Caretta Caretta: Introduction to Biology and Conservation
Despite being the most intensively studied marine turtle, still relatively little is known about loggerhead sea turtles (Caretta caretta), and indeed, all marine animals, in relation to their terrestrial counterparts. This paper attempts to survey and synthesize the known body of knowledge on loggerheads. A brief introduction outlining the taxonomy and morphology of the species is presented, followed by a summary and map of the loggerhead’s vast global range. An extensive discussion of the life history and habits follows, wherein it is clear that there is a great deal more research surrounding terrestrial nesting habits of hatchlings and adults than there is knowledge of juvenile and adult netric and pelagic behavior. Recent genetic connectivity research has important policy implications indicating that conservation should be implemented on a population, rather than species-wide, basis. Human impacts, including bycatch mortality, nesting habitat destruction, contamination, and climate change are analyzed in depth with a view toward addressing the scope and synergistic effects of anthropogenic influences, and an overview and assessment of the efficacy of existing national and international protections is presented. Lastly, several suggestions for future research are proposed.
The Loggerhead Sea Turtle, scientific name Caretta caretta, are relatively poorly understood (Crowder 1994), given that biologists and taxonomists have been studying them since the mid-1700s (Linnaeus, 1748)(See Appendix for translation of Carl von Linne’s original description).
Table 1: Loggerhead Taxonomy (Linneaus, 1748).
Caretta caretta is easily distinguished from other sea turtle species. Loggerhead sea turtles have a relatively large head, equipped with powerful jaws (NOAA Fisheries Office of Protected Resources 2013). The carapace is slightly heart-shaped and reddish-brown; the plastron is pale yellow (NOAA 2013). The turtle’s neck and flippers are dull brown to reddish brown on top and medium to pale yellow on the sides and bottom (NOAA 2013). Layers on the carapace reflect the turtle’s age, akin to the way tree rings tell the age of an individual tree (Klinger & Musick 1995). The average adult weight of a loggerhead sea turtle is 250 lbs (113 kg), and the average length is three feet, or approximately one meter (NOAA 2013).
Image 1: Caretta caretta
Source: NOAA Fisheries Office of Protected Resources Loggerhead Turtle
Species Range and Distribution:
In terms of geography, loggerhead turtles occur throughout temperate and tropical regions of the Atlantic, Pacific, and Indian Oceans. In the Atlantic, the loggerhead’s habitat ranges from Newfoundland all the way to Argentina but is typically found from North Carolina to Florida (NOAA, 2013). Loggerheads primarily nest in the Atlantic and Indian Oceans (NOAA, 2013). Although they also technically have a wide range in the Pacific Ocean, most records of loggerheads in the Pacific are sightings of juveniles off the coasts of California and Mexico. There are also records of loggerhead populations in the Mediterranean Sea. Some populations remain primarily in the coastal continental shelf regions near their nesting sites, while other populations are known to migrate across deep waters (NOAA, 2013).
The depth range of loggerhead sea turtles ranges from shore (sea level), where sea turtles hatch and return to lay their eggs, to the pelagic ocean, where adult sea turtles spend the majority of their time. Less is known about where young and juvenile turtles spend their lives, but it is believed that the young take refuge in nearshore coastal areas, and that once they reach the open oceans, juveniles still live primarily in the upper fifteen feet (five meters) of the water column (NOAA, 2013).
Image 2: Approximate Range of Caretta caretta:
Source: (NOAA Fisheries Office of Protected Resources, 2013).
Hatchling loggerheads scuttle from their nest to the ocean, guided by moonlight reflecting off the water. After swimming perpendicular to shore for several days, hatchlings take refuge in seaweeds, such as Sargassum, at locations where ocean currents converge to form areas of downwelling. (Dodd 1988; Witherington 2002). There they forage for food and can remain for months until currents transport them offshore where they dwell in the currents of the pelagic environment.
