Population Trend and Spatial Distribution of Nesting

Current conservation activities for the leatherback turtle in the 4 index beaches of the Mexican Pacific comprise 64 km of coast (Table 1). If we consider the secondary beaches, in which such activities have not been continuous, the protection efforts for the leatherback in Mexico span 214 km. The daily track counts done from 1982 to 2004 showed a declining trend for the number of leatherback nests in the Mexican Pacific (Fig. 2). The best nesting season was 1985–1986 with 5000 nests recorded in the southeast portion of Mexiquillo (4 km); by adjusting for the proportion of nests that occur each season on the rest of the beach, we estimated as many as 10,000 nests on the whole beach in that season. Population size reached the lowest level in the 2002–2003 season, when we recorded only 120 leatherback nests on the index beaches combined with a maximum nesting density of 12 nests/km. In later seasons, the nesting numbers fluctuated but did not recover in spite of more than 2 decades of protection activities.

Table 1. Index and secondary nesting beaches for the leatherback turtle in the Mexican Pacific.

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Leatherback nesting, although occurring along the whole Pacific coast of Mexico, was not homogeneous but concentrated in rather defined areas. Sarti et al. (1998) estimated that the 4 index beaches hosted about 42% of the total number of nests in the Mexican Pacific. If conservation projects for the secondary beaches were permanent, as much as 73% of the total nesting of leatherbacks in the Mexican Pacific would be protected in an average season. Nevertheless, in later seasons, we recorded changes in the pattern of the nesting distribution (Fig. 3), with a lower concentration of nests in the index beaches and more scattering toward unprotected areas. It is possible that some females from the major nesting beaches are moving to areas without protection activities, and this effect is more evident with an extremely depleted population.

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We found a relatively high interchange of females between some beaches when we combined the tagging data from all index beaches. Individuals may switch nesting beaches both within and between seasons; the degree of interchange depends on geographic proximity of the beaches. Tierra Colorada and Cahuitán are 25 km apart and can share up to 18.7% of the females that nest in both beaches in a season (9% on average), functioning as a single unit for those females. Mexiquillo is about 475 km from the closest index beach (Tierra Colorada), and we did not find any female interchange within seasons, although a female tagged in Mexiquillo in 1999–2000 was recaptured nesting in Tierra Colorada in 2001–2002 and a female originally tagged in Tierra Colorada was recaptured in Mexiquillo in 2003–2004. The movement of females between index beaches is a strong reason for a coordinated and standardized long-term conservation project in these nesting sites, because relevant reproductive data would be lost otherwise.

Tagging Program and Nesting Biology

To date, the teams working on the index beaches tagged 5314 leatherback females (Table 2). Most were not recaptured in later seasons, especially in the early days of the project when only flipper tags were used. However, we recognized many females as remigrants because of the scars in their flippers caused by a lost tag. The use of PIT tags on Mexican leatherbacks started in 1996, and this is a successful method for identifying individuals on the long term (McDonald and Dutton 1996).

Table 2. Results of the tagging program on the index beaches of the Mexican Pacific.^a

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Mexiquillo is the beach with the longest record of leatherback tagging in Mexico and can be used as a model for the remigration behavior of the Mexican leatherback population (Fig. 4). The average remigration interval recorded in this beach is 3 years, with the largest interval (9 years) found for a female tagged in the 1981–1982 season (García-Muñoz 2000). Given the presence of scattered nesting along the entire Pacific coast of Mexico, it is probable that she nested on beaches with no surveillance during that period. We observed a decline in the proportion of remigrants found in Mexiquillo from 1995 to 1999 (Fig. 4), which coincided with low nesting numbers in all the index beaches. From 1999 to 2004, the proportion of remigrants rose again, coming closer to pre-1995 levels. The average percentage of remigrants for all index beaches was 22.4% from 1996 to 2004.

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The Mexican Pacific leatherback females ranged in CCL from 120.0 to 168.0 cm, with a mean of 143.8 cm (Table 3). This population had an average ECF of 5.5 (1.9 SD; range = 3–12), with an average CI of 9.7 days (1.2 SD; range =  8–14 days). We estimated that the average total fecundity per female was 341 eggs, with a maximum of 744 eggs in a season.

Table 3. Statistical parameters of curved carapace length, curved carapace width, and clutch size for nesting leatherbacks on the Mexican index beaches.^a

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Conservation

From the beginning, protecting most of the eggs, females, and hatchlings was a priority among the conservation activities of all the institutions working with the leatherback in Mexico. Since the early 1980s, at least 639,270 eggs were protected and a minimum of 270,129 leatherback hatchlings were released to the wild population at the priority beaches. Projects that focus on other species (e.g., olive ridley and black turtle) on beaches where leatherback nesting is scarce usually protect the leatherback clutches they find and release an unknown number of hatchlings; this number is low but important, because it occurs all along the entire distribution range of the leatherback in the Mexican Pacific.

