Parque Nacional Marino Las Baulas was created in the Pacific Northwest region of Costa Rica in 1991 and ratified by law in 1995. This National Park consists of 3 beaches: Playa Grande, Playa Ventanas, and Playa Langosta (Fig. 1). The park is ranked 1 of the 4 most important nesting sites for leatherback sea turtles (Dermochelys coriacea) in the world (Spotila et al. 2000). Despite the efforts of conservationists and scientists around the world, leatherback sea turtles are disappearing from the oceans. Spotila et al. (1996) reported the decline of leatherbacks in the Pacific Ocean. This situation was linked to intense poaching of eggs, incidental capture of adults by fisheries, and killing of adult turtles on nesting beaches (Pritchard 1982; Spotila et al. 2000).

View Figure

• Download as Powerpoint
• Download Figure

The number of leatherback turtles that come to nest each season at the beaches within Parque Nacional Marino Las Baulas dramatically declined over the past 16 years. In the 1988–1989 nesting season, a total of 1504 females were estimated to have nested in Las Baulas National Park; this total was down to just 68 during the 2002–2003 season. Furthermore, the situation has been aggravated by the imminent threat of development along the thin strip of land behind Playa Grande (the main nesting beach at Las Baulas). Uncontrolled development would considerably increase the environmental impacts on the area and would be detrimental to one of the most important leatherback nesting beaches in the eastern Pacific Ocean.

For 10 years, the Sea Turtle Conservation Project at Playa Langosta studied sea turtle conservation biology, with a particular focus on the leatherback turtle. We documented the number of nesting females, remigration intervals, internesting periods, and reproductive success of leatherback turtles at Playa Langosta. Here we present data collected during 5 nesting seasons (1999–2000 through 2003–2004) to determine the importance of this beach within the Parque Nacional Marino Las Baulas for the survival of the Eastern Pacific leatherback.

METHODS

Playa Langosta is a 1.3-km sand beach located in Guanacaste Province of Costa Rica, approximately 2 km south of the city of Tamarindo (Fig. 1). We divided the beach into 13 sections, 100 m each (by using labeled wooden markers), from south to north. Playa Langosta is dedicated exclusively to conservation and research activities, and tourism, unlike on Playa Grande, is not permitted at night during leatherback nesting season. A 24-hour vigil is maintained on this beach by local and international volunteers to protect the nests from poachers and predation.

We collected nesting data at Playa Langosta every season (October to March) by nightly patrols that started at 1900 and ended at 0400 hours. Nesting females were identified by PIT (passive integrated transponder) tags that we injected in the left and right shoulders of the turtles during oviposition following the method of Dutton and McDonald (1994). The PIT tags used in this study (AVID Marketing Inc., Norco, CA) had 9 or 10 digit identification codes that could be checked by using a tag reader (scanner; AVID). Both PIT tags and scanners were provided by the U.S. National Marine Fisheries Service, Southwest Fisheries Science Center.

We collected morphometric data by using a flexible measuring tape during the oviposition or covering processes. We measured curved carapace length (CCL) from the nucal notch to the terminus of the caudal peduncle and curved carapace width (CCW) immediately behind the armpit extremes, at the widest part of the carapace. More than 70% of the turtles were measured 3 or more times during the nesting season, and we used the mean measurement from each season for comparison. Other nesting data collected included date, time, nesting activity when first observed, and egg count.

A 1-way analysis of variance determined if the turtles grew between seasons and if the size differences between turtles of different nesting seasons were statistically significant.

We recorded the location of every nest on the beach in 3 ways. First, we estimated nest location based on the proximity of the 2 nearest section markers. Second, we located the nest by a zone code. We divided the beach into 3 horizontal zones: zone 1 was located below the high tide line, zone 2 extended from the high tide line to the beginning of the vegetation, and zone 3 consisted of everything behind the vegetation line. Decisions about nest relocation were made contingent upon the turtles' nest-site selection. We relocated turtle clutches in high-risk locations (such as within the tidal zone, in areas of high pedestrian traffic, in the vegetation, or close to the estuary) to the hatchery or the dunes to prevent the death of the clutch. We used large plastic bags to collect and then relocate these eggs. Third, we marked the location of each nest with color-coded, water-resistant aluminum tags that included the date the nest was laid and the nesting female's PIT tag numbers. The color of the tag corresponded to the month and status (relocated or in situ) of the nest.