Juveniles continue to live primarily near the surface of the ocean, within about fifteen feet from the surface until they are approximately 5-12 years old (not quite yet sexually mature), at which point they move in toward nearshore coastal waters known as the neritic zone, including bays and estuaries throughout their vast global range (NOAA 2013; Dodd 1988). There they feed on crustaceans, mollusks, and fish (Klinger & Musick 1995). These sub-adults may migrate with their adult conspecifics, although some studies have found that in some cases these young adults simply stay put, burying themselves in mud to avoid the cold (Dodd 1988). There is also some evidence that adults periodically return to their juvenile foraging grounds and that juveniles themselves exhibit strong fidelity to “their” particular feeding grounds, although some individuals also seem to wander and spend more time in open ocean waters (Casale et al. 2012).
Adults inhabit neritic waters and cross open-ocean during migration. Exactly when adults reach sexual maturity is difficult to ascertain, and it has been suggested that age and size at maturity may vary by population (Dodd, 1988). Likewise, relatively little is known about the characteristics, life history, and behavior of male turtles, which are more difficult to observe (Ishihara & Kamezaki 2011; Dodd 1988). Adult males are distinguishable from females by their longer tails and claws, slightly flatter and wider carapace and wider head (Dodd 1988).
Loggerhead sea turtles appear to be polyandrous, with multiple mates at once (Dodd 1988). Mating occurs during the several months prior to nesting season and may be restricted or most common at specific grounds or mating habitats (Dodd 1988). During summer nights along the sandy beaches in their range, female sea turtles return to their natal beaches to lay their eggs (this fidelity is called philopatry) (Witt et al. 2010). Each female may make several visits each summer, during which she will build a single nest with approximately 120 eggs in it (Pinkney, 1990). Nesting season is May-August in the Northern Hemisphere, and October through March in the Southern Hemisphere, with longer durations of nesting season in lower latitudes, and is correlated with rainy seasons (Dodd 1988). To nest, females crawl up to the beach, dig a shallow nest with their flippers, lay eggs 1-3 at a time, bury and disguise their nest, and return to the sea, all in under about 2 hours (Dodd 1988). False crawls are common, and may occur when a nesting turtle is disturbed (for example by humans) or there is a problem with the beach, as often occurs with artificially “renourished” beaches (Dodd, 1988).
As discussed above, sea turtle habitat varies with age or life stage. The most important habitat factor is the availability of appropriate nesting sites. Loggerhead sea turtles prefer beaches that are sheltered, either with dunes or some vegetation behind the beach (Dodd, 1988). Moreover, as previously mentioned, sea turtle nesting is correlated with rainy seasons, and some loggerhead populations will nest outside the normal nesting timeframe in order to nest during the wet season, for example in the Gulf of Mannar in India (Dodd 1988). Although the relationship between nesting and rainfall does not appear to be well-studied, it is possible that the correlation is significant, and that decreased rainfall associated with climate change may impact nesting cycles. However, too strong rains or storms impair reproduction (Dodd 1988) and conditions that are either too dry or too wet may reduce reproductive success (Foley et al, 2006). Studies have also found that higher levels of salinity may be correlated with decreased hatchling success (Foley 2006), which may point to the reason nesting is correlated with rainfall if freshwater flows play an important role in reducing salinity during nesting season.
Inundation from too-high tides can reduce hatchling success, but nests are capable of withstanding some inundation (Foley 2006). In fact, because temperature and other incubation variations play an important role in the biology of the unhatched turtles, for example in determining sex, size, and locomotive performance, it may be important to maintain variation in the range of nesting locations within the tidal zone (Foley 2006). Thus it may not be a prudent management strategy to relocate all tidal area nests to sites above the high tide mark (Foley 2006).
Temperature likewise plays an important role in determining habitat suitability. Cold-stunning is a condition of intense hypothermia that can affect marine turtles, resulting in strandings and floating turtles (Burke & Standora, 1991). Cold-stunning occurs when water temperatures dip below ten degrees Celsius for more than a few days (Burke, 1991). While threats from climate change and other anthropogenic impacts will be discussed in greater detail below, it is worth noting that cold-stunning frequency may increase in higher latitudes if climate change results in slower thermohaline circulation and reduced ocean current flows. In 2010, a cold snap in the Gulf of Mexico resulted in over 5,000 cold-stunned marine turtles, including loggerhead turtles (Jones et al, 2012).
