Leatherback Turtles, Dermochelys coriacea, Nesting in French Guiana, 1978-1995

Marc Girondot1 and Jacques Fretey2

1 Unité de Recherche Associée "Évolution et Adaptation des Systèmes Ostéo-Musculaires", Centre National de la Recherche Scientifique et Université Paris 7, 2 place Jussieu, 75251 Paris cedex 05, France
[Fax: 01 44 27 56 53; E-mail: mgi@ccr.jussieu.fr];
2 World Wildlife Fund-France, 151 Boulevard de la Reine, 78000 Versailles, France

ABSTRACT.- Ya:lima:po beach in French Guiana exhibits the highest concentration of leatherback (Dermochelys coriacea) nesting in the world. Scientists from more than 25 countries have worked within the Kawana marine turtle project for 16 years. Numbering and tagging of leatherbacks has been the main activity of this project. Number of nests per year has been estimated for 13 nesting seasons between 1978 and 1995. More than 50,000 nestings were recorded annualy in 1988 and 1992, but only 10 to 15 thousand annually in 1978-1986, 1993, and 1995, with intermediate numbers of 20 to 30 thousand annually in 1987, 1989, 1991, and 1994. A general increase in the number of nests is observed in the last few years as compared to the first years, but the actual trend is unclear. Analysis of the database of tagged leatherbacks (over 31,000 data points) elucidates behavior of this species within and between nesting seasons.

KEY WORDS: - Reptilia; Testudines; Dermochelyidae; Dermochelys coriacea; sea turtle; nesting; status; population; conservation; migration; French Guiana.

Four species of marine turtles nest frequently in French Guiana : Dermochelys coriacea, Chelonia mydas, Lepidochelys olivacea, and Eretmochelys imbricata. For every 1000 nests of D. coriacea, approximately 100 nests of C. mydas, 10 of L. olivacea, and 1 of E. imbricata are seen (these values are only approximations and should not be used for further calculations). Exceptionally, females of Caretta caretta are also observed but at a rate of less than one per year. Turtles nest on beaches located along the entire 400 km coast line of French Guiana. However, many of these beaches appear and disappear at an approximate cycle of every 15 years due to the displacement of mud banks from the Amazon River in Brazil. The only stable beach in French Guiana is located on the estuary of the Mana and Maroni Rivers on the border to Surinam, between two Amerindian villages, Awa:la and Ya:lima:po. Approximately 90 to 95% of all the leatherbacks nesting in French Guiana are seen there.

The nesting seasons of the various marine turtles in French Guiana extend from early March to mid-August for D. coriacea, March to June for C. mydas, and April to August for L. olivacea. Nesting seasons for E. imbricata and C. caretta cannot be defined due to the too small numbers of these species. For D. coriacea a second nesting season small numbers of females per night has also been observed in December and January. The same observation has been made in Surinam (H. Reichart, pers. comm.).

The time of arrival for nesting on the beach by leatherbacks is dependent on the localization of the beach and the tide level. Turtles nest during the entire night on beaches outside the Mana-Maroni estuary but mostly around high tide for beaches within the estuary. However, time of nesting is also sensitive to the exact localization within the estuary and the tide level (Fretey and Girondot, 1989a).

Near Ya:lima:po village, a center (Kawana project) established 16 years ago for the study of marine turtles has welcomed scientists and persons interested in the protection and the study of marine turtles (Fretey, 1996). There, from 1984 to 1993, a turtle hatchery has permitted the study of artificial incubation of leatherback eggs (Lescure et al., 1985; Girondot et al., 1990) and the influence of temperature on sex determination (Rimblot et al., 1985; Rimblot-Baly et al., 1986; Desvages et al., 1993). Since 1993, the artificial incubation of eggs has been discontinued for ethical reasons (Mrosovsky and Godfrey, 1995; Girondot and Pieau, 1996; Lovich, 1996). Data obtained on D. coriacea during the past 18 years (1978-1995) will be reviewed in this paper.

Materials and Methods

Numbering.- Turtles are numbered by nightly beach patrols from mid-April to late September. Outside this period, two local inhabitants count turtle tracks in the morning. In D. coriacea, a very large proportion of females landing also nest and the number of turtle tracks is therefore a good estimate of the number of nests. However, this method is not suitable from May to July because too many turtle tracks are present on the beach and only visual counts of turtles seen during the night are used.

Since 1988, a strategy has been developped to allow a good estimate of the total number of nests without counting turtles each night. Data from the 1987 season were used as a reference because the number of females was known exactly for all nights of that season. The percentage of error between the actual number of nests and its estimate using interpolation was calculated. The error was minimized if counts were performed every 7 days with 5 days without count between them (Fig. 1) and therefore this strategy has been used since 1988.


Tagging and Identification.- From 1978 to 1984, plastic tags were used, but females lost them very rapidly. From 1985, females were tagged using one titanium tag on the front left flipper (1985-86), one titanium tag on the rear left flipper (1987-88) or one monel tag on the rear left flipper (1990-93). In 1994, two monel tags were put on each rear flipper and in 1995, PIT (passive integrated transponder) tags were put in the right shoulder of 250 females.


