The Travels of the Hawksbill Turtle
Marine scientist and Technical Advisor to Save Our Sea turtles (SOS) Tobago, Dr Michelle Cazabon-Mannette, describes the fascinating life cycle of hawksbill turtles, the long journeys they make, the different habitats they inhabit, and the challenges they face. This article first appeared in the Tobago Newsday on Thursday 20h July 2017
We start the tale on a warm sandy beach in the tropics, where a female hawksbill has laid a clutch of about 150 round white eggs each the size of a golf ball. The eggs will incubate for about 60 days, before hatching in synchrony. Once the eggs hatch, it takes several days for the hatchlings, with an average shell length of just 4cm or 1.5 inches, to climb to the surface of the sand. They work together to wriggle their way through the sand, but once they hit the surface, it’s “every man for himself” as they make a mad dash for the sea. Natural instincts kick in and the hatchlings head for the brightest source of light (the sea) and then from the water’s edge they head into the oncoming waves. It is at this stage that they are most vulnerable to their natural predators which include various species of birds, mammals and even crabs and ants. Those hatchlings that survive their treacherous trip across the beach face more predators waiting for them just offshore, including birds and several species of fish. Many young turtles will be lost as they run this gauntlet of predators, but this loss is compensated for by the high numbers of eggs that each female turtle can produce in her lifetime - laying on average 4-5 clutches of 140-158 eggs each, every 2-4 years, over a period of several decades.
The hatchlings continue to swim away from shore, and go on to spend their first few years of life in the surface waters, out in the open ocean. This period of their life is often referred to as the “lost years” since they are so difficult to study at this small size and so little is known about where they go. Their route across the ocean basin is determined by a combination of the ocean currents, and their swimming behaviour. They can travel for hundreds to thousands of kilometres during this time, and are often found in association with masses of seaweed such as Sargassum, where they shelter from predators and feed. Unfortunately plastics and other pollutants that drift at the surface can also accumulate in the same areas, which make these young turtles prone to accidentally ingesting them.
Only when their shell is about 20-35cm (8-14 inches), a few years after leaving the nesting beach, do we begin to see juvenile hawksbill turtles close to shore feeding at coral reef, hard bottom and cliff-wall habitats. This represents a major change in their lifestyle, from drifting far from shore and feeding near the surface, to living close to shore and feeding almost exclusively on sponge (simple invertebrate organisms that grow fixed in one place) at the seafloor. This is quite a unique diet – few other animals feed on sponge in such great quantities. As a result of their dispersal at sea, hawksbills at any feeding ground would have originated from several different distant beaches. Once juvenile hawksbills make this shift to a nearshore habitat, they have already travelled great distances. But, their travelling ways change for the time-being – they remain within a relatively small home range for several years, until they approach maturity at the age of about 25-35 years. During this period, these juvenile turtles can still be prone to large natural predators like sharks as they spend their days foraging for food, and their nights resting on the reef.
On approaching maturity, at about 70cm shell length (28 inches), hawksbills migrate to an adult foraging ground, which may be closer to the nesting population to which they belong, but resembles the same type of habitat where they would have spent their juvenile years while developing, therefore there is no major change in diet or lifestyle. From there, females set out on their breeding migrations every 2-4 years on average. Each female returns to the beach where she was born, in an amazing feat of navigation, over distances as much as hundreds or even thousands of kilometres, to mate and lay her own eggs on, or very close to, the very same beach she left as a hatchling about 30 years before. Less is known about adult males, but it is believed that they make shorter breeding migrations every year, to nearby nesting beaches that are not necessarily their beach of origin. Adults continue making these migrations for at least 2-3 decades. And so the circle of life continues as each new generation of ocean ambassadors is deposited in the warm tropical sand, waiting to begin their journey.
 |
| A hawksbill hatchling makes its way to the sea. Photo courtesy Ryan P. Mannette |
We start the tale on a warm sandy beach in the tropics, where a female hawksbill has laid a clutch of about 150 round white eggs each the size of a golf ball. The eggs will incubate for about 60 days, before hatching in synchrony. Once the eggs hatch, it takes several days for the hatchlings, with an average shell length of just 4cm or 1.5 inches, to climb to the surface of the sand. They work together to wriggle their way through the sand, but once they hit the surface, it’s “every man for himself” as they make a mad dash for the sea. Natural instincts kick in and the hatchlings head for the brightest source of light (the sea) and then from the water’s edge they head into the oncoming waves. It is at this stage that they are most vulnerable to their natural predators which include various species of birds, mammals and even crabs and ants. Those hatchlings that survive their treacherous trip across the beach face more predators waiting for them just offshore, including birds and several species of fish. Many young turtles will be lost as they run this gauntlet of predators, but this loss is compensated for by the high numbers of eggs that each female turtle can produce in her lifetime - laying on average 4-5 clutches of 140-158 eggs each, every 2-4 years, over a period of several decades.
The hatchlings continue to swim away from shore, and go on to spend their first few years of life in the surface waters, out in the open ocean. This period of their life is often referred to as the “lost years” since they are so difficult to study at this small size and so little is known about where they go. Their route across the ocean basin is determined by a combination of the ocean currents, and their swimming behaviour. They can travel for hundreds to thousands of kilometres during this time, and are often found in association with masses of seaweed such as Sargassum, where they shelter from predators and feed. Unfortunately plastics and other pollutants that drift at the surface can also accumulate in the same areas, which make these young turtles prone to accidentally ingesting them.
Only when their shell is about 20-35cm (8-14 inches), a few years after leaving the nesting beach, do we begin to see juvenile hawksbill turtles close to shore feeding at coral reef, hard bottom and cliff-wall habitats. This represents a major change in their lifestyle, from drifting far from shore and feeding near the surface, to living close to shore and feeding almost exclusively on sponge (simple invertebrate organisms that grow fixed in one place) at the seafloor. This is quite a unique diet – few other animals feed on sponge in such great quantities. As a result of their dispersal at sea, hawksbills at any feeding ground would have originated from several different distant beaches. Once juvenile hawksbills make this shift to a nearshore habitat, they have already travelled great distances. But, their travelling ways change for the time-being – they remain within a relatively small home range for several years, until they approach maturity at the age of about 25-35 years. During this period, these juvenile turtles can still be prone to large natural predators like sharks as they spend their days foraging for food, and their nights resting on the reef.
 |
| Juvenile hawksbill foraging on a reef. Photo courtesy Ryan P. Mannette |
On approaching maturity, at about 70cm shell length (28 inches), hawksbills migrate to an adult foraging ground, which may be closer to the nesting population to which they belong, but resembles the same type of habitat where they would have spent their juvenile years while developing, therefore there is no major change in diet or lifestyle. From there, females set out on their breeding migrations every 2-4 years on average. Each female returns to the beach where she was born, in an amazing feat of navigation, over distances as much as hundreds or even thousands of kilometres, to mate and lay her own eggs on, or very close to, the very same beach she left as a hatchling about 30 years before. Less is known about adult males, but it is believed that they make shorter breeding migrations every year, to nearby nesting beaches that are not necessarily their beach of origin. Adults continue making these migrations for at least 2-3 decades. And so the circle of life continues as each new generation of ocean ambassadors is deposited in the warm tropical sand, waiting to begin their journey.
Comments
Post a Comment