Another habitat requirement for loggerhead nesting is the need for darkness. Not only are nesting females easily disturbed by lights and silhouettes, but hatchling turtles are easily distracted in the presence of artificial lights and will make for the lights of beach houses, towns, or highways, mistaking them for the light of the moon reflecting off the ocean water (Dodd 1988).
Hatchling and young juvenile sea turtles also rely upon the health of the sea grass communities that provide them with shelter and foraging habitat during their most vulnerable life stages (Dodd 1988). Harmful algal blooms or eutrophic conditions inhibit light penetration to sea grass communities, reducing the availability of this essential turtle habitat (NMFS 1998).
Besides aggregations for mating purposes, Caretta caretta display no known social behaviors. However occasionally some aggregations of small adults or sub-adults have been observed, in some cases with several copulating pairs; thus this communion is thought to be related to mating and reproduction (Dodd 1988). Otherwise, loggerheads are solitary.
Loggerheads inhabit a vast range of marine habitats, but are separated into distinct populations (Dodd 1988). Some of these populations are more endangered or threatened than others. In 2011, the National Ocean and Atmospheric Administration and the U.S. Fish and Wildlife Service revised the loggerhead Endangered Species Act listing from a single “threatened” species to nine separate populations, each separately designated as “threatened” or “endangered” (76 Fed. Reg. 58668, 58952).
Populations do seem to remain distinct from one another. For example, Atlantic loggerheads share a Mediterranean feeding ground with their old-world counterparts, but remain a distinct population without significant genetic transfer (Carreras et al 2011). Studies focusing on genetics must evaluate male genetic material, because male-mediated gene flow is common among loggerheads, meaning that many genetic markers are passed on only by males. Previous studies have indicated that the migration of Atlantic loggerheads to the Mediterranean region is undertaken primarily by males, and thus there is a possibility of genetic mixing based on male-dominated gene flows. Conventional DNA analysis is not entirely precise, often finding homogeneity. Mitochondrial DNA (mtDNA) analysis, therefore, is a better indicator of genetic diversity. Using mtDNA, Carreras et al found that the two populations have remained distinct. The use of advanced techniques to classify disparate loggerhead populations has practical conservation applications as well (Carreras 2011).
Another study by Encalada et al (1998) seeks to evaluate the genetic diversity among Atlantic and Mediterranean loggerhead populations to determine whether nesting turtles return to their native beaches based on some kind of homing mechanism, or whether a behavioral pattern called social facilitation occurs, where first-time nesting females follow other females to the shore. Homing would produce genetic homogeneity, whereas social facilitation would not (Encalada 1998). This knowledge is valuable for conservation management practices, because nests can be moved or relocated, and fishing closures and other protections can be erected around known nesting sites (Encalada 1998). Although the article did not definitively answer the question posed at the outset, the authors determined six distinct loggerhead populations (although NOAA and FWS determined nine for the purposes of the Endangered Species Act): (1) North Carolina, South Carolina, Georgia and northeast Florida, USA; (2) southern Florida, USA; (3) northwest Florida, USA; (4) Quintana Roo, Mexico; (5) Bahia, Brazil; and (6) Peloponnesus Island, Greece (Encalada 1998). The use of genetic markers is particularly significant because the US populations can be distinguished no other way (Encalada 1998). The use of genetic studies has implications for constructing conservation practices that preserve the highest levels of genetic biodiversity, and may be used to better assess the life histories of each population to allow for better conservation management throughout its life cycle and across a breadth of habitat.
Threats to loggerhead sea turtles can be broken down into four areas: fishing/bycatch, nesting disturbances, marine debris/pollutant contamination, and climate change related impacts.
The greatest threat to the survival of loggerhead sea turtles is the frequency with which they are caught and killed as bycatch. Loggerheads are a long-lived marine species, exhibiting the biological and physical characteristics associated with long-lived species, and therefore are especially vulnerable to high exploitation rates (NMFS 2001).
