggesting that the species may be relatively resilient to habitat change. Nevertheless, hawksbills residing in marinas in Seychelles, Hawaii, and some sites in the eastern Pacific often appear emaciated and unhealthy, suggesting limits to their tolerance for habitat degradation.
Though hawksbills inhabit coastal waters in more than 108 countries, their movements are among the least studied of all sea turtle species. In some regions, postnesting hawksbills tend to migrate shorter distances than postnesting green turtles (e.g., the Indian Ocean, Hawaii, and the eastern Pacific, where hawksbill movement corridors are often highly coastal), but this pattern does not hold in the Caribbean or the western Pacific regions (see map of Hawksbill Turtle Satellite Telemetry below). Developmental migrations made by immature hawksbills may be even more extensive than the reproductive migrations of adult hawksbills in the same region. Satellite tracking, molecular genetics, and flipper tagging have demonstrated that within each life stage of a hawksbill population, some individuals may engage in particularly extensive migrations. Moreover, larger hawksbills, including postnesting females, seem to venture deeper, farther from shore, and into lesser-known foraging habitats.
Hawksbill hatchlings typically enter a pelagic foraging phase that transitions to benthic foraging at sizes that vary depending on the ocean basin. Those transitions occur at 20, to 25-centimeter carapace length in the Atlantic Ocean and around 30-centimeter length in the Indo-Pacific. Remarkably, in the eastern Pacific some posthatchling hawksbills skip the oceanic stage altogether and remain within the mangrove estuarine habitat that hosts their natal nesting beaches. In such tidally dominated ecosystems, hatchlings grasp floating debris as a dispersal strategy, which may provide energy-saving transport and safety as they cryptically hitchhike with the current.
The Persian/Arabian Gulf experiences dramatic annual fluctuations in sea temperature, ranging from a low of 17°C to a high of 37°C. Although hawksbills appear to be adept at avoiding or tolerating the temperatures in the upper range, many succumb to cold stunning at the lower range. In the United Arab Emirates, hundreds of small hawksbills wash up covered in barnacles and algae each year, and many are subsequently rescued and rehabilitated. Green turtles in the same region are not similarly afflicted, which highlights the tropical nature of hawksbills compared to other sea turtle species.
Mangrove channels provide habitat for juvenile hawksbill turtles in the lagoon of Europa National Natural Reserve, Eparses Islands, France, Indian Ocean. © H. Sauvignet
Conservation Challenges
The following issues present challenges to hawksbill conservation efforts worldwide.
Direct Take and Trade
Hawksbills have been classified globally as critically endangered on the IUCN Red List of Threatened Species since 1996, and a reassessment is currently under way. They were brought to the brink of global extinction in the twentieth century by the unrelenting international tortoiseshell trade. In 1977, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) banned the trade in hawksbill products among its signatory states. But, because some signatory parties had legally registered reservations for hawksbill sea turtles, CITES had little impact until the 1990s, when all reservations against the ban were dropped. This change led to some wonderful success stories.
In 1994, the government of the Republic of Seychelles reversed a devastating decline in hawksbills by purchasing all worked and unworked tortoiseshell in the country and prohibiting trade in sea turtle products. Then, in 1998, the stockpile of tortoiseshell was publicly burned. Today, you will not find hawksbill shell products for sale anywhere in Seychelles. In March 2022, the government of Cuba also demonstrated its commitment to nonconsumptive, sustainable use of sea turtles for tourism, research, and ecological services by announcing the destruction of its 8.1–metric ton stockpile of hawksbill shell procured in that country prior to 2008, when hawksbill capture became illegal.
The battle to save the hawksbill from human depredation continues despite these and other successes. Illegal international trade and destructive levels of domestic consumption of hawksbill shell continue in many parts of the world. Animals continue to be taken for meat and eggs throughout much of their range, and demand for hawksbill in new and re-emerging black markets in Southeast Asia (including China, Japan, Vietnam, Taiwan, and Hong Kong) is on the rise. This new demand is exacerbated by the growing Chinese presence and purchasing power in the Pacific, combined with illegal, unregulated, and unreported fishing—particularly by the Chinese distant-water fleet. While this demand remains, communities and other small-scale fisheries are becoming increasingly involved and trapped in this vicious trade cycle, whether they are targeting or opportunistically benefitting from it. Also, the marine turtle supply chain has become increasingly fragmented and opaque in some countries, shifting to covert markets that thwart policy responses and enforcement efforts. Using genetics and DNA-based wildlife forensic science, nongovernmental organizations (NGOs), along with local communities, universities, government partners, and international networking efforts, are building a program called ShellBank, a database and platform to help governments, researchers, and conservation managers track the turtle trade from sale to source and to improve the enforcement of bans and identify which populations are most at risk.
