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Grande Riviere beach is <1000 m long, slopes steeply as it enters the sea and is backed by a rural fishing village situated on the north coast of Trinidad. The shoreline is delimited by a variety of trees including mangrove forest. Grande Riviere beach is a high-density nesting site used by thousands of leatherback females each year (Chu Cheong 1990). This high density results in the accidental excavation of nests laid earlier in the season by other females nesting later, which in turn leads to predation and egg loss (Lee Lum 2005). It has been estimated that feral dogs consume upwards of 40% of all eggs each year (Forestry Division 2010). Adult leatherbacks are subject to mortality due to gillnet entanglement mainly on the northern and eastern coasts, with as many as 3,000 annual entanglements occurring in Trinidad and of which approximately 35% results in turtle mortality (Eckert & Lien 1999; Lee Lum 2003). Low hatching success on Grande Riviere likely compounds the population-pressure problem of what appears to be unsustainable adult mortality caused by turtle interactions with fishing gear operations (Spotila et al. 2000).

There is a dire need to protect eggs deposited early in the season to help offset egg loss caused both by females nesting later in the season and also seasonal beach erosion (pers. comm. L. Peters). Relocating freshly-laid eggs to protected structures (often called hatcheries) has been used with varying degrees of success in different parts of the world (Mortimer 1999). Egg clutches have also been incubated in Styrofoam™ boxes, which support high rates of hatching success but require careful manipulation of temperature and moisture conditions (Mortimer 1999). At Grande Riviere, it is difficult to establish an in situ hatchery above the high tide mark, where leatherbacks usually lay their eggs, because the beach is narrow and suffers unpredictable erosion throughout the turtle nesting season.

The objective of the present study was to investigate the suitability of using sand-filled containers constructed with wooden lathes to incubate leatherback eggs. The wooden lathes in each of the three containers were separated either by 4 mm, 5 mm or 6 mm on the sides and bottom, as a means of facilitating exchange of gas and moisture. We monitored temperature and moisture in the containers from October 2012 to February 2013 and compared them to temperature and moisture data collected at a control area. We did not incubate leatherback eggs in these containers; rather we simply monitored incubation conditions, as a first step in evaluating the suitability of this ex-situ incubation technique. Based on recommendations from members of the Grande Riviere Nature Tour Guide Association (GRNTGA), we placed the boxes in a hatchery site at a slight elevation along the beach front, protected from tidal inundation and potential trespassers and where sunlight was not impeded.

Figure 1. Hatchery enclosure showing experimental containers and rain gauge.

The hatchery site was a 6.8 m x 5.4 m area, enclosed by a 38.4 cm high plastic chain link fence attached with tying wire to wooden posts and with a small padlocked entrance to limit access by nonproject personnel (Fig. 1). The hatchery housed three containers that were 90 cm high x 50 cm wide x 90 cm long. Container 1 had 4 mm spaces between the lathes, Container 2 had 5 mm spaces between the lathes and Container 3 had 6 mm spaces between the lathes. Each container was lined with fine mosquito netting, in order to hold the sand (Fig. 2). The spacing between lathes allowed airflow into the containers since gas exchange is imperative to the development of sea turtle embryos (Koch et al. 2007). The spacing also allowed water from precipitation to flow through the boxes. The dimensions of the wooden containers were adopted from the leatherback hatchery nest design at Pacuare Sea Turtle Rescue and Rehabilitation Center, Costa Rica. There, nests are incubated in a 50 cm x 50 cm area, within a total area of 150 cm x 150 cm and with one clutch centered in the 3 x 3 matrix. This arrangement ensures that the incubating environment facilitates gas exchange and metabolic activities within nests that would otherwise be compromised if the nests were too closely spaced (pers. comm. D. Chacon).

Figure 2. Experimental incubation containers lined with fine mosquito netting.