The internesting period, or the number of days between observed ovipositions, was calculated for each female observed nesting multiple times on Playa Langosta. These calculations excluded intervals of more than 14 days and less than 7 days, as well as aborted nesting attempts, based on Miller (1997), who determined a minimum internesting period of 6 days. We calculated the mean remigration interval (the number of years between 2 sequential nesting seasons) for turtles that were PIT tagged only on Langosta and nested during more than 1 season on this beach. We also calculated the percentage of remigrating turtles vs. new turtles. The number of ovipositions observed during the season for an individual was the observed clutch frequency (OCF).

We opened unhatched eggs and examined them for signs of embryonic development. Two days after the first hatchling emerged or the expected time of emergence (60 days) had passed, we excavated nests to determine the hatching and emergence success following the method of Miller (2000). We staged embryos according to the classification used by Alvarado and Figueroa (1990), and Leslie et al. (1996). Stage 1 was eggs that showed signs of blood vessel formation and eggs that contained a small, unpigmented embryo less than 10 mm long. Stage 2 was embryos that had pigmented eyes and were 10 to 20 mm long. Stage 3 was embryos fully pigmented and greater than 20 mm long. Nest excavation data from 2001–2002 to the 2003–2004 seasons included the hatching and emergence success of natural, as well as relocated, nests.

RESULTS

Nesting

The use of PIT tags allowed identification of each nesting female leatherback on Playa Grande since 1993–1994 and on Playa Langosta since the 1996–1997 nesting season. We recorded 904 encounters and 792 nests at Playa Langosta during the 5 nesting seasons included in this study (from 1999–2000 to 2003–2004). Females on Langosta nested successfully 85.6% of the total emergences observed. The average internesting period was 9.6 days. The number of nesting females peaked during the 2000–2001 season (Fig. 2).

View Figure

• Download as Powerpoint
• Download Figure

Tagging and Remigration

For the 5 seasons, we identified 252 individual leatherbacks, 158 (63%) of which were new turtles (untagged before). Of these new turtles, 79 (50%) were PIT tagged on Playa Langosta and 43 nested exclusively on Langosta, whereas the remaining 36 nested on both Langosta and Grande. During the 1999–2000 nesting season, we identified 58 leatherbacks, of which 46 were new. Of these, 18 were PIT tagged on Langosta and 16 nested exclusively there. Twelve of the 58 turtles identified during this season were remigrants that had nested on the park beaches during previous seasons (20.7%), and 7 (58.33%) had nested before on other park beaches (Playa Grande or Ventanas) but not on Langosta.

During the 2000–2001 nesting season, we identified 105 nesting leatherbacks of which 65 (62%) were new. Of these, 28 were PIT tagged on Langosta and 14 (50%) nested only there, whereas the rest nested in the same season on both Langosta and Grande. Forty turtles (38%) were remigrants to the park beaches, and 23 (57.5%) nested on Langosta for the first time.

In 2001–2002, we identified 13 new turtles, 8 (61.53%) of which were PIT tagged on Langosta and 4 (50%) nested on both Grande and Langosta in the same season.

During the 2002–2003 season, 7 (70%) of the 10 new turtles that were identified were PIT tagged at Langosta, and only 2 nested exclusively on Langosta. During the 2003–2004 nesting season, 24 new turtles were identified, 18 of which were PIT tagged on Langosta, and 7 of which nested only on Langosta.

The total number of individual leatherbacks that nested at Playa Langosta during the 1999–2000 season was 58, for the 2000–2001 season, we recorded 105 females, and during 2001–2002 and 2002–2003, the number of females declined drastically to 22 for each respective season, but the number increased to 44 nesting females in 2003–2004 (Fig. 2).

The percentage of remigrating turtles compared with that of new turtles was 20.7% (n = 12) for the 1999–2000 season, 38.1% (n = 40) for the 2000–2001 season, 40.9% (n = 9) for the 2001–2002 season, 54.5% (n = 12) for the 2002–2003 season, and 45.5% (n = 20) for the 2003–2004 season.