Fisheries Bycatch
Some fisheries pose an especially serious threat to hawksbill sea turtles. Gill nets are particularly problematic, given the hundreds of thousands of small vessels that deploy them in shallow waters throughout the world. Lobster are problematic, as well, since they capture hawksbills opportunistically; so even when hawksbill numbers are low, the pressure on them continues unabated as long as the more lucrative resource (lobsters) remains reliable.
In the Indian Ocean, fish aggregating devices (FADs) associated with tuna purse seiners pose yet another threat. FADs float at the surface and cast a shadow into the water to attract aggregations of tuna but are often constructed of discarded nets and ropes that hang tens of meters below the surface, where they unintentionally entangle marine life—including sea turtles. In the southwestern Indian Ocean, FADs often drift into shallow waters, where they snag onto coral reefs and kill hawksbills. Discarded fishing gear and ghost nets relentlessly plague global waters, especially targeting young pelagic turtles that forage within these artificial habitats. Hawksbills are also killed by blast fishing in many parts of their range, another destructive technique that uses explosives to kill or stun fish and collaterally causes permanent damage to coral reefs and other sensitive habitats.
Negative Impacts on Nesting Habitat
Hawksbills are vulnerable at all stages of their life cycles, but especially at their breeding beaches, where nesting females and their eggs are easy to exploit. Unregulated coastal development can destroy nesting habitat when buildings and artificial lighting are placed too close to the beach, sea walls and other coastal armoring interfere with sand flow and beach access for nesting turtles, and the coastal vegetation under which hawksbills nest is removed. Daytime nesting hawksbills are shy animals that are vulnerable to disturbance from human activities as innocuous as picnics, sunbathing, swimming, and boating, even when the activity involves well-meaning tourists and local residents. Thus, critical stretches of nesting habitat need to be better incorporated into nature reserves that will be maintained in perpetuity and protected from unsupervised human access. Well-managed nature reserves can also produce revenue from ecotourism.
At many sites, both inside and outside formal nature reserves, hawksbill population decline caused by over-exploitation for meat and eggs has been effectively reversed by combining protective legislation with long-term monitoring of nesting beaches (see maps below). Beach monitoring programs not only collect population data, but they also serve as a socially responsible approach to conservation when livelihood incentives are tied to conservation outcomes in low-income regions, and as a deterrent to illegal activity. Those programs are particularly effective when implemented by community members because they help create public awareness and support economic well-being. Such programs, however, are often used to provide index site data to represent the long-term status and trends of sea turtle nesting populations for the wider region, and this can be misleading. For example, index beaches that demonstrate increasing trends may not represent the population status of the wider region, in which many more turtle populations and their nesting habitats remain unmonitored and unprotected and may actually be in decline. Regardless of the situation, the expansion of monitoring programs across more nesting sites will allow for an increased understanding of hawksbill population trends and threats.
Genetic Mixing
In the Brazilian state of Bahía, hybridization between hawksbill and loggerhead turtles poses an unusual threat to both species. Genetic studies there have confirmed that first generation hybrid females produced by the mating of male hawksbills and female loggerheads have successfully produced viable hatchlings, resulting in a population in which turtles now share the mixed DNA of both species. More than 40 percent of the sampled hawksbill nesting population was found to comprise multigenerational hybrids that display loggerhead mitochondrial DNA haplotypes but morphologically appear to be hawksbills. Similar results were also found in a nearby loggerhead rookery, accompanied by evidence of lower survival of hybrid offspring. Considering the vastly different ecological roles of the two species, which were separated more than 20 million years ago, this is a remarkable and troubling phenomenon. Such findings raise conservation concerns about the evolutionary and ecological implications of hybridization and the processes that may be driving it.
Climate Change and Other Human Activity
Though difficult to quantify, climate change poses a threat to nesting beaches through sea level rise and erosion, as well as higher incubation temperatures that may feminize, decrease fitness, or result in hatchling mortality. Rising temperatures have already resulted in the destruction of critical habitats (such as coral reefs) on which hawksbills depend. And, like other marine turtles and marine life in general, hawksbills are threatened during all of their life stages by boat strikes and countless human-generated chemical toxins (see article on inorganic pollutants). Plastic debris in the sea and washed onto the beach is another challenging threat.
A Hopeful Future
On the bright side, many people and organizations are now taking action to learn more about hawksbills and to tackle the problems the animals face at local, national, and international levels around the world.
