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Marine Turtle Newsletter 73:5-7, © 1996

Olive Ridley Turtles (Lepidochelys Olivacea) Show Signs of Recovery at La Escobilla, Oaxaca

Rene Marquez-M.,¹ Cuauhtemoc Peñaflores² and Javier Vasconcelos^2
1INP-CRIP-Manzanillo, Playa Ventanas (AP 591), Manzanillo, Colima CP 28200, México
²INP-Centro Mexicano de la Tortuga-Mazunte, AP 16, Puerto Angel, Oaxaca CP 70902, México

The number of sea turtle nests laid per year can be interpreted as an indication of population size. The number of nests laid at a rookery fluctuates from year to year, but in general a persistent decline is taken as a sign that the adult population is depleted, frequently due to over-exploitation. Conversely, a steady increase in the number of nests laid can be interpreted to mean that the adult (breeding age) population is increasing. At La Escobilla beach in Oaxaca, México, we have seen many changes both in the size of the arribazones (the number of turtles arriving en masse to nest, sometimes referred to as an arribada) and the number of nests laid per annum since intensive monitoring began in 1973 (Figure 1). A general decline in nesting was observed until 1988. In 1975, 295,000 nests were laid during six arribazones, whereas 55,730 nests were laid in four arribazones in 1988. However, from this low point we have seen a steady rise in numbers. After 1990, when a total ban on hunting was proclaimed (Diario Oficial de la Federación, México 1990), population recovery has been strong, reaching 718,800 nests in 13 arribazones between May 1994 and February 1995 (Figure 2). The trend has continued, with 670,480 nests laid in eight arribazones between May and December 1995 [N.B. nests laid in early 1996 had not been tallied before going to press].

Unexpectedly, in the same year that the ban was declared (1990), arribazones became more abundant and more precise in their timing, as was the case in earlier years, occurring nearly always at the third quarter of the moon (Márquez et al., 1976). Consequently, in fewer than five years the number of arribazones and the number of nests now exceed our records from two decades ago. Moreover, arribazones now start in May or June and continue through February or March of the next year. We believe that the rapid rise in numbers is the logical result of terminating the excessive commercial harvest (which, at its height, may have exceeded 100,000 breeding adults per year) in waters offshore the nesting beach. At same time, government programs protected eggs on the beach and so we anticipate that recruitment will increase as the years pass. Because of the dramatic rise in the numbers of females arriving and the numbers of nests laid in recent years, fishermen once dependent on the harvest are pressing the government to reopen the quota system and allow regulated exploitation of some populations which they argue are no longer endangered in Oaxaca state.

Intensive exploitation began in 1963 in Sinaloa and Nayarit, increased rapidly with 1050 tons harvested in 1964, and peaked in 1968 with 13,600 tons (ca. 350,000 turtles) harvested throughout the country and 10,262 tons from these two states. As a result, Pacific olive ridley populations plummeted in México. Two of three arribazon populations were decimated. In response, the government enacted a ban in 1971-1972. In 1973 the fishery was reopened and ca. 3400 tons were harvested that year from the states of Jalisco, Guerrero and Oaxaca. In addition to a domestic market for meat, eggs and skins, nearly 150,000 kg of skins were exported to Japan between 1976-1986 (Milliken and Tokunaga, 1987). In 1990, the Government of México closed the industry entirely (Aridjis, 1990) and enacted measures to protect remaining turtles, especially at La Escobilla. It is rare that conservationists can see the fruit of their labors on behalf of sea turtles, but at La Escobilla the olive ridley turtles have responded in a very positive way to INP's conservation and marine surveillance programs since the middle of the 1960's, restrictive regulations in the early 1970's (see Márquez, 1976), and a national moratorium declared in 1990. The population appears to have regained a healthy state, and may now exceed pre-harvest levels.

The data from La Escobilla show that a sea turtle population can respond to protection, and that recovery is possible if a well organized (and enforced) program is developed. It is not an easy task to maintain a high level of population abundance. The task requires a sustained investment of money and hard work, the enforcement of strong restrictions on exploitation, and the support of government agencies, conservation groups, and resource users. When the task has been completed, however, we must evaluate the potential for reopening a regulated harvest. We do not yet know the carrying capacity of La Escobilla (that is, how many turtles and nests the beach can productively support), but the next step in this conservation program will be to determine this density and use that knowledge to propose new harvest quotas for eggs and/or adults for the benefit of the people of Oaxaca. For now, it is important to continue to monitor this population to ensure that what we are witnessing is indeed a prolonged recovery and not just a transient upward trend in an otherwise declining population.

Acknowledgements: Researchers too numerous to name have been involved in this research since the Marine Turtle Program was started in 1966. Some of the pioneers include Antonio Montoya, Manuel Grande Vidal, Aristoteles Villanueva, and Emmanuel Vargas. Many students assisted in the work, in particular from the Universidad Autón~oma de Oaxaca "Benito Juarez". Tremendous assistance was given by the Méxican Navy and Fisheries Inspectors. All these people must be recognized for their role in the survival of this valuable species. Also, the Fisheries Cooperatives in Oaxaca that were harvesting the species for many years are now the principal players in the species' conservation.

Aridjis, H. 1990. México proclaims total ban on harvest of turtles and eggs. Marine Turtle Newsletter 50:1-3.

Diario Oficial de la Federación, México. 1990. "Acuerdo que establece veda para todas las especies y subespecies de tortugas marinas en aguas de jurisdicción nacional de los litorales del Océano Pac¡fico, Golfo de México y Mar Caribe." Diario Oficial de la Federación, México. Mayo 31, 1990:21-22.

Márquez-M., R. 1976. Estado actual de la pesquer¡a de tortugas marinas en México, 1974. Inst. Nac. de Pesca, México, Serie Inf. 46. 27 pp.

Márquez-M., R., A. Villanueva and C. Peñaflores. 1976. Sinopsis de datos biológicos sobre la tortuga golfina, Lepidochelys olivacea (Eschscholtz, 1829). Instituto Nacional de la Pesca, México, INP Sinop. Pesca. No. 2. 61 pp.

Milliken, T. and H. Tokunaga. 1987. The Japanese Sea Turtle Trade 1970-1986. Prepared by TRAFFIC(JAPAN) for the Center for Environ. Education, Washington D.C. 171 pp.