Posted by WorkingAbroad Projects on Wednesday, 13th December 2017
Marine turtles are charismatic animals that as a group, have experienced large population declines worldwide.
Sea turtles face a myriad of threats. The primary threat they face is over-exploitation, as their eggs and meat are coveted for food in many coastal cultures, and their shells have been used as a commodity. Many also die as a result of incidental catch in fishing lines or nets, as well as exposure to pollution like plastics or spilled hydrocarbons. Like many imperiled species, they are also experiencing habitat loss, as many of their coastal nest sites have been destroyed or altered.
The biology of sea turtles make their populations susceptible to these threats. Sea turtles have low reproductive rates and a high age of maturity. As such, their populations are slow to recover from increased rates of mortality. While sea turtles are extremely vulnerable to human impacts throughout their life cycle, adult females, their eggs, and recent hatchlings are particularly susceptible to harvesting and incidental disturbance during the nesting phase. Furthermore, as females typically return to the nest site at which they were born, they are less adaptable to habitat alteration at these sites.
As sea turtles inhabit the ocean outside of marine exclusion zones, nest in over 80 countries worldwide, and face numerous threats, their conservation is truly a global challenge. Before discussing sea turtle conservation further, here’s a brief introduction to the seven living species of sea turtle:
Leatherback (Dermochelys coriacea): The leatherback is the largest species of turtle, reaching up to 2.2 m (7.2 ft) in length, and can weigh over 700 kg (1,540 lbs). Unlike the other species, it lacks an exterior bony shell, instead features a hard leathery exterior. It is the best adapted sea turtle for high-speed movement through the open ocean, reaching speeds of 35 kmh (22 mph), as it possesses an extremely hydrodynamic profile, and large flippers that can measure up to 2.7 m (8.9 ft) in length. It can travel into colder waters than the other sea turtles, as it maintains an internal body temperature higher than the surrounding waters by keeping a high activity rate. The diet of the leatherback is comprised primarily of jellyfish, making the species susceptible to accidental ingestion of plastics. The leatherback is a cosmopolitan species, meaning it is found in all the major oceans, including parts of the Arctic Ocean. While the IUCN has ranked the species as Vulnerable, some populations (eg. eastern Pacific, southwestern Atlantic) have been assessed as Critically Endangered.
Loggerhead (Caretta caretta): The largest hard-shelled sea turtle, the loggerhead can reach up to 2.1 m (7 ft) in length, and can weigh up to 545 kg (1,202 lb). The loggerhead has the largest nesting range of all sea turtles, and inhabits regions of the Atlantic, Indian, and Pacific Oceans, and the Mediterranean Sea. This species is omnivorous, but feeds largely on marine invertebrates. Upon hatching, juveniles swim to the open ocean, where they often inhabit floating mats of the algae Sargassum, and return to warmer coastal waters as they mature. Although the IUCN has ranked the Loggerhead as Vulnerable, some populations (e.g. eastern and western Pacific) are considered Critically Endangered.
Green (Chelonia mydas): The green sea turtle is found in tropical and subtropical seas worldwide, though the two major populations exist in the Atlantic and eastern Pacific. This species is unique in that it is mainly herbivorous, feeding primarily on seagrasses. The IUCN has ranked the green sea turtle as Endangered.
Hawksbill (Eretmochelys imbricata): The hawksbill has a worldwide distribution, though it is primarily found in warm waters within shallow lagoons and coral reefs. The species is named after its sharp, curving beak, which is an adaptation to a diet mainly composed of hard-bodied sea sponges. The hawksbill’s shell has overlapping scutes, which gives it a serrated appearance. The shell has a unique coloration of continuous brown and black bands, which has made the species coveted for the production of tortoiseshell. The hawksbill has been overexploited for the production of this decorative material, and is therefore considered Critically Endangered by the IUCN. Interestingly, the hawksbills is the first documented bioflourescent reptile, a discovery made in 2015.
Olive Ridley (Lepidochelys olivacea): The olive ridley, one of two species of ridley sea turtle, is the most abundant sea turtle. It is a smaller species, typically growing to 0.6 m (2 ft), and weighing under 50 kg (110 lb). It is primarily found in warm waters within the Pacific and Indian oceans, though populations also exist within the southwest Atlantic. It is mostly a carnivorous species, eating a diversity of invertebrates, adult fish, and fish eggs. The olive ridley is famous for synchronized nesting in mass numbers, typically at night. While globally abundant, it has experienced population declines exceeding 30%, and has been ranked as Vulnerable by the IUCN.
