Leatherback Turtles (Dermochelys coriacea) in the Gulf of California: Distribution, Demography, and Human Interactions
ABSTRACT
Since 1943, a total of 40 leatherbacks have been documented in neritic and offshore waters of the Gulf of California, Mexico: 13 as fisheries by-catch, 11 in indigenous ceremonies, 8 coastal strandings, 4 at-sea sightings, 3 observed by fishing fleets, and 1 via satellite telemetry. Leatherback hatchlings were observed on 3 occasions in the northern Gulf of California. The range of curved carapace lengths for nonhatchling leatherbacks was 113 to 160 cm curved carapace length (mean = 139 ± 12 cm). All but 1 leatherback were reported between November and May, a period of cooler water temperatures for the region.
The leatherback, Dermochelys coriacea, is a highly impacted and inadequately understood marine turtle, with a distribution spanning tropical, temperate, and sometimes subarctic waters (Dutton et al. 1999; Spotila 2004). Leatherbacks are declining in many parts of the world, and it has been suggested that these declines are the result of decades of egg poaching at nesting beaches, coupled with a more recent by-catch problem associated with artisanal and industrial marine fisheries (e.g., Eckert and Sarti-Martínez 1997; Sarti Martínez et al. 2007; Alfaro-Shigueto et al. 2007). In the Pacific, the depletion has been so extreme that the eastern Pacific populations of leatherbacks have been considered in imminent danger of extinction (Spotila et al. 2000). The species is currently listed as critically endangered in the World Conservation Union (IUCN) Red List (IUCN 2004) and is included in Appendix I of the Convention on International Trade in Endangered Species of Wild Flora and Fauna. This imperiled status has prompted calls for increased research and protection (National Marine Fisheries Service and U.S. Fish and Wildlife Service 1998); however, the development of appropriate management strategies has been hindered by a lack of empirical information on the distribution and demography of leatherbacks, particularly in marine habitats of the eastern Pacific Ocean.
Leatherbacks have played an important role in human cultures of Pacific rim countries, perhaps nowhere more so than along the shores of the Gulf of California (also known as the Sea of Cortez), a warm-temperate to tropical sea located between the Baja California peninsula and mainland coast of Mexico (Fig. 1). Known as mosnípol (black sea turtle) by the indigenous Comcáac tribe of Sonora (also known as Seri) and tortuga laúd (lute turtle) or siete filos (7 ridges) in local Mexican culture, the leatherback is cherished in traditional beliefs and oral history. While the Comcáac continue to hold these giant reptiles in special spiritual esteem (Felger and Moser 1985; Nabhan 2003), Mexican fishers retell their stories of leatherback encounters with passionate detail as if these are the most amazing marine creatures ever encountered (W. Nichols and J. Seminoff, unpubl. data). Nevertheless, despite the cultural importance of leatherbacks around the Gulf of California, few data on their occurrence in marine habitats are available.
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[137:LTDCIT]2.0.CO;2
A better understanding of leatherback biology and conservation status is clearly needed for the Gulf of California. To partially address these aspects, here we describe the distribution, size, and condition of leatherback turtles based on information collected from the Comcáac, Mexican fishers, and modern researchers. This compilation is the first of its kind for the region, and, hopefully, it will provide wildlife managers with an additional tool for regional conservation planning.
Methods
Data on leatherbacks were gathered from a variety of sources, including governmental fisheries by-catch reports; published and unpublished ethnobiological accounts; and unpublished stranding, observational, and satellite telemetry data. For each account, we recorded date of observation, location, condition of turtle, size, and sex. All turtle sizes are reported as curved carapace lengths (CCL). We paid close attention to the accuracy of each record and rejected any questionable data. As noted in a summary of juvenile leatherback sightings (Eckert 2002), the unique morphology of the species makes misidentification unlikely, we were thus confident that each record had been correctly labeled as a leatherback. Further substantiating the reliability of identifications is the fact that all informants were either experienced wildlife biologists, fisheries managers, or fishers, and were familiar with leatherbacks. The condition of leatherbacks in this report included 5 categories: 3 for live turtles (indigenous ceremony, at-sea observation, or satellite tracked turtle) and 2 for dead animals (beach stranding or artisanal net by-catch).
