Behavior, Range and Outlook of Loggerhead and Green Sea Turtles

This topic submitted by Eric T. Nagy ( nagyet@miamioh.edu) at 3:59 PM on 6/9/04.

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Loggerhead sea turtles, Caretta caretta, are a widespread marine animal living in tropic, subtropical and temperate oceans all over the world. The largest nesting site for these animals in the Atlantic Ocean, and probably the entire world, is the Southeastern United States. It is estimated that 50,000 to 70,000 clutches of around 115 eggs are deposited here every year (Bolten et al., 1998). The green sea turtle, Chelonia mydas, is another species found in the Caribbean, but its migration routes are not as widespread as loggerheads. An estimated several hundred thousand green sea turtles survive in the Caribbean today (Davidson, 1998). At one time, there may have been as many as 600 million green sea turtles calling the Caribbean their home (Davidson, 1998). Sea turtles were among the first casualties in the Caribbean and in coral reefs around the world and no one knows for sure how many there were. The turtles breeding habits made them perfect targets for over hunting, which all but removed the animal from the ecosystem hundreds of years ago (Davidson, 1998).

The loggerhead sea turtle reaches an average carapace length (CCL) of 126 cm and weigh an average of 115 kgs. The green sea turtle has an average CCL of 153 cm and can weigh up to 180 kg (Heithaus et al, 2002). Both species are classified as threatened according to the endangered species act (NMFS, 2004). The species are threatened by human activities including: nesting habitat degradation, shrimp and fish trawling, and pollution of the oceans (NMFS, 2004). The same risks are affecting both turtle species and their habitats. The loggerhead turtles have a much wider range, inhabiting close to all of the worlds’ oceans. Much of the conservation effort has been placed on this animal. There is more research on the loggerhead sea turtles habits, so this paper will focus on its behavior, range, habitat, and predation (Witherington, 2002). The green sea turtle is also widely studied in the Caribbean and is discussed in the paper. Finally, the paper will then look at how human influence is driving down sea turtle populations world wide, and what can be done to halt their decline.

The data on loggerhead sea turtles is limited because the turtles disperse from nesting beaches into the open ocean, where they spend most of their lives (Witherington, 2002). A (mt) DNA study done on Japanese loggerhead nesting populations, the only known nesting site in the northern Pacific Ocean, shows significant genetic differences between four distinct Japanese nesting populations (Hatase et al., 2002). These facts indicate that female loggerhead sea turtles return to their precise beach and lay their eggs. The sex of the infant turtles is determined by the warmth of the nest making trees along the beach become important ecologically to the sea turtle’s survival. The hatchling turtles emerge from eggs on the beach and head instantly for the open ocean where they try to survive until maturity. The mother sea turtles return to the sea after laying several clutches of eggs on a beach. Addison et al. tracked female sea turtles dispersing from nesting sites using sonic and radio telemetry. They found that the initial post nesting movements of these turtles were mostly away from land, and said that more far reaching studies have shown a post laying migration to feeding areas (2000).

Many call the ten or so years following hatching until the turtles show back up as adolescence in the Caribbean as “the lost year.” Modeling studies on the loggerhead hatchlings show that they are caught in the Gulf Stream, and over several years drift to shallow eastern Atlantic feeding grounds (Witherington, 2002). Pacific loggerhead hatchlings from Japan make similar migrations to an area off of Baja California (Hatase et al., 2002). Evidence to support this model include: occasional hatchlings washed ashore during storms, hatchling carcasses found in oceanic fish stomachs, and hatchlings are observed at sea (Witherington et al., 2002). Three open ocean downwelling habitat types were observed in Witherington’s experiment including: fronts (shear boundaries between two water masses between two water masses), slicks (the downwelling above and behind crests of large, slow moving waves), and windrows (wind-generated lines of floating ocean life). The approximate density of loggerhead sea turtles in downwelling lines turns out to be 9000 per square mile (Witherington, 2002). In addition, no green sea turtles were observed in the open ocean, strengthening the hypothesis that they differ in their behavior. The small turtles forage on plants, cynobacteria, algae, hydroids, copepods, shrimps, winged insects, gastropods, and oceanic insects (Witherington, 2002). Many of the small turtles also inject plastic and tar, which is toxic in high levels. These downwelling lines concentrate the food the hatchling turtles need, but also concentrate their predators and toxic substances (Witherington, 2004).

Further, the model of the transatlantic drift of hatching loggerhead sea turtles is supported by mt DNA sequence analysis (Bolten et al., 1998). It seems 100 percent of the juvenile turtles found in Mediterranean and eastern Atlantic feeding grounds matched the DNA of nesting populations in the Southeastern United States. This large migration route makes protection for the turtle difficult. Bolten et al. (1998) say even if the turtles are completely protected in one region, it may not be enough to save the species from extinction. The turtle needs transatlantic and transpacific management plans to ensure its survival.

