Study Site

This study took place on Blackbeard Island National Wildlife Refuge, Liberty County, Georgia. The study area consisted of approximately 11 km of beach located on the east side of the island. Blackbeard Island has one of the longest loggerhead management programs in the country and historically has the densest loggerhead nesting population in the state of Georgia.

Experimental Design

Research was conducted in 2005 and 2006, May through August, during the loggerhead nesting season. Prior to the start of this study a number set was created to represent the number of nests laid per season. Each nest used in this study was designated 1 of 2 treatments, in situ or relocated. In situ nests were left in the original location where the female deposited the eggs. Relocated nests were moved above the SHTL and into areas that do not typically experience erosion or tidal influences. To eliminate human bias on which nests were left in situ or relocated, each nest number was randomly assigned one of these treatments. In order to obtain nests with variable conditions that may occur throughout the nesting season every third nest was targeted to be part of this study.

To begin the process each nest chamber was located and excavated. In order to maintain the original dimensions of the nest chamber, the top of the chamber was measured from the surface (in situ, 20.85 ± 1.13 cm, n  =  35; relocated, 23.97 ± 1.15 cm, n  =  34), all of the eggs were counted and removed (in situ, 118.65 ± 3.16 eggs, n  =  35; relocated, 124.26 ± 3.21 eggs, n  =  34), and then the bottom of the chamber was measured from the surface (in situ, 47.71 ± 0.90 cm, n  =  35; relocated, 50.20 ± 0.91 cm, n  =  34). All nests were reconstructed to the dimensions of the original nest.

As the clutch was removed from the chamber, the diameters of 20 eggs were measured from each nest (in situ, 42.12 ± 0.24 mm, n  =  35; relocated 42.68 ± 0.24 mm, n  =  34). To obtain a distribution of eggs from throughout the nest only every sixth egg was measured. After egg diameters were measured, half of the eggs were placed back into the nest, and another measurement was taken from the middle of the nest chamber to the surface (in situ, 34.88 ± 0.91 cm, n  =  35; relocated, 38.26 ± 0.92 cm, n  =  34).

A temperature data-logger was then placed in the middle of the nest chamber and covered with the remaining eggs. After all of the eggs were deposited back into the nest they were topped with sand and covered by metal screens to prevent predation.

One out of every three nests that received a data-logger also received a sister data-logger. The sister data-logger is defined as a data-logger put into the ground 1 m north of the nest location and placed at the same depth as the nest data-logger. The sister data-loggers were used to observe the temperature of the immediate nesting areas where they were placed and to compare this temperature with nests' temperatures throughout their incubation. All data-loggers were set to record the temperature every 2 hours and remained in their locations for the entire incubation duration.

After a mass emergence (a large percentage of hatchlings come out of the nest at once) or an extended time period without a mass emergence (which would indicate low hatch success or a failed nest) the nests were excavated. During this excavation data-loggers were retrieved, and all data were downloaded. Incubation duration was determined and temperature data were analyzed. Hatch success (percentage of turtles that hatched per clutch) was determined by subtracting the number of eggs that did not hatch from the clutch size.

Twenty hatchlings were targeted to be measured from each nest. Natural incubation durations for loggerhead nests in the southeastern United States average 53 to 68 days (National Marine Fisheries Service [NMFS] and US Fish and Wildlife Service [USFWS] 1991); therefore, after 45 days of incubation a cage was placed over the nests to collect hatchlings upon emergence. In the event of a mass emergence, hatchlings were randomly chosen to be measured. Hatchlings' straight carapace lengths (SCL) were measured using calipers.

Precipitation was observed throughout the nesting season by the use of rain gauges that were placed at the northern, middle, and southern regions of the nesting beach. These data were used to compare precipitation to beach and nest temperatures.

Statistical Analysis

Shapiro–Wilk W Test was used to test for normality. Two-way analysis of covariance (ANCOVA) was used to test the differences among treatments and among nesting areas (above and below SHTL) as the season progressed. The data for hatch success were not normally distributed; therefore, the differences among nest treatments and among nesting areas were tested using Wilcoxon/Kruskal–Wallis tests. Nests and sister data-logger temperatures during embryonic stages 7 to 27 of development were compared using a t test. These particular stages of embryonic development were compared because after stage 27 temperature sex determination has already taken place, and metabolic heat from growing embryos rapidly increases the temperature of the nest (Miller 1985; Godley et al. 2001).

