MTN 74:2-5 Decline of the World's Largest Nesting Assemblage of Leatherback Turtles

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Decline of the World's Largest Nesting Assemblage of Leatherback Turtles

Laura Sarti M.¹, Scott A. Eckert², Ninel Garcia T.³ and Ana Rebeca Barragan³
¹Lab. Tortugas Marinas, Facultad de Ciencias, UNAM Depto. Biología, Circuito Exterior, Ciudad Universitaria, México D.F. 04510 Mexico and Instituto Nacional de la Pesca, Pitogoras 1320 5º Piso, Col. Sta. Cruz Atoyac, México D.F. 03200 Mexico
²Hubbs-Sea World Research Institute, 2595 Ingraham Street, San Diego, California 92109 USA, ³Lab. Tortugas Marinas, Facultad de Ciencias, UNAM Depto. Biología, Circuito Exterior, Ciudad Universitaria, México D.F. 04510 Mexico

The leatherback turtle (Dermochelys coriacea) is classified as Endangered throughout its global range (Groombridge, 1993). Increases in the number of nesting females have been noted at some sites, such as South Africa, but these are far outweighed by local extinctions, especially of island populations, and the demise of once large populations, such as in Malaysia. Gathering reliable quantitative information on national, regional or global population size is difficult, and at the very least requires many years of dedicated field work. We are fortunate that in México, which, until recently, was the most important country in the world for nesting by endangered leatherback turtles, the requisite databases exist. The story they tell, however, is grim.

According to reports from the late 1970's and early 1980's, beaches located on the Méxican Pacific coasts of Michoacán, Guerrero and Oaxaca sustained a large portion, perhaps fully one half, of all global nesting by Dermochelys (Márquez, 1976; Márquez et al., 1981, Fritts et al., 1982; Pritchard, 1982). Monitoring of the nesting assemblage at Mexiquillo has been continuous since the 1983-1984 breeding season, making it the longest leatherback moni-toring program in México, and one of the most complete databases for this species in the world. Since 1986 the population at Mexiquillo has been in a continuous decline, with a particularly low year in 1993-1994 (Sarti et al., 1994) (Figure 1).

Figure 1. Number of nesting crawls (nests and false crawls combined) made by leatherback sea turtles each year at Playa Mexiquillo, Michoacán.

By the early 1990's, communication among Mexican field workers suggested that a declining trend was the norm for all leatherback nesting populations on the Pacific coast. In the absence of ample coverage of the entire shoreline, however, investigators could not rule out the possibility that the geography of nesting had shifted and that, for reasons unknown (but perhaps related to intense El Niño episodes in the late 1980's and early 1990's), females were favoring a different suite of beaches than they had previously been the case. With an aim to census the entire (national) population and gather data crucial to a serious evaluation of the status of Dermochelys in México, an ambitious study was undertaken during the 1995-1996 nesting sea-son which utilized comprehensive aerial surveys and ground patrols to achieve the first coast-wide census of leatherback nesting colonies in Pacific México.

Starting on 2 November 1996, daily ground surveys were conducted for each of four major nesting beaches: Mexiquillo (Michoacán State), Tierra Colorada (Guerrero State), and Chacahua and Barra de la Cruz (Oaxaca State). Weekly nesting crawl counts were recorded at two secondary nesting beaches (La Tuza and Cerro Hermoso, Oaxaca State). These six beaches served as Index Beaches for the purpose of this national survey. Near the end of the season (3-24 January 1996), an aerial survey was conducted from Manzanillo (Colima State) to Puerto Madero (Chiapas State), as well as along the entire coast of Baja California. Simultaneous to the aerial survey, a ground survey was conducted at each of the six Index Beaches to determine aerial observer error.

We identified three sources of error in estimating the seasonal total number of crawls with body pits when using a single ground or air survey: 1) underestimation due to the aging and disappearance of body pits, (2) underestimation due to an observer failing to count a visible body pit, and (3) underestimation due to post-survey nesting. After correcting for these sources of error, the total number of nests was estimated using the average nest:false crawl ratio calculated from nocturnal patrols of the Index Beaches during the 1995-1996 nesting season. We conclude that during the 1995-1996 nesting season, 5,222 leatherback nests were laid on the Pacific coast of México. Based on an average annual frequency of 5.3 nests per female (Mexiquillo; Sarti et al., 1988), fewer than 1000 females nested during the 1995-1996 nesting season.

