Natural threats and predators
Adult sea turtles have few predators, except large sharks and orcas, because these predators can bite through the turtle shell.
Eggs, newly hatched and juvenile sea turtles, on the other hand, are exposed to a wide variety of predators. Not only do crabs, dogs, cats and seabirds dig for their eggs, so do pigs, monitor lizards and other animals. The nests of sea turtles can be destroyed by fungal infections or by flooding during high waves. Once the young turtles have made their way into the sea, they can be eaten there by many kinds of predatory fish.
Furthermore, diseases (especially fibropapillomatosis), fouling, and parasites also trouble the turtles.
However, sea turtles are well adapted to these threats through millions of years of evolution; they do not pose a threat to the continued existence of sea turtles. However, the situation is different due to the massive man-made threats in the last few centuries.
Estimates speak of over 1 billion adult individuals of sea turtles of all seven species before the appearance of humans (including about 600 million green sea turtles). Humans have used sea turtles as a food source since prehistoric times. However, it only became a real threat with the onset of industrialization about 200 years ago. In the last 100 years alone, the global sea turtle population has declined by 60%. Current global populations of adult Kemp’s ridley sea turtles are probably in the single digits of 1,000; leatherback turtles a few 10,000. Man-made (anthropogenic) threats can be distinguished between direct and indirect threats.
Direct action on sea turtles: Hunting and exploitation of eggs
Although sea turtles are already protected in many parts of the world, they are still hunted and their meat eaten. The hawksbill turtles are additionally hunted for their coveted shell and are therefore now particularly threatened with extinction. In many countries, people dig up sea turtle eggs from the beach to eat or sell as a delicacy.
Since direct hunting of sea turtles and mass collection of eggs has already been largely eliminated in most countries, indirect threats are becoming increasingly important. Indirect threats are not directed specifically against sea turtles, but arise from the activities and influences of an ever-increasing human population. In contrast to direct threats, indirect threats continue to increase, sometimes massively. They are often much more difficult to counter with protective measures than direct threats, which are now almost always illegal and therefore subject to criminal prosecution. Of the indirect threats, the following are the most important:
Tourismus und Küstenverbauung
All sea turtle species are increasingly losing their habitat due to tourism and coastal development; beaches where the turtles come to nest are built up; at night, for example, deck chairs on tourist beaches prevent the female animals from coming ashore and finding a suitable place to lay their eggs. If they nevertheless nest on such beaches, the nests are endangered by the tourists or even locals themselves, who, for example, stick their sunshades into the sand at these places where nests are located and thus destroy them. It can also happen that due to the vibrations caused by people on the beach, the turtles hatch too early and during the day instead of at night and then dry out in the sun. Obstacles on the beach such as deck chairs, as well as tire tracks from cars, can become an insurmountable obstacle for a sea turtle. The track left by a car tire in the sand is often too deep for a newly hatched sea turtle to get out on its own.
Artificial light poses an additional threat on built-up beaches. When turtles hatch at night, they run toward the brightest area, which is usually the sea, and find their way into the water. Artificial light sources e.g. from houses, hotels or restaurants, disorient the animals, they do not find their way into the sea and dry up in the hot sun.
Destruction of marine habitats
Seagrass beds and coral reefs are important food sources for sea turtles. Their increasing destruction by humans is therefore a very serious problem not only for sea turtles.
Many sea turtles die as so-called “bycatch”, i.e. they get caught in nets or on baited lines set out by fishermen, become entangled and suffocate because they can no longer swim to the surface to catch their breath. In some areas, such as the Gulf of Mexico, so-called “TEDs” (Turtle Excluder Devices) are already being used in trawling for shrimp; a special device prevents sea turtles and other larger marine animals and seabirds from being accidentally caught.
Boat and ship traffic
Sea turtles are often run over by motorboats and can be fatally injured by the propellers.
Also the increasing environmental pollution is a serious danger; in general, the animals become weakened and more susceptible to diseases. Again and again, turtles are found entangled in floating garbage, e.g. plastic lines or nets. Since many turtles feed on jellyfish, plastic bags floating in the sea can be their undoing: they mistake the plastic for jellyfish and eat it; however, it cannot be digested, it accumulates in the turtle’s intestine, clogging it and eventually leading to their death. Dead sea turtles are often found with their intestines full of plastic. Other types of trash are also accidentally swallowed as food – propelled trash, on the other hand, can make entire beaches or sections of beaches inaccessible to turtles. The 2010 Deepwater Horizon rig fire in the Gulf of Mexico also brought renewed attention to the dangers posed by the extraction and transportation of oil. The incipient recovery of the population of the rarest sea turtle, the Kemp’s ridley sea turtle, which is found only in the Gulf of Mexico, has suffered a severe setback in the process.
The beginning change of the world climate will also have consequences for the sea turtles, which are not yet foreseeable. This concerns on the one hand the change of the coasts and beaches as vital nesting areas. In addition, the development of females and males depends on the average breeding temperature. If this is too high, no more males are produced, which is of course fatal for a population. Countermeasures here can only be artificial, temperature-controlled semi-natural breeding sites (hatcheries) to a limited extent.