In 2000, the National Marine Fisheries Service reinitiated consultation under section 7 of the Endangered Species Act on certain fisheries, meaning that it evaluated those fisheries for their impacts on takes of endangered species, and concluded that the pelagic longline fishery as it currently operates will “likely jeopardize the continued existence of the loggerhead… sea turtle” (NMFS 2001). The specific language of the finding is significant because it triggers stringent conservation requirement efforts under the Endangered Species Act, under which loggerhead sea turtles are a listed species (43 Fed. Reg. 32800; 76 Fed. Reg. 58668). It is during the pelagic phase of their life history that loggerheads are most susceptible to risks from longline fishing (NMFS 2001). Estimates for loggerhead takes from the fishery in the US range from 293 to 2,439 animals per year. At a conservative estimate assuming 50% mortality, the impact of the fishery on loggerhead populations is an annual population reduction of 147-1,220 (NMFS 2001). Other studies have likewise indicated the severity of the threat that pelagic longline fishing poses to loggerhead sea turtles. For example, in one study of bycatch in Brazil, the catch per unit effort (CPUE) for loggerheads ranged from 13.6-24.5 loggerheads per every thousand hooks, about 95% of which were juvenile or sub-adult individuals (Sales et al. 2008).
The only fishery with a greater impact on loggerhead sea turtle survival is the shrimping industry (NMFS 2001). Although turtle excluder devices (TEDs) have been required off and on seasonally the past few decades, the required designs and opening sizes have not been sufficient to ensure the escape of even all juvenile loggerheads (NMFS 2001). Several attempts have been made over the years to tighten restrictions and improve TED efficacy, most recently in 2007 when the agency issued another advance notice of proposed rulemaking on possible amendments (72 Fed. Reg. 7382), but ultimately these efforts have gained little traction. Prior to the development and implementation of turtle excluder devices, management strategies primarily focused on egg and hatchling survival projects (Crowder et al. 1994). However, studies have shown that the greatest benefit for sea turtle populations generally would be incurred from protecting large juvenile and adult turtles than could be expected from a similar expenditure of effort at improving recruitment or fecundity (Crowder 1994). The same studies noted that large juveniles were the most prevalent life stage of stranded turtles, and that these strandings were largely a result of drownings associated with shrimp trawls. In evaluating the potential efficacy of turtle excluder devices on reducing large juvenile mortality of loggerhead turtles, Crowder et al. modeled several scenarios and concluded that seasonal usage of TED devices (as was the case prior to 1992) would result in extremely slow population growth, requiring approximately seventy years to for the population to increase by even one order of magnitude (1994). Year-round implementation of TED regulations would be anticipated to double that rate of growth.
Additionally, although other marine turtles are considered more palatable and thus face greater threats from direct harvest, loggerhead turtles, and especially their eggs, have nevertheless been historically harvested for subsistence or local consumption in many parts of the world (Dodd, 1988).
Plastic litter and other marine debris also threaten loggerhead turtles, because they are often ingested. Studies have found plastic strips, plastic bags, pieces of glass, bark, rope, tar, nylon thread, polyethylene, oil/tar balls, Styrofoam, and more, in turtles’ stomachs (Dodd, 1988). Other types of contamination also have significant impacts on turtles in the pelagic and nearshore environments. Nutrient pollution causes eutrophication that chokes sea grass and sargassum habitats on which juvenile and hatchling turtles depend (NMFS 1998). In the Gulf of Mexico, crude oil from both small accidental releases and the 2010 Macondo BP oil spill have had drastic impacts on both pelagic and nesting beach loggerhead environments (NMFS 2008; NRDA 2011). Additional chemical contaminants from oil spill dispersants, atmospheric deposition from industrial manufacturing and power plants, and urban stormwater runoff contaminants have unknown impacts on turtle populations.
Nesting turtles and new hatchlings also face threats from human impacts. Nesting females approaching the shore and in the initial nesting stages are particularly vulnerable to human disturbance – any perceived threat will cause them to “false crawl” and return to the sea without nesting (Dodd 1988). Hatchlings are extraordinarily sensitive to photopollution impacts because they depend on the light reflecting off of the water to direct them toward the ocean. However, light from anthropogenic sources, such as street lamps, towns, or beach homes may direct them in the wrong direction, resulting in hatchlings falling victim to predators or even becoming roadkill (NOAA Threats 2013).
