Threats to Sea Turtles
Sea Turtles and Climate Change
The following effects of climate change will have critical implications for sea turtles:
More severe storms, such as hurricanes and tropical cyclones, could increase beach erosion rates, endangering sea turtle nesting habitat. Often severe storms could increase the chance that sea turtle nests will flood, decreasing nesting success rates.
The sex of sea turtle hatchlings is influenced by the temperature of the sand in which the eggs develop. Climate change can cause increased temperature. Higher temperatures cause the sand to heat up and lead to a higher proportion of female to male hatchlings. One study concluded that it is likely that southern populations of turtles in the U.S. will become ultra-biased towards female populations if temperatures increase by even 1 C.
Sea Level Rise
As climate change melts ice and warms the oceans, sea levels rise. Sea level rise for the end of the 21st century range from 0.18 to 0.59 meters. Even a small rise in sea level could result in a large loss of beach nesting habitats..
Climate change is altering ocean currents, which are the highways that sea turtles use for migration. With changes in ocean circulation, sea turtles may have to alter their movements and possibly shift their range and nesting timing.
Other Threats to Sea Turtles
Along with fishing gear and climate change, there are numerous human activity threats to sea turtles. These threats include coastal development, pollution, direct harvest, invasive species and vessel strikes.
With the encroachment of hotels, parking lots and housing along nesting beaches, female turtles are forced to use suboptimal nesting habitats. After emerging from their nests at night, newborn hatchlings find their way from nest to sea using the light of the moon.
Artificial lighting, such as street lamps and hotel room lights, confuses these hatchlings, sending them landward in the wrong direction. This is a lower chance for survival due to dehydration, exhaustion, predators and automobiles.
Solid waste, chemicals and pollutants from human activities enter the ocean, causing injury, illness and even death to sea turtles. The pollution sources range from wastewater discharge released by cruise liners to fishing nets that are lost by fishermen to fertilizer runoff that comes down rivers from farms. This means that even people living in the middle of the European Union can have an impact on the health of the oceans and sea turtles.
Thousands of chemicals contaminate the marine environment, many of which accumulate in the tissues of sea turtles, affecting their locomotion, brain functioning and reproductive success. Sea turtles also consume plastics and marine debris which are harmful to their digestive systems. Scientists believe that there may be a link between exposure to agricultural pollutants and the spread and prevalence of fibropapillomatosis, a disease in sea turtles that causes tumors to grow on the eyes, intestinal tracts, lungs, mouth, heart and other organs.
Each year, more than six million tons of trash enters the oceans, with plastics comprising up to 90 percent. Marine trash poses an entanglement and ingestion hazard to turtles, threatening their ability to forage, grow and, ultimately, survive. A study conducted on young loggerhead turtles found that 15 percent of the turtles had ingested large amounts of plastics that blocked their stomach from functioning.
Invasive species are non-native species of plants, animals or insects that have been introduced into an area where they are not found naturally. They can wreak havoc on native species by competing them for space and food or by acting as predators.
Examples of invasive species include a stowaway barnacle from a ship ballast, a hidden plant seed in the sole of a hiking boot worn on a remote island or a pet iguana released in the wild. For sea turtles, non-native species, such as fire ants, rats, red foxes and domestic dogs, can be very dangerous on the beach, where they dig up nests and eat sea turtle eggs.
Sea turtles cannot breathe under water and their regular ascent to the surface puts sea turtles directly in the path of boats. Commercial and recreational vessels are major turtle harzards, particularly in shipping lanes and during peak tourism months when millions of recreational boaters congregate in coastal areas.
Boat collisions are one of the main identified source of trauma for turtles found stranded and dead on Mediterranean beaches. Injuries from boat propellers include amputated flippers, fractured shells, brain injuries and broken bones. Although not all of these injuries cause immediate death, they may provoke elevated levels of stress or injuries, which ultimately affect a sea turtle’s ability to forage, migrate, escape from predators and reproduce.
