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Reproduction and Sex Determination
Sea turtles as a whole use a high fecundity, high mortality approach to reproduction (Davenport 1997). Compared to other reptiles, sea turtles lay large numbers of eggs (50-200 eggs per clutch, 6-7 clutches per mating season), but this is necessary because very few hatchlings survive to adulthood. Upon hatching, a nest of sea turtles travels en masse to the open ocean, making them an easy target for predators. Also, sea turtles, depending on the species, take anywhere from 6-50 years to reach sexual maturity, and most females don’t breed every year. For these reasons, those females that do live long enough to reach sexual maturity must produce large numbers of eggs to sustain the population.
Another important part of sea turtle reproduction is the selection of a nesting site. After mating in near-shore waters, females store sperm for several weeks (Davenport 1997) and lay their eggs on warm, sandy beaches. However, the selection of a nesting beach is not random but instead depends on several factors (Weishampel et al. 2003). First, sea turtles exhibit natal homing, meaning they tend to build nests in the general region where they were born. Additionally, sea turtles show a high degree of accuracy in returning to previous nesting grounds year toyear. The coarse-scale regional homing ability is termed philopatry, while the fine-scale ability to locate a specific beach is termed fixity. Considering that some sea turtles migrate up to11,000 km per year, this nest specificity is quite a feat. Finally, within a region, several physical cues may dictate which beach a sea turtle ultimately selects. These include marine cues such as scent, surf noise, magnetic fields, offshore currents, and features such as reefs and rocks, and terrestrial cues such as beach slope and width, sand texture, dune vegetation, and lighting. A ten year study of a 40.5 km stretch of beach in Florida illustrated this nesting specificity, as nest distribution was non-random and nests were predictably located on specific beaches from year to year (Weishampel et al. 2003).
A final important aspect of sea turtle nesting is the manner of sex determination. Like many reptiles, sea turtles exhibit temperature-dependent sex determination (TSD), in which the incubation temperature of the eggs determines whether the embryo develops into a male or female (Davenport 1997). Therefore, while most species have genetically determined sex with predictable ratios, sea turtle sex ratios can vary with temperature. The temperature during the middle third of incubation determines the sex, with high temperatures yielding females and low temperatures yielding males. The temperature that yields a one-to-one sex ration, termed the pivotal temperature, is around 29° C for most species of sea turtles. Biochemically, all embryos produce testosterone during early development, but high temperatures trigger the expression of the enzyme aromatase, which converts testosterone to the female sex-hormone estradiol, while low temperatures trigger the expression of the enzyme reductase, which converts testosterone to the male-determining hormone dihydrotestosterone.(Crews 1996)
After about 2 months of incubation, sea turtle eggs hatch at night to reduce the risk of predation and to avoid hyperthermia (Davidson 1998). The timing of hatching is governed by photoperiod, and hatchlings use the decreasing temperature of the sand to assure that they emerge at night. Upon emerging, hatchlings use light patterns and the Earth’s magnetic field to orient them in the right direction, and then commence their frantic scurry to the open water. Subsequently, juvenile sea turtles spend at least 3-5 years in the open ocean, but because of the difficulty in tracking them, very little is known of their biology during this period. As a result, this period of pelagic existence is referred to as the “lost year,” even though it can be up to 10 years.
After spending several years in the open ocean, juvenile sea turtles “reappear” in shallow coastal areas to feed and mature. Since they occupy a narrow, near-shore habitat range during this second juvenile phase, researchers have been able to extensively document the behavior of these turtles. Using ultrasonic telemetry and time-depth recorders to monitor the habitat usage of juvenile sea turtles, Makowski et al. (2006) studied this phase of sea turtle life and made several important observations. First, juvenile sea turtles occupy a fairly narrow habitat range, usually less than 5 km2. Also, juvenile home ranges are primarily determined by food availability, and individual home ranges show considerable overlap. During this time, juvenile turtles feed extensively on marine algae, in particular the red algae Gracilaria mammillaris, and they also occasionally feed on sponge fragments. Within their habitat, juvenile turtles spend their day between surface foraging areas and a nocturnal resting site under reef ledges. While the foraging area is shared among several turtles, the nocturnal resting sites are not shared, and each turtle typically has only 1-2 resting sites. Finally, while juvenile sea turtle forage both during the day and at night, they are much more active during the day. During the day, they forage nonstop, while at night they spend more time resting, presumably to avoid nocturnal predators such as sharks. These feeding grounds that juvenile turtles return to are the same ones that they will migrate to and from as breeding adults.