Climate change may also threaten the survival of loggerhead sea turtles. One study suggests that rising sea surface temperature may alter the phenology, or seasonal timing, of loggerhead nesting patterns, resulting in earlier nesting seasons that may impact fecundity and hatchling viability, although results are mixed and longer summer periods may conversely increase foraging time and growth periods for adult turtles (Mazaris et al. 2008). Other studies have shown that earlier nesting seasons actually decreases the duration of nesting season by approximately 43 days, and that individuals do not lay additional clutches because of physiological limitations (Pike & Stiner 2006). Moreover, as discussed in previous sections, many factors are dependent on temperature cues, including hatchling sex ratio, incubation duration, and hatchling viability (Mazaris 2008). Lastly, sea turtle nesting habitat, already strained from anthropogenic habitat destruction, may become further threatened as sea level rises. Studies suggest that about half of eligible nesting sites will be lost, especially in island areas or in areas where human development meets the rising sea (Witt, 2010).
The anthropogenic threats to sea turtle survival are formidable, and require bold solutions. Unfortunately, sea turtle conservation has experienced limited success, hampered by bureaucratic strings, economic disincentives, and jurisdictional enforcement limitations.
The US government listed loggerhead sea turtles under the first incarnation of the US Endangered Species Act (ESA) (16 U.S.C. §1531 et seq.) at its inception in 1978. 43 Fed. Reg. 32800 (1978). Previously listed as a globally threatened species, NOAA and FWS, the agencies jointly responsible for marine turtle management, have divided the species into nine “distinct population segments,” four of which are listed as threatened (the Northwest Atlantic, South Atlantic, Southeast Indo-Pacific, and Southwest Indian oceans), and five of which are designated under the heightened standard, endangered: (the Northeast Atlantic, Mediterranean, North Indian, North Pacific, and South Pacific populations) (76 Fed. Reg. 58868).
Marine turtles, including loggerheads, are also governed by the Magnuson-Stevens Fisheries Conservation and Management Act (15 U.S.C. §1801 et seq.). The Magnuson Act governs fisheries, and it is under the authority of the Magnuson Act that TED requirements are imposed on shrimp trawlers (57 Fed. Reg. 57348) and circle hooks in longline fisheries (50 C.F.R. 223.206-207).
Internationally, loggerheads are listed under the Convention on International Trade in Endangered Species (CITES), the Convention on Migratory Species, and the Inter-American Convention for the Protection and Conservation of Sea Turtles, among other international agreements. However, these agreements are extremely difficult to enforce, and the United States is not even a signatory to the Convention on Migratory Species. As such, the efficacy of these international agreements is substantially undermined.
However, one international agreement is particularly effective. The General Agreement on Tariffs and Trade (GATT), governing parties to the World Trade Organization (WTO), generally prohibits restrictions on trade among member countries. However, Article XX to the Agreement outlines exceptions, including exceptions for environmental conservation. For example, Article XX(b) provides that trade discrimination may be justified “to protect human, animal, or plant life or health,” while section XX(g) specifically authorizes trade barriers in pursuit of efforts “relating to the conservation of exhaustible natural resources if such measures are made effective in conjunction with restrictions on domestic production or consumption.” It is under the auspices of these GATT exceptions that the US Congress was authorized to pass the Shrimp Import Prohibition for marine turtle conservation (16 U.S.C. 1537). The Prohibition proscribes the importation of shrimp from countries that do not implement turtle excluder device and other requirements to reduce sea turtle bycatch. Countries wanting to export shrimp to the US must be certified through the US Department of State and NOAA, and NOAA provides extensive training and assistance with policy development. Thus, the GATT provides a rare opportunity to extend environmental conservation extra-jurisdictionally to facilitate global conservation.
