|Juvenile loggerhead sea turtle being rehabilitated. Photo by author.|
Given the large body size reached by this stage in the life cycle, mortality due to natural predation is quite low. That is, these turtles are now too large for most ocean predators to be a real threat. And turtles have been shown to be highly adapted physiologically to survive environmental fluxes (food availability, water temperature, storms and etc.) by employing various strategies including reserving resources, slowing metabolic processes, entering hibernation-like torpor states, survival within this stage should be quite high (Felger et al. 1976, Heppell et al. 2003). However, survivability during both the juvenile and adult neritic stage of life is largely dependent on exposure to anthropogenic impacts – especially fishing (intentional and incidental take) and pollution (Crouse et al. 1987). This will be a focused on more later.
During breeding migrations, both male and female turtles from multiple foraging grounds migrate and converge on breeding areas near the same beach where they hatched out decades earlier! This is called their "natal beach". Mating occurs in the water near the nesting beaches, and also during migrations – which is one way turtles are thought to increase gene flow among nesting groups. And while variation among species and populations exists, individuals from most species do not breed every year. Instead, female turtles migrate and nest in two to three year cycles, and this is based on tagging and tracking of female turtles. There is still much to be learned about the breeding and behavior of adult male turtles, who are harder to tag and study since they never come ashore the way that the females do to lay their nests.
|Sea turtle nest and female turtle tracks - see the V-shape tracks. Photo by author.|
In 40-60 days (again, depending on species, location, and other environmental factors), the hatchlings will emerge from the nest and begin their own journey through the life cycle.
Conservatively, a female turtle may start nesting when she is thirty years old, and may continue nesting for 30 to 60 years; therefore, a reproductively mature female who has survived to sexual maturity has a very high reproductive value (Crouse et al. 1987, Heppell et al. 2003). That is to say, a full grown female turtle is very valuable to the continued existence of that sea turtle population.
A handful of nesting site studies range longer than 30 years, including Tortuguero in Costa Rico, Hawaii in the USA, and Michoacán in Mexico, and our understanding of sea turtle reproductive and life expectancy is still in progress. This is also further complicated by the lack of technique to determine the age of a turtle. The age of living sea turtles is still a great unknown. Captive-raised turtles have been known to live for nearly 30+ years (Schwartz 1997, Snover et al. 2007b), but we know turtles live much longer than that. The age of a sea turtle can be estimated based on its size, and after a sea turtle has died, a method called skeletochronology can also estimate age. Skeletochronology is similar to the process used to age trees by cross-section (dendrology) and marine mammals by tooth cross-section (Zug 1985, Zug et al. 1986).
|Green sea turtle foraging (eating) in the Caribbean. Photo by author.|
Yet to this day, there is no way to accurately age a living wild sea turtle, as a result, age-at-size estimates are most commonly used to study life history and to construct population structure and abundance models (Crouse et al. 1987, Heppell et al. 2003). In 1986, George Zug and colleagues described the process of skeletochronology for sea turtles, and since then, multiple studies have been conduced describing growth and age information for several marine turtle populations using this technique.
Studies include green turtles Chelonia mydas, in Hawaii (Zug 1985, Zug et al. 2002), and the Caribbean/Atlantic (Goshe et al. 2010, Zug and Glor 1998, Bjorndal et al. 1998), loggerheads Caretta caretta in Gulf of Mexico/Atlantic (Zug et al. 1986, Coles et al. 2001, Snover and Hohn 2004, Snover et al. 2007a, Parham and Zug 1997), and the Mediterranean (Casale et al. 2011) and in the Pacific (Zug et al. 1995), for Kemp’s ridleys Lepidochelys kempii in the Gulf of Mexico (Avens and Goshe 2007, Snover and Hohn 2004, Snover et al. 2007b, Zug et al. 1997), leatherbacks Dermochelys coriacea in the Atlantic (Snover and Rhodin 2008) and olive ridley’s Lepidochelys olivacea in the Pacific (Zug et al. 2006). Similarly, Reich compared skeletochronology of loggerheads and green turtles together with layers of scute material (layers of the sea turtle's shell, or carapace) of green turtles to estimate age at settlement for green turtles in the Atlantic (Reich et al. 2007).
Skeletochronology will be one of two primary techniques I will use for my research, and is what I've been learning all about during my time here in North Carolina.
Avens L, Goshe LR. 2007. Comparative skeletochronological analysis of Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) humeri and scleral ossicles. Marine Biology (Berlin) 152(6):1309-1317.
Bjorndal KA, Bolten AB, Bennett RA, Jacobson ER, Wronski TJ, Valeski JJ, Eliazar PJ. 1998. Age and growth in sea turtles: Limitations of skeletochronology for demographic studies. Copeia(1):23-30.
Casale P, Conte N, Freggi D, Cioni C, Argano R. 2011. Age and growth determination by skeletochronology in loggerhead sea turtles (Caretta caretta) from the Mediterranean Sea. Scientia Marina 75(1):197-203.
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Snover ML, Hohn AA. 2004. Validation and interpretation of annual skeletal marks in loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii) sea turtles. Fishery Bulletin (Seattle) 102(4):682-692.
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Snover ML, Rhodin AGJ. 2008. Comparative ontogenetic and phylogenetic aspects of chelonian chondro-osseous growth and skeletochronology. Biology of Turtles:17-43.
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Zug GR, Balazs GH, Wetherall JA, Parker DM, Murakawa SKK. 2002. Age and growth of Hawaiian green seaturtles (Chelonia mydas): an analysis based on skeletochronology. Fishery Bulletin 100(1):117-127.
Zug GR, Chaloupka M, Balazs GH. 2006. Age and growth in olive ridley seaturtles (Lepidochelys olivacea) from the north-central Pacific: a skeletochronological analysis. Marine Ecology-an Evolutionary Perspective 27(3):263-270.
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