There were a total of 73 observed nests in this study. Three of these nests were laid by a female who had duplicate nests in the data-set. To avoid pseudo-replication the duplicate nests were removed for statistical analysis. Final statistical analysis was performed with 35 in situ nests and 34 relocated nests.

Incubation Duration

There was no significant difference in incubation duration between in situ nests (54.34 ± 0.61 days, n  =  29) and relocated nests (54.69 ± 0.57 days, n  =  33) (F  =  0.53, df  =  1, 59, p  =  0.4655), between in situ nests above the SHTL (54.00 ± 0.93 days, n  =  13) and relocated nests above the SHTL (54.69 ± 0.57 days, n  =  33) (F  =  0.0036, df  =  1, 43, p  =  0.9527), or between nests above the SHTL (54.50 ± 0.48 days, n  =  46) and nests below the SHTL (54.62 ± 0.82 days, n  =  16) (F  =  7.06, df  =  1, 59, p  =  0.2706).

Nest Temperature

There was no significant difference in average nest temperature between in situ nests (29.84 ± 0.11°C, n  =  35) and relocated nests (29.93 ± 0.11°C, n  =  34) (F  =  2.05, df  =  1, 66, p  =  0.1563), or between in situ nests above the SHTL (30.08 ± 0.17°C, n  =  14) and relocated nests above the SHTL (29.93 ± 0.11°C, n  =  34) (F  =  0.0006, df  =  1, 45, p  =  0.9804). There was a significant difference in average nest temperature between nests above the SHTL (29.98 ± 0.09°C, n  =  48) and nests below the SHTL (29.68 ± 0.14°C, n  =  21) (F  =  6.28, df  =  1, 66, p  =  0.0147).

Average Critical Period

There was no significant difference in average critical period temperature between in situ nests (29.96 ± 0.13°C, n  =  29) and relocated nests (30.06 ± 0.12°C, n  =  33) (F  =  2.83, df  =  1, 59, p  =  0.0976), or between in situ nests above the SHTL (30.23 ± 0.20°C, n  =  13) and relocated nests above the SHTL (30.06 ± 0.12°C, n  =  33) (F  =  0.0003, df  =  1, 43, p  =  0.9511). There was a significant difference in average critical period temperature between nests above the SHTL (30.11 ± 0.10°C, n  =  46) and nests below the SHTL (29.74 ± 0.17°C, n  =  16) (F  =  9.10, df  =  1, 59, p  =  0.0038). These temperatures would support a female-biased sex ratio being produced on this beach.

Hatch Success

There was a significant difference in hatch success between in situ nests (61.32% ± 5.03%, n  =  31) and relocated nests (81.21% ± 5.03%, n  =  34) (Z  =  1.99, p  =  0.0455), and between nests above the SHTL (78.60% ± 4.15%, n  =  49) and nests below the SHTL (53.65% ± 6.43%, n  =  20) (Z  =  −2.50, p  =  0.0123). There was no significant difference in hatch success between in situ nests above the SHTL (72.27% ± 6.25%, n  =  14) and relocated nests above the SHTL (81.21% ± 5.03%, n  =  34) (Z  =  −0.48, p  =  0.6258).

Hatchling SCL

There was no significant difference in hatchling SCL between in situ nests (44.91 ± 0.26 mm, n  =  24) and relocated nests (45.20 ± 0.23 mm, n  =  31) (F  =  0.77, df  =  1, 52, p  =  0.3824), between in situ nests above the SHTL (45.06 ± 0.41 mm, n  =  11) and relocated nests above the SHTL (45.20 ± 0.23 mm, n  =  31) (F  =  0.13, df  =  1, 39, p  =  0.7127), or between nests above the SHTL (45.16 ± 0.19 mm, n  =  42) and nests below the SHTL (44.78 ± 0.35 mm, n  =  13) (F  =  0.89, df  =  1, 52, p  =  0.3493).