For the last 20 years, the average hatch success fluctuated between 35% and 52%, with minimum values between 0% and 30% and maximum values between 58% and 78%, with a few clutches reaching 100% hatch success. The number of hatchlings released is proportionately small compared with the original reproductive potential of the population: an estimated 4,184,938 eggs, considering the total number of nests on index beaches, 92% oviposition success, and an average of 62 eggs per clutch. In the 1980s, only Mexiquillo produced leatherback hatchlings every season, and even there the protection effort comprised only less than half (30%–45%) of the total number of clutches, the rest were lost to poaching.

Population Trend and Spatial Distribution of Nesting

As a conservation program, Proyecto Laúd represents the most solid source of knowledge about the biology and the ecology of the leatherback turtle in Mexico. The project showed that the population trend of the leatherback in the Mexican Pacific has had a drastic decline for several years. This situation greatly supported the listing of the leatherback turtle as critically endangered by the World Conservation Union (IUCN) in 2000 (Sarti 2000a). Such a decline is comparable with the one recorded for the Playa Grande leatherback population, in Costa Rica, the last remaining large nesting colony in the Eastern Pacific (Steyermark et al. 1996).

The aerial surveys conducted since 1996 refuted the hypothesis of the possible movement of females to unknown areas as a cause of the decline in the nesting numbers on index beaches. Cahuitán was “discovered” as an important nesting area in 1996 during the first aerial survey, but interviews with local residents confirmed that the leatherbacks had nested there as far as the oldest persons could remember and that the nesting numbers had declined as on other beaches. Therefore, no new nesting areas were documented in the Mexican Pacific from 1996 to 2004.

Tagging Program and Nesting Biology

The tagging program suffered several modifications for the past 20 years. In the 1980s and mid-1990s, only females at Mexiquillo were consistently tagged by using flipper tags (Table 2), mostly Monel and sometimes plastic. If any females were tagged on other Mexican beaches, the information is not available, although the high number of tags of unknown origin recaptured in Mexiquillo indicates that this is the case. In those days, the tagging program was far from achieving saturation. We estimated that an average of 65% of the females that nested in Mexiquillo between 1983 and 1989 received a tag. Conducting a saturation tagging program in the Mexican Pacific is not an easy task because of the length of the beaches and the dispersion of the nesting events; during the latest nesting seasons, more than 95% of the females on the index beaches were tagged each year with Monel and PIT tags.

The long-term tag-recapture program and the use of better technologies for identifying individuals gave us a better understanding of the situation of the leatherback population in Mexico; we have just begun to elucidate the dynamics of the declining process. The decline in numbers of tagged females seen in Table 2 is a direct reflection of the decline in the nesting population, because actually more kilometers of beach are covered for the full season each year and is not an artifact from a differential tagging effort among beaches.

The proportion of remigrants seen on the Mexican index beaches during the late 1990s was similar to the one reported for Playa Grande, Costa Rica, from 1996 to 1999 (24.8% on average; Spotila et al. 2000) but lower than the one reported for St. Croix, U.S. Virgin Islands, during the same period (52.5%; Dutton et al. 2005). The low proportion of remigrants observed in the Eastern Pacific could indicate a high mortality of larger adults in the pelagic environment. Few turtles were arriving at the nesting beaches and most of them were new ones (apparent neophytes, unless they nested before on a beach with no tagging program). In contrast, the St. Croix population has a high estimated annual survival probability for adult females (Dutton et al. 2005).

The severe population decline for the Mexican leatherback population was attributed to an increase in adult mortality from the fishing effort of South American swordfish fisheries occurring around the same time, which captured Mexican leatherbacks in the past (Frazier and Brito-Montero 1990; Eckert and Sarti 1997). The impact of the high-seas industrial fisheries on the eastern Pacific leatherback population is discussed elsewhere (Spotila et al. 1996, 2000); nevertheless, the interaction of local artisanal fisheries with leatherbacks close to shore is poorly documented. Dead leatherbacks wash ashore sometimes at the index beaches, but we need a systematic evaluation of the impact of near-shore artisanal gill nets and longlines in Mexico.

The results of carapace measurements are in concordance with the differences between the Pacific and Atlantic leatherback populations reported by other authors (Pritchard 1971; Fretey 1978; Eckert and Eckert 1985; Boulon et al. 1996). The leatherbacks in the eastern Pacific are indeed smaller and lay fewer eggs than the ones in the Atlantic; the information presented here is the most thorough and accurate compiled for the Mexican Pacific leatherback population, standardized for its complete distribution range and for several nesting seasons.

The estimation of the number of nesting females each season is based on the total number of clutches and the average ECF per season on the index beaches. The average ECF obtained for the Mexican population is similar to the average ECF reported from 1993 to 2000 in Playa Grande, Costa Rica, (5.6 ± 0.2 SE; Reina et al. 2002) but lower than the average ECF reported for Culebra, Puerto Rico (6.7 ± 2.7 SD; Tucker and Frazer 1991). However, caution must be taken while comparing these values, because the methods used for calculating ECF in the 3 cases use different assumptions.

