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Sea turtles range in size from about 100 pounds (Olive Ridley) to over 1,000 pounds (Leatherback). Their shells are composed of the upper carapace and the lower plastron. Large scales cover each species (excluding the Leather back) and are used readily in identification of species.
Generally solitary creatures, sea turtles show little interaction besides courtship and mating. The time period when maleÕs court and mate with females is a very brief period just prior to a femaleÕs first nesting emergence. Using genetic allele frequencies, recent research has shown that sea turtles may practice both polyandry and polygyny in the wild (Crim et. al. 2002). Males court females by nuzzling her head or gently biting at the back of her neck. The male attaches himself to the back of the femaleÕs shell (if the female does not flee) by gripping her top shell with claws in his front flippers and folds his long tail under her shell (CCC 2005). This often leads to observations of females with scratched and bleeding shells on nesting beaches. Females may mate with more than one male, retaining the sperm of each partner for several months. Her eggs will have been fertilized by many males by the time she lays them, leading to high genetic diversity. During the inter-nesting interval, female and male leatherbacks have been shown to exhibit different social interactions, with females many times avoiding males in off shore waters (Reina et. al. 2005). Cumulative mating time is on the average of 25 hours (Hamann et. al. 2003). While it has been studied that the period from mating to nesting is on the order of 35 days, the time from ovulation to oviposition has never been studied.
Sea turtles generally nest in the spring on beaches in a variety of locations in the tropics. Little is known about why sea turtles nest on the same beaches extensively and other, similar, beaches, infrequently. Possible explanations relate to historical sea surface temperatures, beach profiles, and predation rates. Recent climate trends indicate that nesting dates are slowly becoming earlier and earlier in the year for loggerhead sea turtles (Weishampel et. al. 2004). Climatic variation, leading to increased sea surface temperatures and fluctuating feeding conditions, have also been linked to differing nesting populations of green sea turtles at Tortuguero, Costa Rica (Solow et. al. 2002).
The tendency of sea turtles to return to nesting areas and foraging grounds is not well understood. The seven sea turtle species exhibit vastly differing migratory patterns and habitat use, thus furthering the confusion as to what cues are used. Some examples are that research suggests that leatherbacks use sea currents, ocean fronts, bathymetric features and magnetic cues while Olive RidleyÕs are suspected of using sea surface temperature (Plotkin 2003). Radio telemetry and displacement experiments have attempted to decipher exactly what cues green and loggerhead turtles use to navigate the often far distances between the two locations (kesson et. al. 2003; Luschi et. al. 2003). Satellite tracking has been used to show that after laying their eggs, loggerheads in the Mediterranean tend to prefer foraging and inshore waters close to the nesting area (Godley et. al. 2003). Body size is also hypothesized to affect the total distance that turtles travel after nesting. Species also differ in their dive patterns, or the temporal and spatial distribution in the water column. Loggerheads and olive ridleyÕs have been shown to have drastically different dive patterns (Polovina et. al. 2005).
The life history of sea turtles varies among species, but fall into three basic categories: complete development in the neritic zone, early juvenile stages in the neritic and further development in the oceanic zone, and complete development in the oceanic zone (Bolten 2003). Flatback turtles are the only known species to exhibit completely neritic development. The loggerhead is the most extensively researched example of neritic and oceanic development, with green, hawksbill, and kempÕs ridley all being suspected of this type. Leatherbacks and olive ridleyÕs are suspected of exhibiting complete oceanic development, but like green, hawksbill, and kempÕs ridley, little is known about the early life stages of these. Studies have shown that juvenile and developing loggerhead and leatherback sea turtles primarily feed at the surface on neustonic species in the open Pacific Ocean waters (Parker et. al. 2005).
There is great concern for the future of sea turtle populations around the globe. Six of the seven sea turtle species are listed as either endangered or critically endangered according the World Conservation Union (IUCN 1995). There are many theories into why sea turtle species have been in such rapid decline and many actions, both natural and anthropomorphic, have been linked to negative impacts on sea turtles.
One natural cause of turtle population decline is natural predation. In the ocean, juvenile turtles and hatchlings are easy targets for certain fish species and even adults may be preyed upon by killer whales. RaccoonÕs, birds, and crabs have been reported to feed on sea turtle eggs, hatchlings crawling to the sea, and in the case of crocodiles, females on shore for nesting. In the Ten Thousand IslandÕs archipelago in Florida, sea turtle nests have depredation rates of up to 100% due to raccoonÕs. An encouraging finding was that removing raccoonÕs on certain islands in the chain, effectively turning depredation rates to zero (Garmestani and Percival 2005). This, though, has recently been found to be even more detrimental to sea turtle populations in that the removal of raccoonÕs actually increased the population of ghost crabs (Pennisi 2006). Though it is not clear that the ghost crab population increase is in fact increasing depredation rates, this highlights how the removal of one species often times has a cascading effect in the ecosystem. Other species of insects, such as beetles, have also been implicated as key predators on sea turtle eggs (Donlan et. al. 2004).