Since loggerhead sea turtles are mostly dispersed in the open ocean, it is very difficult to understand the effects of predation on the animal. Heithas et al. studied predation of sharks upon loggerhead and green sea turtles in Shark Bay, Western Australia (2002). White, Bull, and Tiger sharks are the main predators of sea turtles and might even effect the populations in some areas. A shark attack on a sea turtle is a rarely observed phenomenon, and little is actually known about the topic (Heithas et al., 2002). Loggerhead sea turtles are injured, and therefore probably killed, by sharks more often than green sea turtles (Heithas et al., 2002). This is probably because green sea turtles are more evasive than loggerhead sea turtles, and live in sea grass beds which might help the turtle hide from sharks (Heithas et al., 2002). The escape ability of the green sea turtle was noted when catching the turtles in shark bay. Loggerhead turtles were much easier to catch than the green (Heithas et al., 2002). Loggerhead sea turtles are more often found over bare ocean bottom and are attacked much more often. Male loggerheads are attacked more often than females, which may be indicative of higher amounts of risky behavior during mating (Heithas, 2002). Green sea turtles have the same attack rates for both male and females, which might be attributed to their better escape ability (Heithas et al., 2002).

Next, the sea turtle’s behavior has led to its decline within the oceans of the world. Humans have increased the amount of plastics and tar into the oceans, which collect in downwelling lines upon which hatchling loggerheads depend. This means a higher mortality rate among young turtles (Witherington, 2002). In addition, harvesting fish, shrimp and sea weed kills many sea turtles each year (NMFS, 2004). The annual, seasonal Sargassum harvest along downwelling lines off the coast of North Carolina is known to kill many young sea turtles (Witherington, 2002). Shrimp trawling in coastal waters in both the Pacific and Atlantic is known to drown many full grown sea turtles. It is unknown how many sea turtles are killed do to commercial fishing. Anywhere from 5,000 to 50,000 sea turtles could be killed annually by this industry (NMFS, 2004). Marine power plant intake systems are thought to kill 2% of loggerhead sea turtles and 7% of green sea turtles (NMFS, 2004). The loss of sea turtle habitat due to dredging, pesticide inputs, nutrient inputs, and PCBs is unknown. However, as these substances cause irreversible damage to reef habitat worldwide the chance for both the loggerhead and green sea turtles to survive declines.

According to Bolten et al. (1998) “the Atlantic loggerhead is protected under a number of international conventions and under legislation in individual nations.” The green and loggerhead sea turtles are becoming more protected in coastal waters, but much of their life is spent in the open oceans. Because the loggerhead turtles are a transatlantic species, a management plan developed by all nations that use these waters is necessary. In addition, the North Pacific and North Atlantic loggerhead turtles are the most heavily studied, little to no information has been gathered about southern hemisphere loggerhead sea turtle populations. In order to fully protect this animal, researchers must uncover their open ocean habitats and “missing life stages” (Bolten et al., 1998).

In conclusion, sea turtles are facing more and more pressure from nesting habitat destruction, increased commercial fishing, and higher ocean pollution. This paper focused on the ocean habitat of sea turtles, but the beaches themselves are under development pressure. Even by cutting the trees next to the beaches can alter the sex of the hatchlings and may upset the species balance. The fishing and shrimping operations kill many sea turtles every year, and sea weed collection causes the mortality of young, ocean drifting sea turtles. The full affects of plastics, tar, nutrients, chemicals, etc. upon sea turtle populations is difficult to study, but can be expected to be highly deleterious. Even though management plans are protecting turtles better than ever before, the populations of sea turtles are today very much in danger. The first step for conserving turtles is fully understanding their behavior and habitat.


Works Cited

Addison, D. S., J. A. Gore, J. Ryder, and K. Worley. 2002. “Tracking post-nesting
movements of loggerhead turtles (Caretta caretta) with sonic and radio telemetry on the southwest coast of Florida, USA.” Marine Biology. 141: 201-205.

Bolten, A. B., K. A. Bjorndal, H. R. Martins, T. Dellinger, M. J. Biscoito, S. E. Encalada,
and B. W. Bowen. 1998. “Transatlantic developmental migrations of loggerhead sea turtles demonstrated by mtDNA sequence analysis.” Ecological Society of America. 8(1): 1-7.

Broward County Flordia. Biological Resources Devision. 29 March 2004.

Davidson, O. G. 1998. The Enchanted Braid. John Wiley & Sons, Inc: New York.

Hatase, H., M. Kinoshita, T. Bando, N. Kamezaki, K. Sato, Y Matsuzawa, K. Goto, K
Omuta, Y Nakashima, H. Takeshita, and W. Sakamoto. 2002. “Population structure of loggerhead tutles, Caretta caretta, nesting in Japan: bottlenecks on the Pacific population.” Marine Biology. 141: 299-305.

Heithaus, M. R., A. Frid, and L. M. Dill. 2002. “Shark-inflicted injury frequencies,
escape ability, and habitat use of green and loggerhead turtles.” Marine Biology. 140: 229-236.

National Marine Fisheries Service. Office of Protected Resources. 29 March 2004.

Witherington, B. E. 2002. “Ecology of neonate loggerhead turtles inhabiting lines of
downwelling near a Gulf Stream front.” Marine Biology. 140: 843-853.


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