Hawksbills (Eretmochelys imbricata) are very complex and specialized marine reptiles. To mature, reach adulthood, reproduce and complete the life-cycle, they need a variety of environments, including beaches, open sea, and coastal and estuarine waters. During the normal life cycle, the turtles disperse and migrate; traveling long distances, often thousands of kilometers long, habitually moving between the open sea and the territorial waters of many nations.
Growth rates vary according to size classes and places, but they are sufficiently slow to cause hawksbills to require decades to mature: the time from egg hatching to a turtle's return to the same beach to reproduce for the first time may take 20-40 years. Under normal conditions, the average hawksbill is capable of living and reproducing for at least 10 years after reaching maturity. Their fecundity, or reproductive output, is usually very high: they lay an average of 140 eggs in a single clutch, depositing several clutches per season and nesting for many seasons, though not every year.
Their high fecundity is offset by elevated mortality during the early phases of the life-cycle. Many eggs do not survive the incubation period, many hatchlings do not reach the sea and of those that do, many do not survive more than a day. In many ways, turtle survival depends on correct reactions at the right moment and finding suitable conditions in particular environments.
This means responding correctly to light on the horizon when emerging from the nest, successfully running the gauntlet from the beach to the sea, avoiding certain bodies of water in the open sea, and choosing a specific environment for food and shelter. After several years of pelagic existence, the immature turtles move to live in benthic waters, where they remain in residence for a limited time, possibly maintaining a territory free of other hawksbills.
Upon reaching maturity, they orient and swim toward a specific nesting beach. A large variety of predators may prey on hawksbills in all stages of the life cycle, but the eggs, hatchlings and small immature individuals suffer the most intense predation. From one development stage to the next, the number of turtles remaining in the population grows progressively smaller, and at the end, probably less than one egg in a thousand survives to produce an adult turtle.
Not enough is known about the sex ratio, but females tend to predominate in populations of immatures. There are few studies on age structure, recruitment and survivorship of different phases of the life cycle, but when a turtle attains large size and matures, the survival rate is potentially high, around 95% per year for some nesting females.
The available scientific data on migrations and genetic markers show that hawksbills are shared international resources. Genetic studies also reveal that each nesting population should be treated as a distinct management unit. Those studies, especially when accompanied by tag recovery data from marked individuals, indicate that each nesting population forms an independent demographic entity, genetically isolated from other populations.
In contrast, the groups in the feeding zones are mixed stocks, and although the individuals may be found together on the same reefs, they usually represent distinct management units, temporarily united by a common biological activity. Therefore, the management of the nesting populations as well as the non-reproductive assemblages depends on international cooperation.
Surprisingly, many known cases of sea turtle hybridization have occurred with hawksbills. The importance of this fact regarding the evolution of sea turtles or the concept of "biological species" is not known. Many of the demographic characteristics of hawksbills characterize them as having "late maturation and longevity".
These characteristics include attributes such as the presence of numerous age classes or superimposed generations in the same population, as well as the relatively large number of immature individuals, which are necessary for maintaining a stable population with a relatively small number of adults.
This kind of population structure is particularly vulnerable to certain disturbances, in particular to the impact of some human actions that compromise the vitality of the adult population segment, as is the case with directed take.
It is impossible to calculate the absolute size of the populations with any certainty, but throughout the world most hawksbill populations are debilitated and diminishing, often precipitously. In addition to overexploitation of their eggs and meat, their reduction has largely been caused by an ongoing demand for the scutes of their shells, which are used to make craft items.
Now the species is considered to be critically endangered with extinction, according to the World Conservation Union (IUCN). In the Caribbean, with few exceptions, most of the units of the genetic mosaic are in decline. As has occurred with other living marine resources, decimated hawksbill populations are subject to the "shifting baseline syndrome," in which people perceive current population levels as normal, due to the lack of collective memory regarding past population levels. It is believed that the reduction in hawksbill abundance has caused changes in the structure and function of the coral reefs.
Sea turtle conservation poses great challenges for modern societies. Although much has been learned about the biology of these animals in the last fifty years, there are still large gaps in our knowledge. Decisions regarding resource management and conservation are inevitably made with information that is insufficient and fragmented.
Nevertheless, we now know that hawksbills travel through the jurisdictional waters of various nations during their migrations and that animals from diverse sources and genetic units converge in feeding zones. Therefore, hawksbill conservation in the Caribbean is a regional challenge that will require multinational coordination and the commitment of diverse players from public and private sectors.
- By Didiher Chacón-Chaverri
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