Figure 1. Percentage of error between the actual total number of nests seen in 1987 and its estimate using interpolation, based on different numbers of days between surveys.

RESULTS AND DISCUSSION

Morphometrics.- In 1987 and 1988, 1328 female leatherbacks were measured and 15 weighed. They had a mean weight of 339.3 kg, SD 41.3 (range 250-415 kg); a mean straight length of 154.6 cm, SD 8.98 (range 127-252 cm); and a mean straight width of 87.3 cm, SD 6.21 (range 67-109 cm).

Numbering.- The actual number of nests from 1978 to 1995 is shown in Fig. 2 (the estimates for 1978, 1982 and 1983 were performed with data of less than 2 months and only half the beach patrolled). More than 50,000 nestings were recorded annually in 1988 and 1992, but only 10 to 15 thousand annually in 1978-1986, 1993, and 1995, with intermediate numbers of 20 to 30 thousand annually in 1987, 1989, 1991, and 1994. A global increase in nesting has been observed since earliest reports for French Guiana (Pritchard, 1969; Schulz, 1971; Pritchard, 1973).


Figure 2. Number of leatherback nests per year on the Ya:lima:po- Awa:la beach, French Guiana.
Moreover, old people of the villages say that leatherbacks did not nest on this beach during the 1950s. From year to year, a large variation of the number of nests is observed and it is not possible to determine a trend for recent years.

According to data from Surinam, concordance in the number of nests per year is observed for the two populations (data from Surinam are from Weijerman et al., 1996). The correlation between the annual number of nests in French Guiana and Gandoca Beach, Costa Rica (Chacón et al., 1995) is high but not significant (test for the data for 1991-92 and 1994-95, r=-0.787, p=0.28). However, it is interesting to note that the year with the highest number of nests in French Guiana (1992) corresponds to the lowest number in Costa Rica and inversely, the year with the lowest number of nests in French Guiana (1995) corresponds to the year with the highest number of nests in Costa Rica. Data for more years are needed to confirm this pattern.

Tagging.- Our database of tagged turtles in French Guiana contains 31,557 data points extending over 9 years from 1987 to 1995. Analysis of this database permits us elucidate some aspects of behavior within and between nesting seasons for D. coriacea in French Guiana, as outlined below.

Behavior Within Nesting Season.- For the 1988 season, the mean number of nests per female was estimated as 7.52 (Fretey and Girondot, 1989b). The estimate for 1987 was lower (1.48; Fretey and Girondot, 1988) but the methodology was also different and the value is also dependent on the tagging effort on the beach. Comparison of the two values for mean number of nests per female is impossible due to the lack of confidence intervals for the 1987 estimate. The 1988 estimate is more accurate but is probably subject to variation from year to year. The mean number of days between two nests varies from 6 to 15 with a mode of 9 to 10 (Fretey and Girondot, 1988. This observation is typical for D. coriacea (National Research Council, 1990). However, peaks of nesting are seen every 15 days during spring tides (full and new moon) on the Ya:lima:po beach (Fig. 3). These two observations appear to be contradictory.

We have analyzed whether females adjust their nesting day as a function of the tide (and by consequence, the moon phase). Using the 1988 tagging data, we analyzed all tagged females that nested between 14 to 11 (Group A) and 9 to 6 (Group B) dyas prior to a full or new moon. The distributions of the internesting intervals were then established for these two groups. Group A (-14 to -11 days) had a longer mean internesting interval of 10.01 days, Group B (-9 to -6 days) had a shorter interval of 9.76 days, with the difference significant (G2=42.6, p<10-4) (Fig. 4). Therefore, females adjust their internesting return date to be closer to a full or new moon and this behavior produces the observed peaks of nesting for spring tides.



Figure 3. Number of leatherback nests during the 1987 nesting season (normal line) and moving average (15 days around each point) for the same data (bold line). Peaks of leatherback nesting are observed during spring tides (full and new moon; dashed lines).

We have very few data concerning shifting of nesting beach. Only two records of females tagged in French Guiana and recovered in Surinam are available (Matthew Godfrey, pers. comm.). One (G36490) nested and was tagged at Ya:lima:po beach in 1993 and was then seen to nest on Matapica beach, Surinam, in 1995. Another (G36866) was tagged and nested twice at Ya:lima:po beach in 1991 was seen to nest on Matapica beach, Surinam, on 5 May 1995, and then nested again two times at Ya:lima:po beach on 27 May and 24 June 1995. The Ya:lima:po and Matapica beaches are 150 km apart from each other. Apparently, a shift of nesting beach within the Guianas region is a possible but not frequent event.

Behavior Between Nesting Seasons.- From year to year, the number of turtles tagged in French Guiana and then recovered elsewhere within the Atlantic Ocean is growing (Table 1).