A final consideration of sea turtle life histories that has conservation implications is growth rate. A high growth rate is important because it results in a larger biomass of adult turtles, which in turn can yield more offspring. However, characterizing growth rates is difficult, because it is impossible to capture and measure every member of a turtle population. As a result, straight carapace length (SCL) is a commonly used parameter to estimate sea turtle growth rates, and it has revealed several important aspects of sea turtle growth rates (Bjorndal et al. 2000). The most important finding is that sea turtle growth rates are density dependent, meaning that as population density increases, growth rates decrease. This differs from the scenario of foraging juveniles discussed above, where there is apparently no competition for resources. The observed density dependence in adults can be explained by a finite carrying capacity of the sea grass beds that adult sea turtles feed on. Also, the density dependent growth rate is enhanced by sea turtles’ feeding habits. Adult sea turtles maintain grazing plots in which they continually recrop new growth until the sea grass is no longer able to grow, after which the turtles move to new plots. Thus, as population density increases, individual turtles aren’t able to maintain large enough plots to sustain a high growth rate, and they also must travel farther to new plots when their current plot expires. A final factor that affects growth rate is the age profile of the population (Hazel and Gyuris 2006). Since hatchlings and juveniles have no reproductive capability, a population comprised primarily of mature adults will grow much faster than one with a younger age distribution.
Each of the above life history traits of sea turtles has implications for sea turtle conservation. Because sea turtle hatchlings are so vulnerable and because nests are readily accessible and manipulated, sea turtle reproduction is the life history trait that is most often applied to conservation efforts. The three most common conservation techniques applied to sea turtle nests are screening, fencing, and relocating (Baskale and Kaska 2005). Screening is used in areas where the risk of predation is high, and it involves surrounding the nest with a wire mesh screen that sits off the ground to prevent predators from digging up the eggs. Fencing is used in areas of high human activity, such as public beaches, to warn people of the proximity of the nest. Finally, relocating involves digging up the eggs that are under the risk of tidal inundation and moving them further from the water. Overall, Baskale and Kaska found that all three techniques significantly increased hatchling success rate.
While the above conservation techniques are largely successful, they may have some unintended consequences. First, the above techniques are somewhat labor intensive and expensive, so in poorer countries such as Mexico, they are largely ineffective because of a lack of funds and cooperation (Garcia et al. 2003) Therefore, an alternative strategy is intensive beach management, which involves heavy patrolling and nest reburial. In this method, trained individuals scour beaches for new nests and promptly relocate them to a centrally located, protected hatchery. This way, all the eggs are in a small area that can be monitored by only a few individuals, instead of having screened, fenced, and relocated nests spread far apart. Also, since it is dangerous to move nests after they have been incubating for some time, the intensive patrolling allows nests to be discovered and relocated before it is too late. Furthermore, screening nests can have some unintended consequences (Irwin et al. 2004). The metal screens used to prevent predation distort the nearby magnetic fields, which may hinder the hatchling’s ability to properly orient and navigate itself. As a result, many conservation programs are switching to magnetically inert materials such as plastic or wood to enclose nests.
TSD is another aspect of sea turtle life history that has an impact on conservation efforts. Global warming has led to female-biased hatchling populations, with as much as 90% of hatchlings being females in some areas (Davenport 1997). In a study of Ascension Island beaches, it was found that the average nest temperature in May has been rising a 0.49°C per 100 years (Hays et al. 2003). In the long run, this creates a shortage of males, and females lay many unfertilized eggs. Further compounding this problem, when nests are transplanted to protected areas, the new beach will most likely have a slightly different temperature profile, which can further skew natural sex ratios. Some sites place eggs in boxes or coolers to lower the incubation temperature, and this results in male-biased populations. While this would seem to combat the female-biased populations, because sea turtles typically breed close to their birthplace (natal homing), the male and female-biased populations don’t sufficiently disperse and mix. To combat these problems, conservationists have fine-tuned some of their conservation efforts with an eye towards maintaining sex ratios. First, conservationists are paying more attention to microclimate data from both natural nesting sites and protected beaches, so that they can try to mimic the natural conditions upon transplantation. Second, some protected beaches are now being set up near condominiums and other buildings that provide shade, which will lead to male hatchlings early in the breeding season, but would have a minimal effect during the warmer, late breeding season.
