Leatherback Turtle Nesting Trends and Threats at Tortuguero, Costa Rica
ABSTRACT
Leatherback turtle (Dermochelys coriacea) nesting activity was monitored, individual movements via flipper tag recoveries and satellite telemetry were determined, and illegal egg collection was quantified at Tortuguero, Costa Rica from 1995 to 2006. Annual nest deposition was estimated at 199–1,623 nests per year; a Bayesian regression model suggests that leatherback nesting decreased by 67.8% between 1995 and 2006. Tag recaptures from fisheries bycatch and strandings have been reported from Cuba, Nicaragua, and the United States. Two leatherbacks were followed with satellite telemetry; 1 swam to Cuba and 1 moved into the northwest Atlantic Ocean. Minimum rates for illegal egg collection from 2000 to 2005 were estimated at 13.0%–21.5%.
Tortuguero beach, Costa Rica (Fig. 1), is well known for its green turtle (Chelonia mydas) nesting population, the largest in the Atlantic Ocean (Carr et al. 1978; Troëng and Rankin 2005). The beach also hosts nesting by leatherback (Dermochelys coriacea) (Troëng et al. 2004), hawksbill (Eretmochelys imbricata) (Carr et al. 1966; Troëng et al. 2005), and, very occasionally, loggerhead turtles (Caretta caretta) (Troëng et al. 1998; Reyes et al. 2001). Carr (1956) described encountering his first-ever leatherback nest at Tortuguero and claims it was the first recorded for Central America. Carr and Ogren (1959) elaborated on leatherback nesting behavior close to the Matina rivermouth (10°07.19′N; 83°11.54′W), located 64 km south of the Tortuguero rivermouth, and documented close to 100% egg collection by local “turtle-egg hunters.” They also described leatherback incubation periods and simple field tests of hatchling orientation at Tortuguero.
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[117:LTNTAT]2.0.CO;2
Leatherback nesting along the Caribbean coast of Costa Rica occurs mainly from late February through mid-July (Chacón 1999). Research and monitoring efforts at Tortuguero were concentrated during the main green turtle nesting season from late June through October, and few leatherback turtles were tagged before the 1980s. In April 1982, in preparation for the first Western Atlantic Turtle Symposium (WATS) meeting, an aerial survey to record leatherback nesting was conducted between the Tortuguero rivermouth (10°35.51′N; 83°31.40′W) and Moin (10°00.18′N; 83°04.90′W), located just north of Limon (Bravo 1983). A total of 405 leatherback nests were recorded, of which 181 were deposited between the Tortuguero rivermouth and Jalova lagoon (10°21.46′N; 83°23.41′W) at the southern limit of Tortuguero National Park (Bravo 1983). In 1985, 4 aerial surveys were undertaken along the same stretch of coast, and 380 of the 659 recorded nests were laid between the Tortuguero rivermouth and Jalova lagoon (Hirth and Ogren 1987). Hirth and Ogren (1987) also conducted night patrols and tagged 82 leatherback turtles at Jalova, 6 at Parismina (10°19.04′N; 83°21.39′W), and 1 at Tortuguero. In 1987, Berry (1987) estimated that 445 leatherback nests were deposited at Jalova (and 8 km north), and total leatherback nesting in Caribbean Costa Rica was 4,987 nests. Approximately 78% or 3,867 of these nests may have been illegally collected (Berry 1987). During 1989–1991, researchers from Drexel and Indiana-Purdue Universities undertook expeditions to study the physiological and nesting ecology of leatherback turtles at Tortuguero. The Caribbean Conservation Corporation (CCC) began an annual leatherback program in 1995 (Campbell et al. 1996).
Observations of tagged individuals show that females can change nesting beaches in Caribbean Central America, at least along the coast from Honduras to Colombia (Troëng et al. 2004). In 2001, aerial surveys along the entire Caribbean coast of Costa Rica and northern Panama were reinitiated, and, in 2002, these surveys were extended to include Chiriqui beach, Panama (9°00.739′N; 81°42.767′W to 8°50.807′N; 81°34.208′W). Four hundred twenty-two leatherback nests were counted between the Tortuguero rivermouth and Moin during these surveys, of which 140 nests were deposited between the Tortuguero rivermouth and Jalova lagoon (Troëng et al. 2004). Based on aerial survey results and annual nest estimates for Tortuguero, Pacuare, and Gandoca beaches, the Caribbean Costa Rica and Panama leatherback rookery is thought to be the fourth largest remaining leatherback rookery worldwide (Troëng et al. 2004). Trend analysis of leatherback nesting at Tortuguero from 1995 to 2003 suggests a decline in nesting (Troëng et al. 2004). Previously described survival threats to the Tortuguero leatherbacks include killing of females on nearby nesting beaches (Troëng et al. 2002), illegal egg collection (Troëng et al. 2004), incidental capture in fisheries in feeding areas and along migratory corridors (Troëng et al. 2004), and, to a lesser extent, the depredation of nesting females by jaguars (Panthera onca) (Troëng 2000).
