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Marine megafauna such as sea turtles, seabirds, and pinnipeds that use terrestrial systems for breeding are particularly vulnerable to overexploitation. Effective conservation of these areas is a crucial aspect of sea turtle conservation efforts around the world. In southern Costa Rica, nesting beaches experience high levels of nest predation from coatis, vultures, dogs, crabs, pigs, and humans (Drake 1996). However, unlike other turtle nesting beaches in Costa Rica such as Tortuguero, Ostional, Nancite and Playa Grande, little is known regarding the extent of predation. This is an important data gap as predation of sea turtle eggs can destroy up to 100% of nests (Spotila 2004) and may have population level consequences in cases where predation is high.

Here, I report on research conducted on four nesting beaches on the Osa peninsula, Costa Rica that aimed to better understand the nesting ecology of sea turtles inhabiting the region. Our objectives were to: 1) document the numbers of clutches laid by each of the four turtle species known to use these beaches and how this varies temporally and spatially; 2) document the frequency of nest predation events and their temporal and spatial variation; and 3) assess a method that seeks to mitigate nesting predation levels. For statistical analysis I used a chi-square test to compare the proportions of different turtle species and nest predators at the different beaches. However, the data should be interpreted cautiously because of the short temporal duration of my study.

Figure 1. Location of four study beaches in the Osa Peninsula, Costa Rica.

Monitoring was conducted between July and December 2010 along four beaches on the Osa peninsula: Playa Piro (N83.339397°, W083.339397°; 2 km), Pejeperro (N084.16336°, W083.393331°; 5 km), Rio Oro (N084.27778°, W083.416789°; 2.4 km), and Carate (N084.37661°, W083.443178°; 5 km, Fig. 1). Each beach was marked with a number at the beginning of each 100 meter section. Daily beach monitoring was carried out with the assistance of volunteers from the organizations ‘Osa Conservation’ (formally ‘Friends of the Osa’) and ‘Frontier.’ The beaches were surveyed each morning from 05:30 am until the whole study area had been surveyed. When a new track or nest was located, track width, track form (symmetrical or asymmetrical-in terms of the flipper marks), and location were recorded. Track width and form were used to identify the species of turtle following Pritchard et al. (1983).

Evidence of nest predation was recorded daily during the morning surveys. We identified paw prints and determined the nest predators. The white-nosed coati (Nasua narica) typically digs a small hole into a nest. Adult coati paw prints are 4.5 cm wide, making them more oblong and at least 1-1.5 cm smaller in width than raccoon paw prints.

Dog prints are customarily accompanied by sand thrown in one or more directions with a broad opening into the side of the nest (Fowler 1979). Human prints usually occur with one large and various smaller holes close by, where a probing stick has been used to search for a nest. Nests were classified as ‘new’ (i.e., from that morning or the night before) or ‘old’ (i.e., a nest laid before the previous night).

Nest Protection - Prior research has shown that placing wire netting over nests successfully reduces nest predation (Yerli et al. 1997). Twelve nests on Playa Piro were protected with a 1-m square section of 10 x 10 cm wire netting and secured with eight metal stakes to assess if this could protect nests and if the level of protection varied by predator species. Netting protection was placed over nests at a 10-cm depth to lessen the chance that predators could locate them. The size of the holes in the netting allowed the passage of hatchlings, and nettings were removed after all hatchlings had left the nest.

Nesting species, numbers and seasonality - From 5 July to 12 December 2010, 1943 nests were documented along the four beaches. Nest production varied greatly between the beaches (Fig. 2). The highest number of nests was recorded at Pejeperro, and the lowest at Rio Oro (Table 1). L. olivacea nesting peaked during August (N = 334) and September (N = 253) (See Fig. 3). These nests accounted for 52% of L. olivacea nests for the whole study period. C. mydas nesting also peaked in August (N = 23) and September (N = 20). These nests accounted for 54% of C. mydas nests for the whole study period. ‘Unknown,’ E. imbricata, and D. coriacea nesting peaked in July (N = 24) and August (N = 29) accounting for 79% of ‘unknown,’ E. imbricata and D. coriacea nests for the whole study period. Due to the high number of L. olivacea nests recorded in July (N = 171), this may suggest that nesting was also occurring before the onset of the surveys.

Figure 2. Sea turtle nests laid per 10 meter section on four beaches during 2011 on the Osa Peninsula, Costa Rica.

