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Marine Turtle Newsletter 53:17-18, © 1991
Dalia Silbersteinand Razi Dmi'el1
Department of Zoology, G. S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978 Israel
[1 to whom correspondence should be directed].
Among sea turtle species, the loggerhead (Caretta caretta) is the most commonly found on the shores of Israel. The main nesting grounds for the Mediterranean population of this species are located along the shores of Greece, Cyprus and Turkey (I.C.S.E.M. Report 1986). Until quite recently, hundreds of nests were found each year along the Mediterranean coast of Israel. In the 1950's, some 200 nests with a density of about 15 nests per km were recorded on 15 km of typical beach in the northern region of Israel (Sella 1982). The picture has changed, however, during the last few decades. The latest reports by the Israeli Nature Reserves Authority (Soffer 1988, Kuler 1990), and the results of our own research, show that the abundance and density of nesting by this species has abruptly decreased. Along some 55 km of coastline, which includes the surveyed area mentioned in Sella's report, only 10 nests were found in 1984, 14 in 1985, 16 in 1986, 16 in 1987, and 11 and 13 nests in the years 1988 and 1989, respectively. Reports conveyed to us as personal communications revealed a similar situation on other Mediterranean beaches in Israel.
In the framework of attempts to recover the loggerhead sea turtle in Israel, we investigated several aspects of its reproduction, with an emphasis on the physical conditions prevailing in the nests. During the 1986-1989 nesting seasons (mid-May through July), we surveyed repeatedly, in collaboration with the rangers of the Nature Reserves Authority, 35 km of beach along the northern Mediterranean coast of Israel. A total of 34 loggerhead nests were found in this area (16 nests were found in 1986, 10 in 1987, 4 in 1988, 4 in 1989). The earliest clutch was laid on 21 May, the latest on 27 July. Most of the nests were located <10-50 m beyond the high-tide line. Only a few were found further, in the dunes, some 80-150 m beyond the high-tide line. The average clutch size was 82 eggs (range 55-149). The average period of incubation was 54 days, ranging from 60-62 days in May (temperature in the center of the nest = 26.5°C) to 50-52 days in late July (nest temperature = 30°C).
During 1986-1989, hatching success varied considerably among nests, ranging from 0%-90%. Hatch failure was attributed to the depth of the nest and to its location with respect to the water line. Typically, nests were dug by the female to a depth of 30 cm (upper egg layer) to 50 cm (lower egg layer) below the sand surface. At this depth, the sand humidity (average water content 3%) and the daily temperature fluctuation (2°C) were stable. Four nests, however, were constructed so that the uppermost eggs were only 10-15 cm below the sand surface. In these cases, strong temperature fluctuations and sand desiccation resulted in an average hatching success of only 6%. Eleven nests, located too close to the high-tide line, were exposed to summer storms that occasionally occur in this area. Seawater inundation resulted in an almost complete cessation of embryo development, resulting in 0-11% hatching success (Dmi'el, unpubl. data).
A sharp decline in the number and density of loggerhead sea turtle nests in Israel is attributable to both regional and local processes. On the one hand, an immense fishing of turtles took place in the eastern basin of the Mediterranean Sea during the first half of this century. Today the intensity of this activity has diminished, but the consequences for the loggerhead population are still very conspicuous. Concomitantly, a local large-scale excavation of beach sand for concrete production took place in Israel. This altered the structure of many beaches in which female loggerheads dug their nests (Sella 1982). The present situation demands that immediate protective measures be taken. The first emergency step should be to rescue eggs laid in poorly located nests, as is done for other sea turtle species (e.g., Eckert and Eckert 1990). These eggs should be transplanted over the shortest practicable distance and reburied under favorable natural conditions (at a beach hatchery site if necessary). As a last resort, and only if beach reburial is not possible, eggs may be artificially incubated in the laboratory. This procedure has been shown to result in high (90%) hatching success (Silberstein 1988). In either case, hatchlings should be released immediately to the sea from the original oviposition site. Other measures should include the protection of nesting and foraging grounds, technology to reduce incidental catch, and a concerted effort at public awareness and education.
Acknowledgement. This research was supported by grant 4/86 to Razi Dmi'el from the Basic Research Foundation, The Israel Academy of Sciences and Humanities.
Eckert, K. L. and S. A. Eckert. 1990. Embryo mortality and hatch success in in situ and translocated leatherback sea turtle (Dermochelys coriacea) eggs. Biol. Cons. 53:37-46.
Kuler, Z. 1990. Summary of the 1989 annual sea turtle nesting survey. Nature Reserves Authority, Central Region, Israel. 10 pp. (in Hebrew).
I.C.S.E.M. 1986. Proc. Working Group on Mediterranean Marine Turtles, Palma de Majorca, October 1986. Intl. Council for the Scientific Exploration of the Mediterranean, Monaco.
Sella, I. 1982. Sea turtles in the eastern Mediterranean and northern Red Sea, p. 417-423. In: Biology and Conservation of Sea Turtles (K. A. Bjorndal, ed.). Smithsonian Institution Press, Washington D.C.
Silberstein, D. 1988. The physical conditions prevailing in nests of the loggerhead sea turtle (Caretta caretta) and their effect on egg development. Master of Science thesis, Dept. of Zool., Tel Aviv University. 71 pp. (in Hebrew with English summary).
Soffer, A. 1988. Sea turtles nesting in Haifa - Caesarea beach. Nature Reserves Authority, Jerusalem, Israel. 31 pp. (in Hebrew).