Human impacts on sea turtle populations have been documented for many years and fall into a few general categories. First, fisheries by-catch has been studied and reported on in relation to the incidental capture of leatherback and loggerhead sea turtles on longline fish boats in the both the Atlantic (Kotas et al 2004; James et. al. 2005) and the Pacific Oceans (Lewison 2004a). The practice of longline fishing may be affecting both loggerhead and leatherback sea turtles due to the hypothesized ontogenetic development in pelagic waters of the Atlantic Ocean. A recent study points out, that the annual survivorship of oceanic, developmental-stage (ages 2-12), populations of loggerhead sea turtles in the North Atlantic within tuna fisheries is actually above 90% (Bjorndal et. al. 2003). This indicates that in this location high survivorship is actually apparent even under stress from human sources. These authors point out, though, that similar studies have been conducted throughout the world that include the portion of turtles under the age of 2 and result in much lower survival rates. Other studies have reported similar survival rates, but only when analyzing nesting female leatherbacks (Dutton et. al. 2005). These contrasting results highlight some of the key issues associated with quantifying by-catch data, that being limited data and spatial disparity and scale (Lewison et. al. 2004b).
Turtle excluder devices (TEDs) are now used throughout the world in longline trawls in order to allow caught turtles to escape. To stem the by-catch of sea turtles by shrimp trawlers, the United States has a law prohibiting the import of any shrimp from nations who harvest them in a manner that negatively impacts sea turtles. This meant that shrimpers needed to install TEDs for catches to be exported to the US. Originally meant to address Caribbean and the Western Atlantic, the decision came to incorporate any shrimping industry globally (Epperly 2003). A recent report indicated that though turtle strandingÕs have increased along coasts where TEDs are required in the Gulf of Mexico (Lewison et. al. 2003). This fact though is misleading. The research shows that turtle populations have grown and that higher stranding rates correlate to low compliance with the TED law. This means that TEDs are effective, in conjunction with other conservation measures.
Fishermen have been used in other ways to study sea turtles and to aid in conservation. In Canada, of all places, because so little is known about leatherback populations along its coasts, researchers actively got volunteer fishermen to help with determining the biology, abundance, and distribution (Martin and James 2005). This way of eliciting data is a novel idea and could be replicated in many other areas. The knowledge of local fishermen should not be discounted.
Sea turtle eggs have long been harvested by coastal communities as a food source and only recently has this become an issue with the reported declines in populations. In one region in Mexico, a study found that over 80% of dead black, olive ridley, and hawksbill turtles were attributable to human consumption (Koch et. al. 2006).
Alterations to coastal habitats are also an important problem for sea turtle populations stemming from human interactions. Artificial lighting from tourist, residential, and business developments deter females from coming ashore and nesting and cause hatchlings to become confused and disoriented. Coastal armoring often blocks females from reaching suitable nesting grounds. Beach nourishment replaces eroded sands on shore lines and is preferable to armoring, but can negatively impact sea turtles if the sand is placed too compactly for females to nest properly and if it is done during nesting season. Dredging leads to the incidental capture of thousands of turtles. Human nighttime activities may cause females to not come ashore or stop nesting if disturbed as can beach furniture (which sea turtles can also become entrapped in). Beach driving has a variety of negative impacts, such as compacting sands, disturbing nesting females, as well as disorienting hatchlings, increasing the time it takes for hatchlings to reach the water, and possibly crushing them along the way (Caribbean Conservation Corporation 2005b).
Anthropogenic contaminants, such as organochlorines, PCBs, and heavy metals, are another human induced difficulty on sea turtle populations. A recent report showcased the endpoints of zinc, nickel, copper, manganese, cadmium, and lead in four turtle species in Mexico. Although there is not a lot of research about what concentrations of these metals mean physiologically for sea turtles, its important to note that this publication reported the highest concentrations of zinc, nickel, and iron in certain organs (Gardner et. al. 2006). Many reports have shown that organochlorine pesticides and PCBs readily bioaccumulate in sea turtle tissues (Rybitski et. al. 1995; Gardner et. al. 2003). Recently, pesticides and PCBs were shown to suppress immune responses in loggerhead turtles in Georgia and Florida (Keller et. al. 2006).