The highest density was recorded at Playa Mexiquillo, followed by Llano Grande (a previously undescribed nesting beach) and Tierra Colorada (Figure 2). These beaches support nearly 35% of all leatherback nesting on the Pacific coast. It is clear that total nesting is far, far below what has been previously reported for México. It is also clear from many lines of evi-dence that this is not simply a trend which is within the range of natural population fluctuation. The data underscore what Mexican field scientists have feared for some time, and that is that the world's largest population of endangered leatherback turtles has collapsed. Our data indicate that the decline at Mexiquillo has progressed at an annual rate of 22.66% for the last 12 years. If Pritchard's (1982) oft-cited population values (which appear to be overestimates) are used, the annual rate of decline sharpens to 54.64%.

Figure 2. Leatherback nest density (1995-1996 nesting season) on the Pacific coast of México.

We believe a combination of uncontrolled domestic harvest and the advent of the high seas driftnet fishery in the 1980's (see Wetherall et al., 1993) gave impetus to the disaster. The details of our study and conclusions are being prepared for peer-reviewed publication.

Acknowledgments: A project of this magnitude requires the cooperation and support of many institutions and individuals. The authors gratefully acknowledge the support of the following institutions: U. S. National Marine Fisheries Service, UNAM Facultad de Ciencias, Instituto Nacional de La Pesca, Instituto Nacional de Ecologia, Animal Alliance, LightHawk, the Dodge Foundation, Polar Services, and Centro Mexicano de la Tortuga. We are also indebted to the efforts of the following individuals: Ernesto Albavera, Juan Carlos Alvarado, Verónica Blanco, Libertad Cuéllar, Leticia Gámez, Patricia Huerta, Arturo Juárez, Dr. Gerald Kooyman, Karsten Kooyman, Carlos López, Cristina Ordoñez, Cuauhtemoc Peñaflores, Humberto Pineda, Guadalupe Sánchez Sandoval, and Javier Vasconcelos. Finally, the kindness of the following communities is deeply appreciated: Barra de la Cruz, Caleta de Campos, Chacahua, El Farito, Llano Grande, and Tierra Colorada.

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Groombridge, B. (Editor). 1993. IUCN Red List of Threatened Animals. World Conservation Union (IUCN), Cambridge. 286 pp.

Márquez M., R. 1976. Reservas naturales para la conservación de las tortugas marinas de México. Inst. Nac. Pesca. INP/ SI:183. 22 pp.

Márquez M., R., A. Villanueva and C. Peñaflores. 1981. Anidación de la Tortuga Laúd, Dermochelys coriacea schlegelli, en el Pacífico mexicano. Ciencia Pesquera 1(1):45-52. INP, México.

Pritchard, P. C. H. 1982. Nesting of the leatherback turtle, Dermochelys coriacea, in Pacific México, with a new estimate of the world population status. Copeia 1982:741-747.

Sarti M., L., B. Jiménez, J. Carranza, A. Villaseñor and M. Robles. 1988. IV Informe Final de Trabajo. Investigación y Conservación de las Tortugas Laúd Dermochelys coriacea y Golfina Lepidochelys olivacea en Mexiquillo, Michoacán. Temporada de anidación 1987-1988. Sec. Desarrollo Urbano y Ecología Delegación Michoacán. 46 pp.

Sarti-M., L., C. López, N. García T. and S. Karam-M. 1994. Resultado de las Actividades de Protección de las Tortugas Golfina, Lepidochelys olivacea, y Laúd, Dermochelyscoriacea, en el Playón de Mexiquillo, Michoacán. Temporada 1993-1994. In: Memorias de Resúmenes del XI Encuentro Interuniversitario sobre Tortugas Marinas en México, celebrado en San Patricio-Melaque, Jalisco, México del 12 al 18 de junio de 1994.

Wetherall, J. A., G. H. Balazs, R. A. Tokunaga and M. Y. Y. Yong. 1993. Bycatch of marine turtles in North Pacific high seas driftnet fisheries and impacts on the stocks. Bull. N. Pacific Comm. 53(III):519-538.