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Marine Turtle Newsletter 29:2-5, © 1984

Monitoring Green Turtles at Tromelin and Europa (Indian Ocean) 1970-1984

J.-Y. Le Gall, A. Lebeau and J. Kopp
Institut Scientifique et Technique des Pêches Maritimes, B.P. 60 - 97420 Le Port - Ile de la Réunion

An evaluation of research conducted over the last 15 years on populations of green turtles nesting on Tromelin and Europa islands (S.W. Indian Ocean) has been completed. The first observations by Hughes (November 1970, published 1974) were followed up by year-round work in 1973-74 on Europa by Servan (1977) and by Batori (1974) for Tromelin. Systematic surveys during the nesting season have been conducted since 1978-1979 on Europa and since 1981-1982 on Tromelin (Table 1). Each season a 2 to 4 month (November to February) project is set up on both islands in order to assess the number and size of the adult females and the production of hatchlings. This long-term research programme is partially motivated by the development of the turtle ranch on La Reunion. Work by scientists at the nesting beaches includes census of tracks, tagging and intra-season recapture analysis, measurement of females, counting of clutch size and estimating survival rates of hatchlings emerging by day and total survival rates.

Daily counts were made early each morning of tracks of turtles returning to the sea; these are summed over 10 day periods for Tromelin (all beaches) and the Meteorological Station Beach as a long-term reference site for Europa (Table 2). These data demonstrate the inter-annual variation of nesting populations but show no very obvious trends. It is to be noted that the number of tracks recorded on Station Beach, Europa (roughly 50 tracks a night) for the last season (1983-1984), is close to the level observed by Hughes in November/December 1970. A preliminary estimate of the total number of adult females nesting on Europa, derived from data for the last four months of the 1983/84 nesting season, is roughly 8000 females. This is an estimate for the number of individuals nesting in a single year, for the whole island of Europa.

For Tromelin island similar records of tracks over 10 or 30 day periods demonstrate inter-annual fluctuations without clear trends. The numbers of tracks for the 1983/84 season (ca. 80 per night) are about the same as those seen by Batori (1974). Population size estimates for 1982/83 and 1983/84 nesting seasons (October to February) are approximately 1000 nesting females each year.

The number of females tagged (Monel tags) by season and inter-annual recaptures by site (Tables 3 and 4) can be summarized as follows:

Recapture rates of 2.1 to 3.6% are in the same order of magnitude as those in other intensive tagging studies. The distribution of time-intervals between two recaptures demonstrate a bimodal distribution (4 and 8 years). As quoted by Hughes (1982), for Caribbean green turtles, females return for nesting after two, three (more frequently) and some in four years, or even more.

This monitoring programme (along with extensive data analysis) is to be continued at this level for 1984-1985 nesting season. A programme involving less frequent but carefully selected sampling times will be set up over the following five year period; more intensive studies are anticipated at times associated with the collection of hatchlings for the turtle ranch on La Réunion.

Table 1: Dates of scientific studies, biologists present, and number of tags applied.

Table 2: Numbers of tracks at Tromelin and Europa Islands.

Table 3: Numbers of turtles tagged and inter-annual recaptures on Tromelin.

Table 4: Numbers of turtles tagged and inter-annual recaptures on Europa.

Batori. G. 1974. Rapport d'activité. Ile Tromelin. note ronégotypée, 15 pages. Service Meteorologique de la Réunion.

Hughes, G.R. 1974. The sea turtles of South East Africa (I,II). Oceanographic Research Institute, Durban, Investigational Reports 35 and 36

Hughes, G.R. 1982. Nesting cycles in sea turtles. In: Biology and Conservation of Sea Turtles. Bjorndal, K., (ed.), Smithsonian Institution Press, Washington, D.C., pp. 397-404.

Servan, J. 1977. Ecologie de la tortue verte à l'Ile Europa. Canal du Mozambique--La Terre et la Vie, 16: 421-464.