The most abundant sea turtle was L. olivacea with 1,300 nests found, whilst C. mydas was the second most common with 98 nests located. At least four E. imbricata nests were found (at beaches Pejeperro and Carate), and only one nesting D. coriacea was encountered (at Carate beach). Of the total, we were unable to accurately identify 526 nests as they were indistinguishable between L. olivacea and E. imbricata. There was a significant difference (χ2 = 28.92, df = 3, P < 0.001) between the proportions of the two main species on the four beaches (numbers of the two other species were too low to analyze).

Figure 3. Seasonality of nesting during 2011 on the Osa Peninsula, Costa Rica.

Nest predation - Of the 1,943 nests in total, 39% (N = 751) were predated (Table 1). Predation events varied significantly between beaches using a chi-squared test (χ2 = 28.92, df = 3, P < 0.001); predation events were highest at Rio Oro and Pejeperro (50% of all nests), followed by Piro (47%) and were lowest at Carate (18% of all nests).

Table 1. Nesting species and nest predation frequency data in the Osa Peninsula, Costa Rica. % values are relative to total number of nests. LO = L. olivacea, CM = C. mydas, EI = E. imbricata, DC = D. coriacea

Overall, nests were most commonly predated by dogs (39%), although the predator was unknown in 24% of cases (Table 2). At Piro and Pejeperro, the most common predators of nests were dogs (36% and 52%, respectively). At Rio Oro, coatis were the most dominant predator (18%), whereas in Carate humans were the most common predator (55%). Pigs were only identified as a predator at Rio Oro beach (15%). There was a significant difference in the proportions of predated nests between the four beaches (χ2 = 153.39, df = 6, P << 0.001).

Table 2. Nest predation data from 2011 on the Osa Peninsula, Costa Rica.

The proportion of nests excavated by predators on all four beaches was higher than the records from work done 14 years prior to this study by Drake (30% in 1996 compared to 39% in 2011). Although this is not much of an increase, especially with such a short temporal duration, our predation rates are very high and this is a cause for concern.

Compared to Drake (1996), the nest predation by dogs has decreased (from 53% to 39%). Poaching has also decreased (from 39% to 28%). However, poaching is still high and has actually increased at Carate beach. This may be due to easier road access to Carate. There are higher predation rates by coatis at Piro than at any of the other beaches in 2011 (N = 26 depredated nests), and the number of coati-predated nests has increased slightly from 12% to 19%.

Fifteen years after it was noted in Drake (1996), the illegal consumption of turtle eggs continues on the Osa peninsula. Humans poached 28% of nests. Rio Oro has been classified as a wildlife refuge. However, many people come here for illegal harvesting, not just to collect eggs for personal consumption but to also sell them in local bars and restaurants. This may change if Ministry of the Environment and Energy (MINAE) patrols become more frequent. Previous studies have shown that their presence on Costa Rican beaches can successfully deter poachers (Fowler 1979).

Conversations with local residents suggested that poachers were covering up excavated nests to hide the fact that they had poached them. Some volunteers were unaware of this at the time and marked the predated nests as ‘unknown,’ which may be why this percentage of nest classifications is so high (48%). Thus, a better understanding of this poaching behavior is needed.

Dogs at Pejeperro caused the loss of 52% of nests on this beach, more than half of the total. This figure is up 16% from 1996 (Drake 1996). Since dogs can locate eggs at all times during incubation (Fowler 1979), nests are never safe as long as dogs are freely roaming the beach.

On the relatively few occasions that researchers were able to patrol Rio Oro beach, a total of nine nests predated by pigs were recorded. The farm at Rio Oro keeps pigs that occasionally escape. Ghost crabs (Ocypode quadrata) are the second most common sea turtle nest predator (Antworth et al. 2006). Nest excavations were not conducted during this study and so we were unable to examine the effects of ghost crabs as nest predators. Coati predation was not high overall. However, this species has previously been described as a predator of the ghost crab, which can reach high densities in the absence of heavy predation. Antworth et al. (2006) noted that natural predators are important components in maintaining community structure and composition, however it is important that coati and ghost crab numbers are monitored.