The objectives of this paper are to 1) provide an updated trend analysis for the 12 years of nest monitoring between 1995 and 2006, 2) describe the survival threats facing leatherback turtles nesting at Tortuguero, and 3) suggest priorities for research and conservation.
Methods
The methodology used to calculate annual nest estimates from track survey results as well as the trend analysis method are described in Troëng et al. (2004). We also calculated 95% confidence limits for each annual nest estimate. Annual nest estimates for 2004–2006 are based on results of 20 track surveys each year, conducted during the leatherback nesting season.
To determine the geographic range of potential Tortuguero leatherback survival threats, migratory movements were studied using flipper tags attached to the rear flippers and satellite telemetry. Flipper tag data were sourced from gray literature reports and the CCC Tortuguero Tagging and Tag Return Databases. Fishermen and sea turtle researchers provided tag recapture locations. If no exact recapture location was provided, we estimated an approximate location based on the description of the recapture site. In 2004, KiwiSat 101 (http://www.sirtrack.com) satellite transmitters were attached to 2 nesting leatherback females using a harness (Eckert and Eckert 1986). Satellite telemetry location data were provided by the Argos satellite system (http://www.argosinc.com).
Since 1998, a sample of leatherback nests have been marked with flagging tapes and monitored to determine hatching and emerging success. Each morning, the marked nests were inspected to determine the fate of the nests. Depredation and illegal egg collection resulted in termination of nest monitoring. Nests were only marked along the 8.6-km beach section between the Tortuguero rivermouth and the mile-5 marker. Nests were excavated 2 days after hatching or 75 days after oviposition (if no hatchling tracks were observed). The numbers of empty eggshells; pipped eggs; live and dead hatchlings; unhatched eggs without embryo, with embryo, and with fully developed embryo; and depredated eggs were determined for each nest. Eggshells amounting to more than 50% of an egg were recorded as an egg. Hatching success is defined as the proportion of yolked eggs producing hatchlings that left the eggshell. Emerging success is defined as the proportion of yolked eggs producing hatchlings that emerged from the sand surface. Hatching and emerging success data were sourced from gray literature reports and published accounts.
Methods for identifying illegal egg collection and estimating annual levels of illegal take of leatherback nests at Tortuguero are outlined in Troëng et al. (2004). The method for identifying leatherback turtles killed by jaguars is detailed in Troëng (2000).
Results
Total leatherback nesting at Tortuguero is estimated at 199–1,623 nests per year. Leatherback nesting decreased between 1995 and 2006 (Fig. 2). The trend analysis suggests an overall decrease of 67.8% in nesting at Tortuguero during the study period.
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[117:LTNTAT]2.0.CO;2
Between 1976 and 2005, in addition to the 657 leatherback females that were flipper tagged (Table 1), 240 female leatherbacks tagged on other nesting beaches in Costa Rica and Panama were registered on the Tortuguero beach. Leatherback turtles originally tagged on the Tortuguero beach have later been observed at Pacuare (B. Dick, pers. comm.) and Gandoca (D. Chacón, pers. comm.) beaches in Costa Rica, and Soropta and Chiriqui beaches in Panama (C. Ordoñez, pers. comm.). These tag returns and those from other beaches in Costa Rica and Panama show that the Tortuguero leatherbacks form part of a regional nesting stock that may extend to beaches as far north as Honduras and as far south as Colombia.
A small number of recaptures of leatherbacks tagged at Tortuguero in fishing activities or through strandings have been reported from Cuba (n = 3), Nicaragua (n = 1) and North Carolina, USA (n = 1) (Fig. 3). The 2 leatherbacks followed through satellite telemetry took distinctly different routes after leaving Tortuguero (Fig. 4). One traveled north to coastal waters south of Cuba, where transmissions indicate it remained for 19 days until contact was lost for unknown reasons. The second leatherback returned to nest 11 nights later, probably on the stretch of beach between the Matina rivermouth and Moin. It then traveled northeast, exited the Caribbean between the islands of Hispaniola and Puerto Rico and swam north into the North Atlantic (Fig. 4). It continued transmitting for 154 days until 27 October 2004.
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[117:LTNTAT]2.0.CO;2
Citation: Chelonian Conservation and Biology 6, 1; 10.2744/1071-8443(2007)6[117:LTNTAT]2.0.CO;2
Emerging success for monitored leatherback nests was lower than the 30.1%–32.5% reported by Leslie et al. (1996) for 1990–1991 during all but 1 of the 1998–2005 nesting seasons (Table 2). During 2000–2005, the minimum annual rate of nests illegally collected varied between 13.0% and 21.5%. Minimum illegal extraction rates were higher (30.6%) on the stretches 5.8 km north and 5.6 km south of the Tortuguero National Park boundaries than within (11.0%) the national park.