From 1999–2000 to 2003–2004, we PIT tagged 79 individuals on Playa Langosta, of which 10 (12.6%) returned to nest in a later season. Only 27.8% (n = 5) of the turtles PIT tagged during the 1999–2000 nesting season returned to nest in a later season, but 3 of them nested exclusively on Playa Langosta again (60%), 1 nested on both Grande and Langosta, and 1 nested only on Grande. Of the turtles that were PIT tagged during the 2000–2001 season on Playa Langosta, 17.9% (n = 5) returned to nest. Two of them nested only on Langosta, 1 nested on both beaches, and 2 nested only on Grande. The mean remigration interval of those 10 turtles that returned to nest during the period of this study was 3.2 ± 0.7 years.

Morphometrics and Reproductive Output

Leatherbacks nesting at Playa Langosta had a range of CCL from 125 to 164 cm, and a range of CCW from 90 to 116 cm. There was no significant difference in mean CCL (ANOVA, F = 0.95, p ≤ 0.4835) or CCW (ANOVA, F = 0.71, p ≤ 0.5811) between nesting seasons. The average number of eggs per clutch during each season ranged from 54.6 to 69.1 (Table 1).

Table 1. Morphometric and reproductive data for leatherbacks nesting at Playa Langosta from the 1999–2000 to the 2003–04 nesting season.^a

View Fullscreen

Nesting Frequency

The OCF for each season was as follows: 1999–2000, mean = 3.01 ± 2.1 SD, range = 1–8; 2000–2001, mean = 2.9 ± 2.2 SD, range = 1–8; 2001–2002, mean = 3.7 ± 2.5 SD, range = 1–9; 2002–2003, mean = 4.6 ± 3.5 SD, range = 1–11; and 2003–2004, mean = 3.3 ± 2.8 SD, range = 1–11.

Hatching and Emergence Success

We recorded 792 nests during this study and relocated 242 nests (31%) to safe places on the beach. The other nests were left in situ to incubate under natural conditions. A total of 220 natural nests, and 86 relocated nests were later excavated to determine hatching and emergence success.

Our results for in situ nests showed a decline in the percentage of both hatching and emergence success over the first 4 seasons, with an increase in these variables in the last season of this study (Table 2). For the first 4 seasons, there was a decrease in percentage of hatching success from 68.9% (1999–2000) to 30.4% (2002–2003) in the in situ nests. A dramatic decline from 57.2% to 9.3% in emergence success for these respective seasons also occurred. Our results for hatching and emergence success for relocated nests were, in general, better than for in situ nests (Table 2).

Table 2. Hatching and emergence success for leatherback nests in situ on Playa Langosta over 5 nesting seasons and relocated nests over 3 nesting seasons.

View Fullscreen

DISCUSSION

Playa Langosta is 1 of 3 leatherback nesting beaches that comprise Parque Nacional Marino Las Baulas, an important nesting area in the Eastern Pacific. It is the only beach in this protected area that has no nighttime ecotourism and is completely protected from poaching, with little or no beach development behind the vegetation. Thus, it serves as a good comparison of relatively undisturbed natural nesting activity for leatherbacks in this area. Of the leatherback turtles recorded on Playa Langosta during the period of this study, 63% were new turtles that had never before been observed nesting on these beaches and 50% of those turtles were PIT tagged on Playa Langosta. As demonstrated by the tagging and remigration data, a large percentage (45.6%) of leatherbacks that were tagged on Playa Langosta also nested on the other beaches within the park (Playa Grande and Playa Ventanas). Of the 18 new leatherbacks PIT tagged during the 1999–2000 season on Playa Langosta, 11.6% nested at least one time on Playa Grande during the same season.

This percentage was similar to that reported by Chavez et al. (1996) for the 1991–1992 nesting season, during which 10% of the turtles that were tagged on Playa Langosta were seen nesting in the same season on Playa Grande. During the following nesting seasons, the percentage of turtles tagged on Playa Langosta that nested on both Langosta and Grande increased to more than 50%, and movements of nesting females between Langosta and Grande are frequent. However, more research is needed in this area to discern the factors influencing beach nesting fidelity.