On Piro beach, 12 wire nettings were placed over new nests. The predation rate of these nests was 0%, although some nests showed signs of attempted predation by dogs. Of the protected nests, three of them had dog paw prints and digging holes around the netting. These 12 nests were compared to five control nests also on Piro with no protection. The predation rate of these nests was 100%, all by dogs. Netting has been used in previous studies to protect in situ nests (Antworth et al. 2006). The 100% success rate during this study suggests that this is a useful method of nest protection, especially when compared to the five control nests which were 100% predated.

Protecting nests in situ may be better from a biological point of view rather than with that of relocation of eggs (Lopez-Castro 2004). However, some studies show that the altered magnetic environment exerted by the metal netting might affect subsequent magnetic orientation and navigation behavior of hatchlings (Irwin et al. 2004). Therefore, for the future it would be better to construct protective cages out of magnetically inert materials. This would provide a way to deter predators without risking unintended behavioral consequences of distorting the ambient electromagnetic field.

Nesting species composition and seasonality - To date, little research has been done on nesting trends on the Costa Rican beaches of Piro, Pejeperro, Rio Oro and Carate. Four sea turtle species are found on the Osa peninsula, highlighting the importance of these four nesting beaches. Pejeperro and Carate beaches receive the greatest number of nests of all four beaches, and olive ridley turtles were the most frequently encountered.

Castro (1986) had noted that hatching success rates at non-arribada nesting beaches, such as the four study sites, might be much higher than at arribada sites. This supports the hypothesis that non-arribada nesting beaches maintain healthy population levels of L. olivacea, which therefore increases the biological importance of nesting beaches such as Piro, Pejeperro, Rio Oro and Carate.

Rio Oro lies at one end of the Pejeperro lagoon. High rainfall levels from August to October sometimes caused lagoon waters to rise so that Rio Oro was inaccessible. I believe that if researchers had been able to visit this site more often, a higher number of L. olivacea nests may have been recorded. Many L. olivacea nests are found near river mouths, lagoons, or estuaries (Drake 1996). There are lagoons at Pejeperro and Carate beaches. This may explain the large number of nests found at these two sites in comparison to Piro, which does not have a lagoon and is the farthest away from one. In addition, this beach is the narrowest, which leaves limited area above the high tide line where most individuals nest.

In the month of September, Pejeperrito lagoon overfilled and broke open, forming a river through the beach and into the sea. This happened at Sector 24 of Carate Beach, which precluded monitoring the remaining 26 sectors. Local residents informed us that many turtle tracks on the inaccessible part of the beach had been seen, suggesting that many nests went unrecorded.

Rain on the Osa peninsula peaks from August-October, when the most females nest (Drake 1996). This is especially true for L. olivacea and C. mydas. Due to the low number of E. imbricata and D. coriacea nesting during the study period, no obvious seasonal trend was identified. There may have been more E. imbricata nesting individuals present than recorded. However, without seeing the actual turtle, it is difficult to distinguish their tracks from L. olivacea. Therefore, because of this, nest counts for olive ridleys and hawksbills should be considered minimum estimates.

I recommend the continued in situ protection of nests with netting at Piro, and the introduction of this technique to Pejeperro, Rio Oro and Carate. This deters dogs, coatis, pigs, and possibly humans from digging up the nests. The efforts involved in taking care of in situ nests may be greater because constant beach patrols are required. This study had a short temporal duration (6 months) with limited access to Rio Oro beach. Because of this, sample sizes were small. The data suggest that there are probably sea turtles nesting on the beaches outside of the study area and period. A more extensive study into the area would be of great benefit. It is important that data collection on these beaches continues to monitor further changes in nest predation rates. Involvement of the local people to protect the nesting turtles and the hatchlings is essential in the running of this project. Eagerness by individuals in helping the area has been demonstrated. Continued and more frequent MINAE-led patrols are essential to deter poachers from taking turtle eggs, especially those poachers who travel from outside of the Osa peninsula and in turn harvest more eggs.

Acknowledgements. I thank Dr. Martin Jones for his guidance during this study and his review of the manuscript. I am grateful to all the leaders and volunteers from the organizations ‘Frontier’ and ‘Osa Conservation’ (formerly Friends of the Osa) for their help in the field. I also thank Tracey Pinel for her help with the statistics. This research was supported by the organization ‘Osa Conservation.’ This study was used to complete an M.Sc. in Animal Behaviour at Manchester Metropolitan University. I also thank the two anonymous reviewers of this manuscript for their helpful comments.

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