Kemp’s Ridley (Lepidochelys kempii): A close relative to the olive ridley, the Kemp’s Ridley is the world’s smallest sea turtle, typically weighing under 45 kg (99 lbs). It breeds mainly within the Gulf of Mexico, and Atlantic Coast of North America. It is a shallow-water benthic feeder, with a diet that consist primarily of crabs. This species is highly imperiled, and is ranked as Critically Endangered. While large increases in nesting numbers occurred between 1985 and 2010, the recovery of this species was impeded by the 2010 Deepwater Horizon disaster, in which hundreds of individuals were killed, and breeding habitat was affected.
Flatback (Natator depressus): The flatback has the most limited distribution of all the marine turtle species, as it is found within waters around the Australian continent shelf, with breeding sites restricted to the north shore of Australia. The flatback is named after its shell, which in profile appears as a smooth, flattened dome. It is a smaller species, typically about 1 m in length (3.2 ft), and about 90 kg (200 lb) in weight. As it does not undergo long-distance migration in open ocean like the other sea turtle species, the flatback is considered the least endangered sea turtle. The IUCN has ranked the species as Data Deficient, meaning that there are data gaps regarding trends of distribution and abundance.
While turtles inhabit large areas of ocean, their populations have been divided into discrete groups relative to individual nesting sites. These areas contain biologically distinct turtle populations, as they differ in terms of genetics, distribution, movement, and demography, largely because of female nest-site philopatry (i.e. females return to sites where they were born, repetitively). These areas also contain different levels of threats, in terms of levels of harvesting, incidental take, pollution, and habitat alteration. Given these differences, conservation efforts have been organized at the level of these distinct population units, referred to as Regional Management Units (RMUs), rather than the entire species.
A 2011 investigation published in PLoS ONE, identified 58 RMUs across the seven sea turtle species. This study employed a risk-assessment to rank each RMU in terms of conservation priority, by comparing the risk level (eg. smaller, declining populations with low genetic diversity are higher risk), as well as the threat level (eg. levels of harvesting, incidental take, pollution, etc.), within each RMU.
Overall, 19 RMUs were ranked as High Risk / High Threat, while 12 RMUs were assessed as 12 Low Risk / Low Threat. Only the Kemp’s Ridley did not have at least one High Risk / High Threat RMU, although this analysis was performed prior to the Deepwater Horizon event. RMUs within the Pacific had the highest risks, but the low threats, while RMUs within the Atlantic had the lowest risks, but the highest threats. The attached map shows the ranking for the RMUs of the loggerhead, reproduced from this article.
Figure 1– Conservation ranking of worldwide RMUs for the loggerhead, Caretta caretta. Figure reproduced from Wallace et al. 2011, using data obtained from OBIS-SEAMAP.
Among the 11 most endangered WMUs, five were in the Indian Ocean, three were in the Pacific, and two were in the Atlantic. In terms of the species of these WMUs, four were hawksbill, three were olive ridley, three were loggerhead, and one was leatherback. The results from this analysis indicate that there are highly imperiled populations of sea turtles worldwide.
A recent (2017) article by Mazaris et al. published in Science Advances, conducted an analysis of trends in turtle abundances, across 299 time series. Among these time series, 95 found a significant increasing trend, while 35 showed a significant decreasing trend. When these abundance trends were summed within 17 RMUs identified in the aforementioned study, 12 were found to have an increasing turtle population, while 5 were decreasing. This efforts suggests that conservation efforts have been effective in some areas to reverse the alarming population declines of marine turtles.
Turtle conservation seems to be most effective in locations where prohibitions on the harvesting and collection of turtles and their eggs are legislated and enforced, where nesting habitat is protected, and where effective monitoring programs are in place.
One success story is the tremendous rebound of Hawaiian green turtles, which nest in the French Frigate Shoals, an atoll in the Northwestern Hawaiian Islands. An effective monitoring program has been active there since 1973. In 1978, the green turtle was protected under the US Endangered Species Act, which ceased the harvesting of green turtles in the area. Furthermore, the area is now protected under the Hawaiian Islands National Wildlife Refuge.
Another example of a path to success, though still underway, relates to the hawksbill turtles of the Arnavon Islands, located in the Solomon Islands. This area was declared a sanctuary in 1976, and was later protected in 1995 as the Arnavon Community Marine Conservation Area. The area has had a beach monitoring and tagging program since 1991, which between then and 2012, over 845 turtles were tagged. These efforts, along with legislation banning the sale of turtle products, have caused the number of nests laid and rate of remigration to the island to double since 1995.
Conservation efforts, such as monitoring and habitat enhancement, are central to the recovery of sea turtle populations. If you are interested in helping sea turtle recovery, WorkingAbroad has a variety of projects related to sea turtle conservation, including:
Images are taken from Wikimedia commons
By WorkingAbroad Blog Writer Sean Feagan
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