The maximum clutch frequency observed in the Mexican population was 12 clutches per female per season (Sarti et al. 2004). Considering that not all of the nesting events are observed along the distribution range and that females may move between beaches, the actual average clutch frequency could be closer to the maximum value reported for the population. Higher values of average clutch frequency mean lower numbers of estimated females, so the Mexican Pacific leatherback population could actually be smaller than estimated.

Conservation

Early accounts (Márquez et al. 1981) and anecdotal information suggest that 25 years ago the harvest of eggs on most of the important nesting beaches of the Mexican Pacific could reach 100%, causing very limited recruitment. This situation combined with the high incidental capture of adults in pelagic fisheries for an unknown period might be the cause of the dramatic crash in the nesting numbers of the leatherback all over the Pacific coast of Mexico, and the reason the population is not yet responding to more than 2 decades of protection efforts.

The hatchlings released by the early protection program in Mexiquillo appear to be the major source of the young nesting females seen on the index beaches in recent seasons. However, time and increased efforts are still needed to see a change in the population trend. Most importantly, current efforts must not be abandoned. Today, the protection activities on all index beaches relocate at least 80% of the clutches to hatcheries, and care is taken to avoid any management problems that can harm the population. We recognize the proven fact that in situ leatherback nests have a better hatch success than relocated nests (Sarti et al. 1987, Boulon et al. 1996); nevertheless, the condition of most of the nesting beaches with regard to poaching and predation by introduced mammals, turn beach hatcheries into the only currently viable conservation option. Studies done in Mexiquillo show that in a properly managed hatchery, the sex of the hatchlings is skewed toward females, in a proportion similar to in situ nests sampled in the same season (Benabib 1984; Ordoñez 1998). The status of the Mexican leatherbacks is dire but not hopeless. The protection results presented above give hope for the future of this important population and are essential for other conservation measures to operate.

One of the major achievements of Proyecto Laúd is the involvement of local community members in conservation activities. After years of reluctance and animosity toward the protection efforts, some people who live in villages close to the index beaches agreed to participate side by side with biologists in patrolling the beaches and relocating the clutches to protected areas. Most of these persons receive remuneration for their work, and some communities organized into protection committees that began to show a true interest in the recovery of the leatherback population, turning their protection efforts into a matter of pride for the community.

Government authorities showed an increased interest in the fate of the leatherback turtle. In September 2003, the governors of Michoacán, Guerrero, and Oaxaca joined federal authorities in the signature of the Tri-State Agreement for the Recovery of the Leatherback Turtle in the Mexican Pacific, in which the parties committed to support all the necessary activities for the conservation of this species along the coast of those states. This was the first high-level government agreement centered on the recovery of a single sea turtle species and included the protection of leatherback individuals, as well as their habitat. As part of the agreement, the technical committee established the Community Network for the Recovery and Protection of the Leatherback Turtle. This network includes several communities located close to the priority beaches and serves as a communication bridge between the communities and the authorities. The network also promotes awareness of the status of the leatherback among the local population and increased training of conservation workers in standard management methods, supported by the best available knowledge of leatherback biology.

In recent years, CONANP promoted the protection of nesting habitat, with 2 of the index beaches (Mexiquillo and Tierra Colorada) designated as natural protected areas (turtle sanctuaries) in 2002, and 3 of them (Mexiquillo, Tierra Colorada, and Cahuitán) were listed as Ramsar Sites in 2003.

The threats faced by the Mexican leatherbacks are no different from the ones faced by the rest of the eastern Pacific population. We know that in most of Central America the harvest of leatherback eggs is more intense than in Mexico, and few protection programs in the area are older than 10 years. This has contributed to the decline of the nesting population throughout the eastern Pacific.

It is very important to establish recovery programs on a regional level, with international cooperation, focused on increasing the production of healthy hatchlings and the survival of juveniles and adults in the pelagic environment. It is worthwhile to note that the number of hatchlings released so far on the Mexican index beaches is similar to the number of hatchlings released to date in the St. Croix conservation program (estimated in ca. 260,000; Dutton et al. 2005) but that population is increasing and the Mexican one is not. Clearly, there are more factors involved, particularly regarding survival probabilities, which need to be assessed.

The resolution for the protection of the leatherback passed at the COP 2 (Venezuela, 2004) of the Inter American Convention for the Conservation of Sea Turtles is a relevant step toward a regional conservation approach. The sum of the actions of all stakeholders as protection of eggs on nesting beaches, reduction of incidental capture on the high seas, along with awareness programs and education of wide audiences in general, will contribute greatly to the recovery of the population of the eastern Pacific leatherback. With these efforts, we would expect to witness positive results and an increment in the nesting numbers in Mexico by 2010 to 2015, given the increase in the production of hatchlings on all the beaches for the past 10 years and by assuming 13–14 years for the leatherback's average sexual maturity but 9 years as a minimum age of maturity (Zug and Parham 1996).

In Mexico, there is an increased concern for the fate of the Pacific leatherback. Proyecto Laúd will continue with the long-term monitoring of the population trend, including possible changes in biological parameters, such as clutch frequency, hatch success, clutch size, body size, and other parameters of the nesting biology, and the health condition of reproductive adults. We will continue producing hatchlings and give this population time for the other conservation measures to yield results.