Females are caught both on west and east Atlantic coasts. For example, the female G41420/G45918 was captured in Spain only 5 months after its last nesting in French Guiana. The females G36302 and G43041/G43042 were also captured only 5 months after their last nesting in French Guiana, but this time on west Atlantic coasts in USA. One year after nesting in French Guiana, females have been seen on east Atlantic coasts (France, Spain, and Morocco) or on west Atlantic coasts (Florida, Georgia and South Carolina).

Female G34567/QQM631/G41593 was remarkable. It was first tagged in French Guiana in 1990, then seen alive in a fishing net and re-tagged in Florida in 1991, and then re-observed nesting twice in French Guiana in 1992 and 1994.


Figure 4. Distributions of the number of days between two successive nesting processes for turtles having nested (Group A) days -14 to -11 before spring tide (full and new moon) and (Group B) days -9 to -6 before spring tide, utilizing data from 1988.


Table 1. List of leatherbacks tagged in French Guiana and recovered elsewhere within the Atlantic.

Tag Number Nesting Dates in French Guiana Date and Location of Capture Number of Months


West Atlantic Captures
G20024/
G31511
1985
14 May 1988
17 Feb 1989
Georgia, USA
9
G35151 29 Jun 1991 8 May 1992
South Carolina, USA
10
G36302 21 Jul 1993
1 Aug 1993
16 Jan 1994
Florida, USA
5
G38947 21 Jun 1990 12 Mar 1993
Florida, USA
32
G42743 21 May 1991 12 Apr 1992
Georgia, USA
10
G43041/
G43042
8 Jun 1994 7 Nov 1994
Georgia, USA
5
G23339 1987 Feb 1989
Georgia, USA
19
G25376 1988 Sep 1988
Newfoundland, Canada
4
G34567/
G41593/
QQM631
29 May 1990
7 Jun 1990
24 Jul 1992
4 Jun 1994
23 Jun 1994
3 Jul 1994
30 Jun 1991
Florida, USA
11
East Atlantic Captures
G41420/
G45918
26 Jun 1994 29 Oct 1994
Spain
5
G45041/
G46738
26 Jun 1994 30 May 1995
Morocco
11
G46279 21 Jul 1992
5 Aug 1992
17 Jun 1994
27 Jun 1994
20 May 1995
France
10

Table 2. Distribution of the number of years between tagging and return to nest in French Guiana according to the year of tagging. The method of tagging and the tag models are described in the text.

Num-
ber of newly tagged fema-
les
Number of tagged females captured per year
8889909192939495
T
a
g
g
i
n
g

y
e
a
r
871322303201000
885417131451242
895000000
9013129119514815
9115361511810930
9226623911941
93127941184
94230320
95342

According to these few data (0.03% of the turtles tagged in French Guiana have been observed outside the region), it appears that females migrate to the north from French Guiana after the nesting season and do not follow any particular route within the north Atlantic. But the data are still relatively scarce and many more are needed to truly understand the pelagic habits of females (Fretey and Girondot, 1996).

Remigration Interval Between Nesting Seasons.- The remigration intervals for tagged females returning to nest have been studied for several years (Table 2). It appears that the distribution of the number of years recorded between two nesting seasons is highly sensitive to the position of tagging and the kind of tags used (compare years 1987-88 and years after 1990 for different types of tagging). From 1985-86, only 4 females out of more than 1000 were seen 3 years after nesting in French Guiana (Fretey and Girondot, 1990) reflecting a high incidence of tag loss.

The remigration distributions are very different for the years 1990, 1991, and 1992 ([chi]2=130.6, 6 DF, p<0.0001; for this test, turtles seen after 4 and 5 years for 1990 and 1991 are grouped with turtles not seen in order to be consistent with the 1992 data). Thus, nesting intervals are not cyclic because it varies for turtles seen a given year and they are not typical of one population because the distributions varies from year to year (see discussion in Frazer, 1989, who proposed that nesting intervals are typical of one population).

Several females have been seen during more than two nesting seasons with 88 seen during 3 seasons and 2 during 4 seasons. For these turtles, the number of years between the two first nesting seasons and between the las two is significantly correlated (figure 5, r=-0.216, p=0.042). The number of years between two nesting seasons is inversely related to the number of years between the previous ones. This could be a compensatory strategy, suggesting that reproductive output should be studied at the level of several nesting seasons. However, the correlation is very slight and needs to be confirmed with further data.


Figure 5. Relationship between two successive nesting intervals for turtles seen at least 3 times.


Acknowledgments - We are grateful to the more than hundred people having participated to the Kawana project in French Guiana from all over the world. This project was also made possible by the organizations and societies having financed this project (WWF, Greenpeace, French Minister of Environment). The inhabitants of Awa:la and Ya:lima:po in French Guiana have made this research possible by receiving us in their villages. We especially thank Daniel William, Jeanne and Félix Tiouka, Michel Thérèse, Paul Henri. We thank also Amy Woodhead and Matthew Godfrey who have transmitted us informations about some tagged females from French Guiana. This paper has greatly benifitted from the corrections and enhancements proposed by the editors of Chelonian Conservation and Biology.

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Received: 8 April 1996. Accepted: 6 August 1996