Sand was transported to the hatchery site from the area of the beach above the high tide line where most leatherback nests are laid, hand-sieved and placed in the containers; sand grain size was approximately 0.4 mm. A “boot-shaped” nest chamber similar to those created by female leatherbacks was dug within each container to a depth of 70 cm, which is the average depth of a leatherback clutch (Chacon & Machado 2005); a similar chamber was created at a control site on the nesting beach. Temperature data were collected over an approximate six month period from October 2012 to April 2013. Temperature data loggers (UA -001-64 HOBO, Onset Computers, USA) were inserted at the center of the “boot-shaped” chamber in each sand-filled wooden container in order to achieve uniform measurements and the loggers were set to take hourly temperature readings. The cavities were then refilled with sand. Temperature data loggers were inserted at the control site to record hourly air temperature; these were attached to a rain gauge inside the hatchery site. The original temperature data logger at the control site was lost from severe wave action and beach erosion, however, it was replaced at another site and readings were recorded for approximately 22 days from March to April 2013 (Fig. 3). Daily rainfall data were collected during the same period as the temperature data, using an automatic rain gauge (MKIII CC-LR, RainWise, USA) located inside the hatchery enclosure.

Figure 3. Mean monthly temperatures (bars show SD) of experimental incubation containers.

For analytical purposes, temperatures in the wooden containers in the hatchery were compared to the thermal tolerance range for sea turtle embryos estimated between 25-35°C (Ackerman 1997, cited in Tapilatu & Tiwari 2007). During the investigative period, air temperatures inside the hatchery enclosure fluctuated between 23.9-29.8°C, the modal temperature was 26°C and the average temperature was 26.1°C. The temperature ranges for the containers 1, 2 and 3 were 24.7-29.9°C, 24.3-32.1°C and 24.7-29.9°C, respectively. A simple ANOVA using SPSS (v. 14) revealed a significant difference among the three containers over the four month period (df = 2, F = 13.298 and P < 0.001). For the duration of the experiment, no significant difference was observed among the mean temperatures for containers 1 and 3. When mean temperature for the three experimental containers for all months were compared, container 2 was higher (Fig. 4). The reasons for this are largely undetermined but may be attributed to the position of container 2 in the hatchery, in relation to containers 1 and 3. At the control site, data collected from 20 March to 09 April 2013 showed a mean temperature of 27.8°C and a temperature range of 26.5-28.8°C, which closely reflect tolerance ranges for leatherback embryos. However, data collection time frames at the control site and for both hatchery enclosure and containers did not coincide, making temperature comparisons difficult.

Figure 4. Mean daily temperature at control site. Dashed line is average temperature for study period.

The hatchery data for the rain gauge could not be retrieved because the internal battery died when the rain gauge was collected and the data were corrupted. No correlations between rainfall and temperature could therefore be established. It was difficult to determine if the hydrous environment within the boxes was optimal, but it was probably maintained by rainfall during the investigation period, which was more pronounced from December 2012 to January 2013.

The incubation of leatherback eggs in experimental wooden containers is an untested idea. Ackerman (1997) (cited in Tapilatu & Tiwari 2007) stated that the tolerance ranges for leatherback embryos were approximately 25-35°C. In this experiment, most of the temperature ranges for all three wooden containers fell within the optimal range/tolerance temperature ranges for leatherback egg incubation, however, the lower limit temperatures within all three containers were <25°C. In the absence of daily moisture readings, it is difficult to ascertain whether the sand-filled boxes provided the optimum moisture content for developing leatherback embryos. Whether or not the eggs buried in these boxes would receive sufficient oxygen for survival cannot be determined at this time. This study was aimed at determining the suitability of using wooden containers for leatherback egg incubation, but not whether this method would affect sex-ratios or hatchlings; these factors will be studied in future.

In the next stage of this investigative study, five sand-filled experimental wooden containers (with 6 mm spacing) will be used for egg incubation, given that the temperature range for this container was comparable to the tolerance ranges/optimal ranges for leatherback egg incubation. Temperature data loggers will be used to capture hourly temperature changes in the five wooden containers and also in five control nests to determine hatching success rate in both. A rain gauge will also be installed inside the hatchery enclosure to capture rainfall data. This experiment is expected to be conducted during the 2013 leatherback nesting season.

Acknowledgments. Support for the GRNTGA is provided by the Wider Caribbean Sea Turtle network (WIDECAST). WIDECAST also provides assistance to the GRNTGA in data collection initiatives and monitoring of turtle nesting activities throughout the nesting season. Approval to handle leatherback eggs at Grande Riviere for research purposes was granted by the Forestry Division of the Ministry of the Environment and Water Resources of the Government of Trinidad and Tobago. The project was participatory with the community and GRNTGA as beneficiaries.

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