Unanswered Questions and Potential Directions for Future Research:
The impacts of climate change on nesting habitat bears further research. Various climate and sea level rise models should be analyzed and mapped over important nesting beaches to determine the level and timeline of the threats that nesting turtles on these beaches will face. Additionally, the impacts of locally increased or decreased rainfall should be assessed, analyzing the degree to which changing patterns are likely to affect relative levels of inundation of nesting habitat. Likewise, sand temperatures should be predicted, and it should be determined whether there is an upper limit that eggs can tolerate. Because organisms tend to already gravitate toward the upper limit of their tolerance range to maximize metabolism and growth rate, even a slight temperature increase may push loggerhead sea turtles outside their tolerance range. Therefore it is expected that sea turtle nesting sites in the tropics may experience adverse effects.
Second, anecdotal evidence has suggested that chemical dispersant applied in the wake of the BP Macondo oil spill has resulted in a range of negative impacts on marine fauna, including high mortality levels among Gulf sea turtle populations. The results of the research and necropsy conclusions should be released to the public so that further research may be developed to assess the likelihood of any ongoing or chronic impacts.
Also related to the BP oil spill, research should be conducted to determine the effects of nest relocation on sea turtle populations. Because nesting loggerhead turtles exhibit philopatry, populations (at least the female portion of populations) would be expected to remain relatively stable, with little genetic mixing, as discussed above. Relocating Gulf nests to the Atlantic coast may have had a range of results, from skewing sex ratios in hatchlings from the temperature differences, to reducing genetic diversity and population viability of Gulf populations. While it seems obvious that relocating the nests was preferable to risking their exposure to the oil spill and dispersant, the effects of that relocation are nevertheless worth examining for future reference. One potential study might monitor nesting behavior and turtle sightings offshore of the respective beaches, and compare those results with any available pre-spill data.
It may also be beneficial to evaluate the efficacy of captive breeding and release management plans to bolster threatened populations to know whether such programs might be a viable way to prevent future population declines. Such a study might be conducted through tag and release programs comparing captive versus wild populations.
Lastly, future research should explore relationship between nesting incidence and rainfall. It may be possible to do this study retroactively, by comparing precipitation data with records of nesting incidences and fecundity, all of which data is publicly available.
Bibliography: see References.
Appendix A: Original Description
Linnaeus, Systema Naturae (1748).
Called Testudo caretta
(See attached PDF files of cover and description)
Carl von Linne, Systema Naturae, 197 (1748) as translated by William Turton, Vol. 1, 641 (Allen, Lackington and Co., 1806) via Google Books.
Amphibia: “This class of animals is distinguished by a body cold and
generally naked; a countenance stern and expressive; voice harsh; colours mostly lurid, and filthy odour: a few are funished with a horrid poison; all have cartilaginous bones, flow circulation, exquisite sight and hearing, large pulmonary vessels, lobate liver, oblong thick stomach, and cycstic, hepatic, and pancreatic ducts: they are deficient in diaphragm, do not transpire, can live a long time without food, are tenacious of life, and have the power of reproducing parts which have been destroyed or lost; some undergo a metamorphosis; some cast their skin; some appear to live promisculously on land or in the water, and some are torpid during winter.”
Order 1: Reptiles: “lungs arbitrary, generally four legs, penis simple”
Testudo: “Body tailed, covered above and beneath with a bony or coriaceous shell, or scales above: upper jaw enclosing the lower like the lid of a box.”
Testudo description: These are held in abhorrence by the Persians; are very fertile and in the egg state the prey of many ravenous animals; feed on worms, the marine ones on sea weeds, and when tamed will eat almost any thing; are extremely slow, and in copulation frequently adhere together a month; are capable of existing a long time in noxious air, and so tenacious of life that if the head be cut off or the chest opened they will live several days; the land ones are torpid during winter, in cold climates. The shell consists of two connected laminae, the upper convex, covered with scutels which of the disk are 13, of the margin 24; the lower concave, particularly in the male, obtuse on the fore-part and notched behind, divided by futures into scutels; between the 2 laminae is an anterior aperture for the head and arms, and a posterior one for the tail and thighs.
Marine Turtles: “legs fin-shaped, the foremost longer.”
Caretta: “Plates of the back gibbous behind; fore and hind feed two-clawed. Inhabits American and Mediterranean islands; affords tortoise shell, flesh rancid. Head middle-sized; mouth large; beak long, stout; back more prominent and gibbous than others; shell thick, elegantly painted.
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