Results and Discussion
Forty-three records of leatherbacks in the Gulf of California were gathered. These included 18 published accounts (Caldwell 1962; Cliffton et al. 1982; Felger and Moser 1985; Nichols 2003) and 25 unpublished records (from S. Benson; D. Burckhalter; P. Dutton; S. Eckert; R. Felger; M. Hall; C. Navarro; G. Nabhan; T. Pfister; A. Resendiz; and J. Seminoff). The conditions of the large immature and adult leatherbacks, summarized in Table 1: 16 were found dead in artisanal fishing nets, 11 were involved in Comcáac leatherback ceremonies (see Felger and Moser [1985] for description of ceremony), 8 were coastal strandings, and 5 live turtles were observed at sea, including 1 that was equipped with a satellite transmitter and tracked into the Gulf (Table 1; Fig. 2). In addition, hatchling-sized leatherbacks were encountered on 3 occasions: in 1961, near San Felipe, Baja California (Caldwell 1962); in the mid-1970s, on a beach in the Infiernillo Channel, Sonora (R. Felger, unpubl. data); and, in October 2006, dead on a beach near Puerto Libertad (D. Burckhalter, pers. comm.) (Fig. 1). The San Felipe report did not indicate if the turtles were in the water or on the beach (Caldwell 1962).
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[137:LTDCIT]2.0.CO;2
Of the 40 records of nonhatchlings, reliable size data were available for 21 individuals (Fig. 3). CCLs ranged from 113 to 160 cm (mean = 139.4 cm, SD = 12.0 cm). Considering that most sea turtles attain maturity at or near mean nesting size (Chaloupka and Musick 1997), this size range is indicative of a population that consists of both immature and adult turtles (mean nesting size in Mexico = 143.8 cm CCL; L. Sarti-Martínez, pers. comm.).
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[137:LTDCIT]2.0.CO;2
The 3 accounts of leatherback hatchlings suggest that this species nests in the Gulf of California. The presence of hatchling-sized turtles may not provide unequivocal evidence of nesting, but these accounts, coupled with the fact that leatherbacks grow beyond the hatchling size in a matter of weeks (Witham and Futch 1977), suggest that these small turtles originated from somewhere near the sites of observation. However, the few records of hatchlings and the lack of observed beach activity by leatherbacks, despite widespread human presence in the region, suggest that nesting in the Gulf is very rare.
Despite the records of hatchling-sized turtles, the question remains: what are the nesting beach origins for leatherbacks found in the Gulf of California? We believe that, although leatherbacks may nest on occasion in the region, it is more likely that the nonhatchlings included in this report originated from nesting sites outside the Gulf of California. The closest concentrated leatherback nesting areas are at the tip of the Baja California peninsula, south of Todos Santos (Fritts et al. 1982; Sarti Martínez et al. 2007; Fig. 1.), and along mainland Mexico in the states of Michoacán, Guerrero, and Oaxaca (e.g., Sarti Martínez et al. 2007), and proximity would suggest that these beaches are the primary nesting sites for leatherbacks in the Gulf. However, none of the females that have been satellite tracked after leaving nesting beaches in Mexico and Costa Rica have been found to enter the Gulf of California (e.g., Morreale et al. 1996; Eckert and Sarti 1997; S. Eckert, pers. comm.). Furthermore, Dutton, Benson, and Eckert (unpubl. data) tracked an adult male leatherback captured on the foraging area in Monterey Bay, California, into the Gulf of California. Over several months in the region, this animal was in the upper gulf, offshore from San Felipe when transmissions ceased. Genetic analysis of a skin sample collected from this turtle indicates its origin is from a nesting beach in the western Pacific, tens of thousands of kilometers away (P. Dutton, unpubl. data; see Dutton et al. 1999). Clearly, additional genetic data are needed from the Gulf to elucidate the leatherback turtle stock composition in the region. The further use of satellite telemetry on females departing from nesting sites in Baja California may also elucidate the nesting beach to foraging area link for leatherbacks found in the Gulf of California.
