Study

Effectiveness of different conservation measures for loggerhead sea turtle (Caretta caretta) nests at Zakynthos Island, Greece

  • Published source details Kornaraki E., Matossian D.A., Mazaris A.D., Matsinos Y.G. & Margaritoulis D. (2006) Effectiveness of different conservation measures for loggerhead sea turtle (Caretta caretta) nests at Zakynthos Island, Greece. Biological Conservation, 130, 324-330.

Summary

For several decades the focus of sea turtle conservation efforts has been on the early life stages in the terrestrial habitat, due to the relative ease of monitoring and protecting eggs and hatchlings. Hatcheries, protective cages and head-starting programs are three of the main conservation techniques developed. This case reports the results of an investigation into hatching success of the loggerhead turtles Caretta caretta, in Laganas Bay for: natural nests; nests transplanted to a hatchery; and nests protected by cages.

Survey & translocations: Loggerhead turtle Caretta caretta nesting studies were undertaken from 1988 to 1995, along the 2,600 m long East Laganas beach on the island of Zakynthos in the Aegean Sea in the east Mediterranean. Loggerheads are the only species of turtle known to nest at this locality. The studies ran throughout the turtle nesting season in each year, with extensive nest searches performed by Archelon (Greek Society for the Protection of Sea Turtles) personnel from May to September.

Nests that were situated within 7 m of the sea, and/or close to specific plant species with root systems known to grow into nests (exc tamarisk Tamarix sp., reed Phragmites communis and pistachio Pistacia lentiscus) were considered to be at threat and were therefore carefully dug up and translocated to a beach hatchery (n = about 75 nests). All nests not considered to be at threat were left (n = around 330).

Hatchery creation: In 1988, a hatchery was established at the upper part of East Lagunas beach located 25 m a.s.l. (to protect from inundation by high tides) in areas where laying frequently occurred. The hatchery (7 m in length by 2.5 in width) was contained within a fenced enclosure and within it, an area of 0.8 mĀ² was available for each nest. To approximate natural conditions, the dimensions of each nest cavity were measured during each excavation and was duplicated in the artificially dug nests. Special care was taken to minimize disturbance of the eggs during translocation by collecting and transferring eggs within 12 hours of deposition, avoiding rotating the eggs when dug up and held, and placing the eggs in cylindrical styrofoam boxes prior to transfer to the hatchery area. The eggs were placed into the artificial nest chamber in an order approximating their natural distribution within the egg chamber.

Natural nest protection: From 1990, natural nests that were subject to increased disturbance from tourists and bathers were protected from trampling by metal cages (n = 90). The cages had a diameter of 50 cm at their base and were surrounded by a metal mesh to protect them from terrestrial predators. Each cage was placed over nests considered in danger from trampling, with the metal mesh buried to about 15 cm in the sand.

Hatching success rate: The contents of all nests were examined eight to 10 days after the last hatchling had emerged. Counts of the numbers of empty egg shells (hatched), unhatched eggs and dead hatchlings were made. The total number of eggs per clutch were calculated by counting the unhatched eggs and assembling the shells produced by hatched eggs. The number of live hatchlings was estimated by subtracting the number of dead hatchlings found from the count of assembled egg shells. Hatching success was then calculated as the proportion of the total number of live hatchlings produced from the total number of eggs recorded for natural nests.

Hatching succes: The hatching success of hatchery (65.8%)and caged nests (66.3%) was significantly higher than natural nests (62.8%). Furthermore the success of natural nests was over-estimated as nests where no hatchlings emerged were not included. Significant differences in hatching success occurred between years within each of the three nest types (hatchery, protected and natural nests, see figure 1 attached). However no parallel fluctuations were found on an annual basis between methods, even when extreme weather events occurred (e.g. extensive rain in 1992 and 1995). In hatchery nests, under control conditions, yearly success also differed significantly. Results thus suggest that micro-environmental conditions may have been the key factors affecting the annual variation of hatching success within each nest type rather than extreme weather events.

Conclusions: This study indicates that the creation of hatcheries and the placement of protection cages can be a valuable turtle conservation strategy, as it significantly increased hatching success. However the conservation value of hatcheries has been questioned as it may bias the sex ratio of the hatchlings (as incubation temperature in the nest determines the sex of the offspring). Although the effect of hatcheries on hatchling sex ratios was not studied, the close duplication and control of the environmental conditions of natural nests could significantly reduce any effect on natural sex ratios. In order to ensure the intervention of relocating nests is beneficial, only nesting sites suffering from severe nest losses due to human or natural causes should be moved.


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