Morphometric data for the females nesting on Playa Langosta during the 5 seasons of this study were similar to data documented by Chavez et al. (1996) during the 1991–1992 nesting season and also were similar to the data recorded on Playa Grande (Steyermark et al. 1996; Reina et al. 2002). The females nesting on Playa Langosta were smaller on average (146 cm) than those on the Caribbean coast of Costa Rica: Tortuguero (156.2 cm) (Leslie et al. 1996), and Jalova (152.8 cm) (Hirth and Ogren 1987). In addition, a comparison of these data with morphometric data from other populations around the world shows that turtles nesting at Las Baulas in the Eastern Pacific are among the smallest in the world (Reina et al. 2002). Hughes (1996) reported the mean CCL for Tongoland, South Africa as 159.6 cm. Leatherbacks at St. Croix averaged 152.9 cm (Dutton et al. 1994) and at Culebra averaged 154 cm (Tucker and Frazer 1991). However, size is not always an indicator of age (especially when comparing Pacific and Atlantic leatherbacks), and it would not be possible to conclude that the average nesting females in this population are younger than those in these other Atlantic populations. Yet, the high percentage of new untagged females probably indicated new first-time nesters that had just matured and were nesting for the first time. Thus, there appeared to be important recruitment to this population, which was probably because of the several years of beach protection that preceded this study.

There was no significant difference in mean CCL or CCW between nesting seasons, indicating that there was minimal to no apparent change in average population body size in the 3-year interseasonal period. According to Price et al. (2004), adult female leatherbacks recorded at Las Baulas from 1993–1994 to 2001–2002 nesting seasons had a very low growth rate.

The observed internesting period of approximately 9 days for leatherbacks at Playa Langosta included in this study was similar to values reported by Chaves et al. (1996) (9 days) for the 1991–1992 Playa Langosta nesting season and Steyermark et al. (1996) (9 days) for the 1993–1994 Playa Grande nesting season, Boulon et al. (1996) on St. Croix, and Tucker and Frazer (1991) on Culebra.

The mean OCF at Playa Langosta for 1999–2000 (3.0) and 2000–2001 (2.9) were lower than mean OCF values reported for 2001–2002 (3.7) and 2002–2003 (4.6). During the first 2 nesting seasons, most of the turtles nested just one time at Playa Langosta. Of the 58 females that nested on Playa Langosta during the 1999–2000 season, 25 were observed nesting once and 10 were observed nesting twice. A similar situation occurred during the 2000–2001 nesting season. However, a gradual increase in OCF values occurred during 2001–2002 and 2002–2003, but these values decreased slightly during the 2003–2004 season, probably because 18 of the 44 turtles reported for this season nested only one time on Playa Langosta. Chaves et al. (1996) reported a mean OCF of 3.3 on this beach, and Boulon et al. (1996) recorded an OCF of 5.6 on St. Croix. Most of the leatherback turtles that nested on Playa Langosta also nested on Playa Grande. The increase in clutch frequency during more recent seasons could be related to longer and more effective survey periods at Playa Langosta.

In the 1999–2000, the hatching success value (68.9%) for in situ nests were similar to those of Leslie et al. (1996), who reported a hatching success of 70% in 1990 at Tortuguero. We relocated only the nests that were laid in high-risk locations to improve hatchling production. Our results showed that the hatching success for relocated nests in 2001–2002 (52.4%), and 2003–2004 (51.4%) were higher than nests laid in situ during the same seasons (33.2% and 47.9%), respectively. Hatching success for leatherback nests at Playa Grande has been about 50% (Bell et al. 2004). Hatching success of our relocated nests was generally higher than the hatching success of in situ nests.

Excavated nests showed high embryonic mortality in the first and third stages of development according to the embryo stage classification used by Alvarado and Figueroa (1990) and Leslie et al. (1996). It is possible that premature death at these stages was caused by extreme embryonic sensitivity to temperature or other environmental factors like moisture. In the 2003–2004 nesting season, hatching and emergence success improved. Interestingly, the lowest hatching successes occurred in 2001–2002 and 2002–2003 when the number of nesting turtles was also the lowest. However, more research is needed in this area to determine what factors, including oceanic conditions, may be impacting developmental success.

Conservation efforts at Playa Langosta reduced nest loss because of poaching, salt-water infiltration, and vegetation infiltration. We are working now to increase the number of hatchlings produced every year. To ensure reproductive success for sea turtles, nesting areas must be protected against uncontrolled beach development. In addition, fishing activity in migratory pathways must also be strictly protected and regulated to allow turtles to migrate safety to the nesting beaches. The research conducted at Playa Langosta and within Las Baulas National Park will hopefully improve the chances of survival for the Eastern Pacific leatherback turtle population. We strongly recommend that the Costa Rican government intensify efforts to consolidate this National Marine Park to protect one of the most important leatherback nesting sites in the eastern Pacific.