Nesting sites have been the center of sea turtle conservation and the subject of much debate among conservationists for many years. Archie Carr, acting as chairman for the Marine Turtle Group of the International Union for the Conservation of Nature and Natural Resources (IUCN), advocated for the total protection of all nesting grounds around the world to stem population declines (Davis 2005). His view, and the view of others like him, is in contrast to some local officials and policy makers. For instance, the Panamanian Environmental Authority has put into place a program where egg harvesting is monitored and only allowed on certain portions of Guanico Abajo beach based on historical hatching rates (Barsimantov 2004). While the program has issues, in that the Authority cannot adequately monitor poaching or market prices of illegally harvested eggs, close to 90% of hatchlings reach the water instead of the past low of 5%.
Other approaches have been implemented to protect nesting sites besides allowing limited harvesting. In Turkey, relocation of loggerhead turtles from areas of heavy predation and human activity, along with screening off predators and fencing off nesting areas showed dramatic increases in hatching success rates (Baskale and Kaska 2005).
Many local and international groups have led a push to protect nesting areas on beaches as well as to stem the poaching of eggs. Intensive beach monitoring programs have been shown to significantly increase turtle populations in other Central American countries besides Costa Rica (Garcia et. al. 2003). Examples of organizations operating within Costa Rica include Earthwatch, the Caribbean Conservation Corporation, Save the Turtles of Parismina, and the Asociacin ANAI. These movements do appear to be working as well. Preliminary analysis of nest protection is being linked to increases in nesting populations of the leatherback sea turtle in the US Virgin Islands (Dutton et. al. 2005). These community-based conservation associations follow a general formula for protecting coastal beaches for sea turtles while still allowing for economic progress. A grass-roots localized organization, through sheer will and community involvement, heads the movement by initiating beach patrols to fend off poachers. Larger, international, attention comes about and research often commences to study the turtle species nesting on the beaches. Through this activity, numerous volunteers come into communities, bringing with them much needed income to coastal areas. Much of the time, this income counteracts (and at times is more than) the income lost when eggs are not collected and sold on the black market. In this way, the turtle species, the beach areas, and the community are allowed to grow and interact in a sustainable fashion.
The Asociacin ANAI is a non-profit organization centered in the Talamanca region of Costa Rica, runs the Sea Turtle Conservation Project at Gandoca Beach. This program focuses on community and volunteer conservation of nesting sites of the four sea turtle species that nest on the beach in southeastern Costa Rica. By 1990, the combined efforts of Project staff and local residents had successfully protected 90% of nests through routine patrolling of the beaches in the area. With the onset of a volunteer and research program, the project was able to bring in more income to the local community than could have been received through the sale of turtle eggs on the black market (ANAI 2005).
The Caribbean Conservation Corporation and Sea Turtle Survival League acts primarily out of Florida, but has an intensive research and conservation program in Tortuguero, Costa Rica. Nest site monitoring studies have been conducted here for over 40 years (after being initiated by Dr. Carr). The research has focused on reversing the decline of green, leatherback, and hawksbill turtle populations in the Caribbean. Much like the ANAI program, volunteers are utilized, along with researchers, to patrol the beaches to ward off poachers and to collect data on nesting females and hatchlings (Caribbean Conservation Corporation 2005a). The long term data collected at Tortuguero has been analyzed and shows how long-term conservation efforts are positively effecting nesting rates in the area (Troeing and Rankin 2005).
Another community-based effort in Costa Rica stems from the Parismina region. Starting in 2001, located just south of Tortuguero National Park, local villagers began patrolling the Parismina Beach because of the unsustainable harvesting of eggs by poachers and egg thieves. Much like Tortuguero, leatherback, green, and hawksbill turtles nest on Parismina Beach. A non-profit organization, Save the Turtles of Parismina, was formed in 2002 and volunteers now number over 200 per year. Again, as with other community oriented approaches, the incoming volunteers have stimulated the villagesÕ economy (Save the Turtles of Parismina 2006).
With the current threats to sea turtles around the world, entire populations and possibly whole species are in danger of being wiped off the earth in the next 20 years. Whether it be natural or anthropogenic, these threats can all be stemmed with the right initiative and research. On the local level, areas in Costa Rica such as Tortuguero and Parismina are heading community-based conservation activities that are positively affecting nesting populations, which in turn aids the overall population of loggerheads, leatherbacks, and hawksbills, to name a few. In conjunction with techniques that are more turtle friendly in commercial fisheries, the prospect of reversing downward population trends is encouraging.
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