More leatherback turtles than green turtles are on Tortuguero beach during February–May. Even so, during 1998–2005, jaguars killed at least 73 green turtles and 2 hawksbill turtles during these months, compared with only 3 leatherback turtles killed during the same period. Two of the leatherbacks killed by jaguars were documented in 1999 (Troëng 2000), and 1 was killed in 2001.
Discussion.
The negative nesting trend observed for leatherbacks at Tortuguero between 1995 and 2006 is a cause for concern. It is possible that a shift away from Tortuguero to other leatherback nesting beaches in the region could be partly to blame for the observed nesting decline. Results from Pacuare and Gandoca beaches in Costa Rica, however, suggest that the nesting decline applies to all 3 beaches with the longest monitoring records (Troëng et al. 2004). We therefore conclude that the nesting trend at Tortuguero represents a real decline in the number of leatherback nests and not only a shift in nesting to nearby beaches. Although the leatherback-nesting decline at Tortuguero is worrisome, the decline is not as severe as for Pacific leatherback populations (Spotila et al. 2000). Possible explanations for the difference in leatherback nesting trends on the Pacific and Caribbean coasts of Costa Rica include 1) higher hatching success on Caribbean beaches due to more disperse nesting, 2) less leatherback bycatch in fisheries in the Atlantic, and 3) less overlap between fishing areas and leatherback habitats in the Atlantic than in the Pacific Ocean (Troëng et al. 2004). The decrease in leatherback nesting at Tortuguero between 1995 and 2006 emphasizes the importance of reducing threats to leatherback survival throughout the population's range to avoid further nesting declines.
The wide-ranging movements by leatherback turtles nesting at Tortuguero, demonstrated by recaptures of tagged animals and individuals tracked by satellite telemetry (Troëng et al. 2006), shows the large geographical area where potential survival threats may impact the population. Fisheries bycatch could cause the decline in leatherback nesting at Tortuguero. Three of 5 tag recaptures and 1 of 2 leatherbacks followed by satellite swam to Cuba. A small number of leatherback turtles are taken in Cuban fisheries (Ottenwalder and Ross 1992), and mortality from fisheries bycatch in Cuban waters has been estimated at 8 leatherback turtles per year, all captured off the southern coast (Moncada et al. 2003). Killing of female turtles on some nesting beaches in Panama (Troëng et al. 2002) is another possible contributing factor to the decline in nesting.
The low hatching success for leatherback nests at Tortuguero in recent years, compared to a growing leatherback population at St. Croix (Dutton et al. 2005), indicates that the Tortuguero population may be unable to withstand additional survival threats and may recover slowly once threats are discontinued. Illegal take of leatherback nests on beaches along the Caribbean coast of Costa Rica and Panama reduces egg survivorship and ultimately will lead to reduced recruitment of leatherback turtles into the adult population. For logistical reasons, we do not promote establishing a hatchery to increase hatching success at Tortuguero, but instead suggest that conservation efforts be directed toward eliminating illegal egg collection and controlling dogs. It appears that current levels of illegal egg take in Caribbean Costa Rica are lower than historical levels of illegal egg take observed in the 1950s (Carr and Ogren 1959) and 1980s (Berry 1987). It is not yet clear, however, if current levels of egg extraction are sustainable given the other survival threats to the Tortuguero leatherback population, such as fisheries bycatch. It does appear that the protected status of Tortuguero National Park, park ranger patrols, and the remote location of the nesting beach have contributed to lower levels of illegal egg take in the national park compared to more accessible beach sections outside the park. To completely eliminate the threat of illegal egg take at Tortuguero and other nesting beaches in Costa Rica and Panama, increased patrols by park rangers and other law enforcers are urgently needed.
It appears that jaguars prefer depredating nesting green turtles rather than attacking the much larger leatherbacks. At the moment, jaguar depredation of leatherback turtles at Tortuguero can only be considered a minor survival threat that on its own is unlikely to cause a population decline.
We conclude that Tortuguero remains an important nesting beach for leatherback turtles. Quantifying the threats to leatherback survival, particularly in water, and increasing enforcement of legislation protecting leatherback turtles and their nests in Costa Rica and Panama represent the most urgent research and conservation priorities.
Location of study area at Tortuguero beach, Costa Rica.
Leatherback nesting trend at Tortuguero, Costa Rica, for 1995–2006 with error bars indicating 95% confidence limits for each annual nest estimate. Solid-line trend curve calculated with a Bayesian regression model, 95% credible intervals marked with dashed lines.
Recapture locations of flipper-tagged Tortuguero leatherback turtles (*).
Tortuguero leatherback post-nesting migrations determined through satellite telemetry.