Sister Data-Logger

There was no significant difference in average temperature between nests with sister data-loggers (29.23 ± 0.12°C, n  =  22) and the sister data-loggers (29.02 ± 0.12°C, n  =  22) during embryonic development (t  =  1.16, df  =  42, p  =  0.2514).

Incubation Duration

Incubation duration has been correlated with sex ratios produced in natural nests (Mrsovsky et al. 1999). Nests observed in this study showed no significant difference in incubation duration for in situ nests, relocated nests, or in nests below the SHTL. Nest temperature can be influenced by abiotic conditions before or after the critical period (Fig. 3), which could slow embryonic development and lengthen incubation duration. The significant decrease in average critical period temperature in nests below the SHTL would indicate that sex ratio can be influenced without a change in incubation duration. Therefore, incubation duration can not be used as a certain index of sex ratio in nesting areas with extreme abiotic conditions.

Nest Temperature

Nest temperature has been correlated with sex ratios and incubation duration produced in natural nests (Mrsovsky 1988; Mrsovsky et al. 1999). It has been suggested that nests relocation can cause changes in nest temperature, which would alter hatchling sex ratios (Foley 2000). The significant difference in average nest temperature in nests above and below the SHTL, and no significant difference in average nest temperature between in situ and relocated nests above the SHTL, indicates that the average nest temperature was influenced by nest location and not by nest treatment. Nests remaining below the SHTL were significantly cooler on average and would likely produce more males if they survived to hatching.

Critical Period Temperature

The pivotal temperature for loggerhead populations is approximately 29°C. Hatchling sex ratio is male biased when the average critical period temperature begins to drop to temperatures of 28.5°C. Hatchling sex ratio is female biased when the average critical period temperature begins to reach temperatures of 30°C (Mrsovsky 1988; Marcovaldi et al. 1997). The average critical period temperatures for in situ nests, relocated nests, and nests below the SHTL show a female-biased sex ratio (Fig. 1). This is consistent with many other studies that have found female-biased sex ratios in the northern and southern loggerhead populations of the United States: Hutchinson Island, Florida (2.5F:1.0M) (Wibbels et al. 1987a, 1991); Chesapeake Bay, Virginia (2.0F:1.0M); Indian River, Florida (1.4F:1.0M) (Wibbels et al. 1987b); Cumberland Island, Georgia (1.9F:1.0M) (Shoop et al. 1998); Virginia (2.1F:1.0M); North Carolina (1.9F:1.0M); South Carolina (2.1F:1.0M); Georgia (1.7F:1.0M); and Florida (1.9F:1.0) (National Marine Fisheries Service 2001).

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The fact that there was no significant difference in average critical period temperature between nest treatments indicates that nest relocation at this study site had no effect on hatchling sex ratio (Fig. 1). The average critical period temperature of nests laid below the SHTL was significantly lower than the average critical period temperature of nests that were laid above the SHTL (Fig. 1). The nest sites below the SHTL experience tidal wash over, which increases the moisture content of the sand, which decreases sand temperature (McGhee 1990; Foley 1998). Therefore the reduction in nest temperatures caused by leaving nests in areas of tidal influence could skew this population's sex ratio by producing more male hatchlings. However, since survivorship in nests left below the SHTL is greatly reduced, this effect would be minimal at best. Previous results support that a female biased sex ratio is being produced on this beach in most years (Drake 2000; LeBlanc 2004).

Hatch Success

It has been suggested that nest relocation has significant detrimental effects on hatch success (Foley 1998; Foley et al. 2000). These findings are not consistent with data from some long-term nest management projects. Nest relocation has been used to aid recovery efforts for sea turtles for decades. Kemp's Ridley nests in Mexico have been actively managed since 1978. Management of this population involves the relocation of almost 100% of all nests (USFWS and NMFS 1992, 2007) to a large protected corral. In many cases, nests are moved a distance of kilometers down the beach. It is known that in this type of relocation to a large nest corral, temperatures are altered and more female hatchlings are produced; this is an extreme management case with a critically endangered species (Wibbels 2007). Nonetheless, these efforts have had hatch success up to 79% and have increased the nesting population from 274 nesting females in 1985, to 4047 nesting females in 2006 (USFWS and NMFS 1992, 2007). Nest relocation has also been part of nest management in South Carolina since the late 1970s. Data from South Carolina sea turtle management projects show 2,780,677 hatchlings were produced from 1981–2001. It has been estimated that the amount of hatchlings produced during the same time period, with no nest management, would have been reduced to 235,340 (Hopkins-Murphy and Seithel 2005). The increase in the leatherback population of St Croix, US Virgin Islands, is directly attributable to an aggressive program of beach protection and nest relocation. This nesting population has increased from about 18 to 30 females during the 1980s to 186 in 2001. The hatchling production for this same population during this time has gone from about 2000 to over 49,000 (Dutton et al. 2005).