The paucity of nesting activity suggests that the primary importance of the Gulf of California for leatherbacks is as a foraging area. The distribution of sighting data indicates that leatherbacks may forage in the uppermost portions of the Gulf, an area of great tidal flux and shallow waters (Fig. 1). Moreover, based on the distributions of leatherback observations and fisheries by-catch events, it is apparent that leatherbacks may enter shallow nearshore waters. All but 1 of the 22 by-caught leatherbacks were captured in commercial, and at the time legal, green turtle entanglement nets set within 200 m of shore and in water depths of less than 15 m (A. Resendiz, pers. comm.). Foraging in coastal waters of the eastern Pacific has been documented previously (Eisenberg and Frazier 1983; Starbird et al. 1995; Dutton, Benson, and Eckert unpubl. data), but, to our knowledge, the occupancy of shallow near-shore waters by foraging leatherbacks as described here is the first such account for Mexico.
The majority of sightings occurred during the months of January to May, during which time all but 3 turtles (30 of 33 sightings), with reliable observation dates were encountered (Fig. 4). Although this temporal pattern should be viewed cautiously because of the small sample size, the preponderance of sightings during winter and spring raises intriguing questions about the temporal distribution of leatherbacks in the Gulf of California. These observations were the result of fishing effort, which can vary throughout the season (J. Seminoff, pers. obs.), but the disproportionately high number of records during winter (December to March), a time when fishing effort is lowest, suggests that leatherbacks are indeed more common in the Gulf during this period. Interestingly, the time of greatest leatherback presence is also the period of coolest sea surface temperatures (SST): whereas winter mean SSTs are coolest in February and March, dropping to below 15°C, summer mean SSTs peak in August and September at 30°C or more (Robinson 1973). In the northeastern Pacific, the seasonal presence of leatherbacks in nearshore waters appears to be associated with 14°–15°C isotherms (Stinson 1984; S. Benson, unpubl. data). In the Gulf of California, perhaps the warm summer waters are thermally challenging to leatherbacks, a species whose large size is efficient for heat retention (i.e., gigantothermy; Paladino et al. 1990). However, studies by Penick et al. (1998) suggest that leatherback tissue metabolism functions normally at temperatures up to 38°C. A second scenario is that elevated SST indirectly affects leatherbacks by negatively impacting the abundance of their gelatinous megaplanktonic prey. Unfortunately, few data are available on gelatinous plankton in the Gulf of California and we are thus unable to ascertain the validity of this theory.
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[137:LTDCIT]2.0.CO;2
The demographical data described here are the first substantial data for leatherbacks in the Gulf of California in over 2 decades (see Cliffton et al. 1982). Despite the many years that have passed since the presidential decree aimed at protecting sea turtles in México (Diario Oficial 1990), the recent accounts of stranded turtles reported here indicate that leatherbacks continue to be adversely impacted by humans in the Gulf of California. Efforts to prevent such mortality will rely, in part, on a better understanding of leatherback presence in this region. This underscores the need for additional biological information that will guide the focus and implementation of future recovery efforts. From a conservation standpoint, we recommend that the Gulf of California be included as a target area in recovery effort planning in the Eastern Pacific Ocean, despite the low number of turtles included in this report.
Distribution of leatherback sightings in the Gulf of California, Mexico.
Satellite tracked movements of a male leatherback turtle tagged near Monterey Bay, California, USA. This turtle was 157.0-cm curved carapace length and was tracked for 762 days from 2002–2004. (Courtesy of Dutton et al. unpubl. data).
Individual size of 20 nonhatchling leatherback turtles killed in fishing nets (n = 16) or stranded (n = 4) in the Gulf of California. An additional 25 nonhatchling turtles were documented in the area for which no reliable size data were available.
Summary of month of observation for 33 leatherback turtles in the Gulf of California. An additional 14 leatherbacks were documented for which no reliable date of encounter was available.