A trawl is a large net that is pulled through the water column or along the seabed, catching anything that is not small enough to pass through the net’s mesh openings. This fishing gear is typically used to catch fish or shrimp. Catching unwanted species is a problem for trawlers because it is a very indiscriminant form of fishing. Trawling can occur with one boat pulling a net or by two boats, each pulling an end of a much larger trawl net.
Sea turtles are prone to drowning when they are caught in trawl nets and are crushed or are unable to escape to the surface to breathe. Turtle excluder devices (TEDs) were designed to protect sea turtles and are required in shrimp trawls in US but not in EU. A TED is a grid of bars in the neck of the net with an opening, reminiscent of an escape hatch. The bars are spaced far enough apart to allow shrimp and fish to pass through to the tail of the net while allowing large species, such as sea turtles, to escape from the net through the opening.
The mandated use of these devices has greatly reduced the death of sea turtles in shrimp trawl fisheries but there are several other trawl fisheries which are not currently required to use TEDs.
Sea scallops, clams and other animals which live on the ocean floor are caught using a method known as dredging. Dredging is a destructive method of fishing that involves dragging a dredge, or heavy metal frame and bag made of heavy metal chain links, often weighing several thousand pounds, along the seafloor. Dredges rake up bottom habitat, including corals and sponges, while collecting scallops and other bottom-dwelling species. Dredges also entrap or crush other organisms that rest and forage on the seafloor, including sea turtles.
Longline fishing is a commercial fishing technique that uses thousands of baited hooks hanging from fishing line, often miles long. Swordfish, tuna mackerel, snapper and grouper are commonly targeted by this gear type. Longlines can be set to hang at varying depths depending on the targeted species.
Leatherbacks, which primarily inhabit the open ocean, are especially threatened by longlines hung near the surface. Loggerheads are often caught by longlines on the ocean bottom since they forage on bottom dwelling species.
Longlines are often set in the water for extended periods of time. By the time deeper set longlines are removed from the water, the hooked turtles may have drowned. Even if a sea turtle survives being hooked on a longline, it will have injuries that can be very serious. There are ways to disentangle turtles from fishing line and to remove hooks, but the process can be time-consuming, complicated and dangerous depending on the size of the turtle and weather conditions at sea.
Gillnets are long walls of netting hung in the water to trap and hold fish. Gillnets can be fished in a variety of ways: anchored, so they float in one space at the top of the water column, set on the ocean bottom or left drifting in the current. A fish swims into the invisible netting and as it swims in reverse to try to dislodge itself, its gills become caught in the twine.
Sea turtles, like fish, become entangled in the netting. Because sea turtles need to come to the surface to breath, they die if held under water for too long. If they are alive when the net is retrieved, they are often injured and stressed. Turtles can lose flippers from entanglement in gillnetting. If a turtle survives from a gillnet, the turtle is then more vulnerable to predation and has reduced ability to carry on normal activities.
Purse Seine Nets
Purse seines are typically fished using two boats — a small skiff and a launch vessel. The skiff encircles a school of fish, dragging one end of the net behind it. After a complete circle is made, the launch vessel tightens a rope to close the bottom of the net. Purse seines are not generally left underwater for extended periods of time, so the resulting risk of turtle mortality from forced submergence is relatively low compared to other gear types. However, bycatch in seines presents an ongoing problem for turtles and other marine wildlife, such as dolphins and sharks.
Pound nets are stationary fishing devices that can extend more than 15 feet in length and are used to catch a variety of species including striped bass, bluefish, crab, croaker and flounder.
The pound net system is divided into three sections: a perpendicular leader that acts as a partition to block fish and other ocean critters from swimming past; a heart-shaped wall of nets that forces animals to swim in the direction of the pound; and a pound net that serves as the actual entrapment basin where fishermen can collect and sort their catch.
The tops of the nets poke out of the surface of the water, making sure that fish and other animals do not escape. As sea turtles swim parallel to shore, their path is blocked by the fence-like leader, where their fins or heads can be entangled, causing serious injury or death from drowning. Recent studies have shown that decreasing mesh size and increasing net stiffness can lower sea turtle bycatch rates in pound nets.