Another major threat to sea turtle hatchlings that is being addressed by conservationists is artificial lighting (Tuxbury and Salmon 2005). As benign as this may seem, it in fact can seriously hinder hatchlings by disrupting the photoperiod cues used for the proper timing of hatching. Also, newly hatched sea turtles use the reflection of light off the ocean to guide them towards the water, so artificial lighting can prevent the proper orientation of hatchlings. One possible solution to this problem would be to place a bright light on the water side of the nest, which would direct the hatchlings away from the dimmer off-shore lights. Another solution would be to temporarily ban of limit artificial lighting in areas adjacent to nests, but this would be difficult to implement and enforce.
While most of the sea turtle conservation efforts have been directed to nests and hatchlings, the increased understanding of juvenile behavior has important conservation implications. With the hunting of sea turtles having been outlawed, the largest remaining threat to sea turtles is incidental capture in fishing nets (Makowski et al. 2006). Thus, understanding the home range and behavior of juvenile sea turtles is essential to design the appropriate conservation strategies for this time of life. By understanding the reef habitat and foraging areas of juvenile sea turtles, these areas can be targeted by conservationists to prevent accidental death due to fishing. Also, new fishing methods have been developed that reduce sea turtle mortality with little effect on fishing output (Watson et al. 2005). For example, circle hooks and mackerel bait greatly reduce sea turtle mortality compared to traditional J-hooks and squid bait. While this information is a start, the next step for researchers and conservationists is to understand sea turtle life history during the “lost year.” This information, combined with what is known about off-shore juveniles, would allow sea turtles to be monitored and protected all the way from birth to reproductive maturity.
A recently identified threat that is just beginning to gain the attention of conservationists is vessel-related mortality (Hazel and Gyuris 2006). Because sea turtles actively forage for much of the day, they are prone to being struck by vessels that are traveling near the shore. On the Queensland east coast, vessel-related mortality accounted for approximately 65 deaths annually, a number that rivals accidental fishing mortality. However, because this threat has only been recently recognized, there have not been any attempts to remedy this problem. One potential solution would be to mark sea turtle “hot-spots” that boaters and ships can avoid. But since there is a lack of reliable data and public awareness concerning vessel-related mortality, it is unlikely that something will be done in the near future.
The final aspect of sea turtle life history, growth rates, is clearly related to conservation. The most effective conservation effort is the one that maximizes growth rates, so understanding the factors that limit sea turtle growth rate is essential for improving conservation efforts. Right now, most natural populations of sea turtles are too low for there to be a negative density-dependent effect, but some populations in protected reserves have reached carrying capacity (Bjorndal et al. 2000). Thus, it may be necessary to expand protected areas once populations reach a certain limit. Alternatively, the relocation of sea turtles from a population at carrying capacity to one far below carrying capacity could effectively increase population growth in both areas. Furthermore, since it is now known that population growth rate is much more sensitive to late juvenile and adult survival, (Hazel and Gyuris 2006), this furthers the need to devote more effort towards protecting juvenile and adult turtles. Also, since the carrying capacity of sea turtles is determined by the availability of sea grass, sea turtle conservation efforts should also focus on protecting sea grass beds. In some locations, over 50% of sea grass beds have been destroyed by human activities such as mining, construction, and intensive fishing (Davidson 1998). Therefore, all of the money and effort spent on protecting sea turtles won’t amount to anything if sea grass beds are ignored in the process.
Overall, sea turtle conservation efforts have been largely effective in improving sea turtle populations worldwide. Since the 19 70’s, populations in some areas have doubled; however, most populations are still far below their pre-Columbian levels. Thus, as researchers begin to uncover more and more about sea turtle life history, this information can be improved to develop new conservation strategies and improve old ones. The greatest deficiency right now is a lack of attention towards juvenile and adult conservation, but greater attention towards protecting reefs and changing harmful fishing practices is beginning to reverse this trend.
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