In the Indo-Pacific, the Memorandum of Understanding on the Conservation and Management of Marine Turtles and their Habitats of the Indian Ocean and South-East Asia (IOSEA MoU) has put in place a framework through which states, territories, governmental entities, and NGOs can work together to conserve marine turtles and their habitats. IOSEA recently published an “Assessment of the Conservation Status of the Hawksbill Turtle in the Indian Ocean and South-East Asia Region” and is currently working to create an action plan for hawksbills in the region. The TImOI project (Tortues Imbriquées de l’Océan Indien, or Hawksbills of the Indian Ocean) is another effort that uses genetics and satellite tracking to look at population connectivity among 13 countries and territories. In the Atlantic/Caribbean region, WIDECAST (the Wider Caribbean Sea Turtle Conservation Network) engages experts from more than 40 nations and territories, and the ICAPO network (Iniciativa Carey del Pacífico Oriental, or Eastern Pacific Hawksbill Initiative) has also made enormous strides over the past decade to amalgamate the efforts of multiple organizations working to engage with local fishers and community members to study and protect hawksbills in that important region (see “¡CAREY! Where Have the Eastern Pacific Hawksbills Gone?!” from SWOT Report, vol. III).
Now in Appendix I of CITES, hawksbills continue to enjoy the protection afforded by a complete ban on legal international trade in hawksbill products by signatory states. Hawksbills are also the focus of a special resolution of the Inter-American Convention for the Protection and Conservation of Sea Turtles (IAC) that promotes national legislation and conservation actions to protect sea turtles in 16 nations in the Americas.
Although problems persist, history has shown that with enough hard work and perseverance, conservation efforts can and will save this most beautiful of sea turtles.
Feature Map: Nesting Biogeography of Hawksbill Turtles
The map that follows displays available nesting data for hawksbill sea turtles. The data include 1,792 nesting sites, which were compiled through a literature review and provided directly to SWOT by data contributors worldwide. For metadata and information about data sources, see the complete data citations here.
Nesting sites are represented by brown dots scaled according to their relative nesting abundance in the most recent year for which data are available. Black squares represent nesting sites for which data are older than 10 years, data were unquantified, or the nest count for the most recent year was given as zero. For uniformity, all types of nesting counts (such as number of nesting females or number of crawls) were converted to number of clutches, as needed. Conversion factors were as follows: a ratio of 3.6 nests to each nesting female in the eastern Pacific and Indian Oceans, 4.3 nests to each nesting female in the Wider Caribbean and Atlantic Ocean, and 3 nests to each nesting female in Australia, plus a ratio of 0.6 nests for every crawl in all regions.
Nesting Biogeography of Hawksbill Turtles (2022). For data sources and other metadata, see the complete data citations.
Feature Map: Hawksbill Turtle Satellite Telemetry
The map below summarizes all available telemetry data from tags deployed on hawksbill sea turtles around the world. The data consist of more than 300,000 locations from 477 individually tracked turtles and were contributed by more than 51 partners (see data citations). Telemetry data are represented as polygons that are shaded according to the number of locations they contain. Darker brown represents a higher number of locations, which can indicate that a high number of tracked turtles were present or that turtles spent a lot of time in that location. Telemetry data are displayed as given by the providers, with minimal processing to remove locations on land and visual outliers. Thus, some tracks are raw Argos or GPS locations, whereas others have been more extensively filtered or modeled. For a complete list of data providers and available metadata, see the complete data citations.
Global Hawksbill Turtle Satellite Telemetry (2022). For data sources and other metadata, see the complete data citations.
This article originally appeared in SWOT Report, vol. 17 (2022). Click here to download the complete article as a PDF.
Author Affiliations
JEANNE A. MORTIMER, Turtle Action Group Seychelles, Mahé, Seychelles, and Department of Biology, University of Florida, Florida, U.S.A.
KATIA BALLORAIN, Centre d’Étude et de Découverte des Tortues Marines (CEDTM), Île de la Réunion, France
CARLOS E. DIEZ, Department of Natural and Environmental Resources, Puerto Rico
NICOLE ESTEBAN, Bioscience, Swansea University, Wales, U.K.
NANCY N. FITZSIMMONS, Australian Rivers Institute, School of Environment and Science, Griffith University, Australia
ALEXANDER R. GAOS, Pacific Islands Fisheries Science Center, NOAA-NMFS, Hawaii, U.S.A.
GRAEME C. HAYS, Deakin University, Victoria, Australia
CHRISTINE A. MADDEN HOF, World Wide Fund for Nature, Coral Triangle Programme, Indonesia; and University of the Sunshine Coast, Queensland, Australia
MICHAEL P. JENSEN, Department of Chemistry and Bioscience, Aalborg University, Denmark, and Australian Rivers Institute School of Environment and Science, Griffith University, Australia
MICHAEL LILES, Asociación ProCosta, El Salvador
RODERIC B. MAST, Oceanic Society, California, U.S.A.
ANNE MEYLAN, Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, Florida, U.S.A.
NICOLAS PILCHER, Marine Research Foundation, Sabah, Malaysia
JEFFREY A. SEMINOFF, Southwest Fisheries Science Center, NOAA-NMFS, California, U.S.A.
SCOTT WHITING, Department of Biodiversity, Conservation and Attractions, Western Australia, Australia