There was no significant difference in hatch success in nests above the SHTL, regardless of nest treatment (Fig. 2). Hatch success in nests below the SHTL had significantly lower hatch success than nests above the SHTL (Fig. 2). The significant difference in hatch success in nests above and below the SHTL and no significant difference in hatch success between in situ and relocated nests above the SHTL indicate that hatch success was influenced by nest location and not by nest treatment. Therefore, increasing the number of nests relocated to above the SHTL will not alter sex ratio of surviving hatchlings; it will simply increase the number of hatchlings that survive to emergence and potentially reach the water across the season.

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Hatchling SCL

It has been found that hatchlings develop more slowly in cooler nest temperatures, but hatchlings in cooler nest temperatures grow larger and can swim faster (Foley 1998). It has been suggested that the increase in nest temperature caused by nest relocation would decrease a hatchling's ability to swim faster and avoid predation, which would ultimately decrease a hatchling's fitness (Foley 1998). Although this study did not observe the swimming ability of hatchlings, it did observe hatchling size. Nests observed in this study showed no significant difference in hatchling SCL for in situ nests, relocated nests, or in nests below the SHTL. No significant differences in hatchling SCL and no significant differences in incubation duration between nests would indicate that hatchling growth rate was not affected by nest treatment; therefore, there is no reason to believe that hatchling mobility would be affected by nest treatment.

Sister Data-Logger

Average nest temperatures between nests with sister data-loggers and sister data-loggers were not significantly different during embryonic development (Fig. 3). The consistency between nest temperatures and temperatures in the nesting environment suggest embryonic development will fluctuate as environmental conditions fluctuate. Precipitation was the main environmental factor that influenced the nesting environment on this site (Fig. 3). Nest and beach temperature decreased with increased precipitation. Hydric conditions can have a profound effect on incubation duration and sex ratio produced depending on when the rain event occurs (LeBlanc 2004). If the rain event occurs during the critical period, this may alter individual nests; however, since all our nests were distributed over this period, no one treatment type received more or less precipitation. If environmental conditions are the driving factor for nest parameters during embryonic development, then nest treatment should not alter these parameters.

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Conclusion

It has been documented that nest relocation can significantly alter hatchling development by changing nest parameters, such as nest dimensions, moisture content, and temperature within nests (Foley 1998; Foley et al. 2000; Carthy et al. 2003). The results of this study do not provide any evidence that short-distance nest relocation altered any nest parameters or hatchling characteristics. Increased hatch success was achieved by alleviating nests of extreme abiotic conditions, not by nest treatment. On this study site, nest relocation was used to protect nests from tidal wash over, tidal inundation, and areas of extreme erosion. This management tool has achieved its purpose of increasing hatch success without altering any of the observed variables of nest site conditions or embryonic development.

This study shows that alleviating extreme abiotic conditions through nest relocation can be an effective management tool for increasing hatch success for sea turtle populations. This study site has proven to be a nesting area with consistent beach conditions above the SHTL, providing nesting areas that are favorable for embryonic development and hatch success. Tidal fluctuations of this region increase moisture content of nesting sites below the SHTL, which have proved to be unfavorable for embryonic development and hatch success.

If a nesting area has conditions that are highly variable, such as temperature, grain size, or sand color, then a cautious approach should be taken when relocating nests. Relocating nests to areas with conditions that are not similar to the original nesting conditions could alter embryonic development and hatch success. Beaches within the same geographical region as this study site should have similar nest site conditions, which would allow short-distance nest relocation to be utilized. To be certain that nest relocation can be used as an effective management technique for a particular nesting site, temperature trends and areas with extreme abiotic conditions must be located, and nest conditions should be closely monitored.