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The Pretoma hatchery project was launched in 2001, and following this, the Playa Caletas-Ario National Wildlife Refuge was established in 2003 and later nationally recognized by the government in 2006. In 2008, Pretoma began hosting student interns to conduct scientific studies through The Science Exchange Sea Turtle Internship Program (<http://www.thescienceexchange.com>). Since the Caletas’s camp and hatchery was established, a decline in nest predation rates from 90% to 20% has been achieved through the relocation of over 6,000 nests to the protected hatchery site. A total of 235,000 hatchlings have been released since 2001 (Pretoma 2011).

Four out of the seven sea turtle species nest at Playa Caletas, however, it is primarily an olive ridley (Lepidochelys olivacea) sea turtle nesting site. Here, this species mainly exhibits solitary nesting (Gaos et al. 2006) with some 1,000-1,500 nests per season (July – March, annually). The olive ridley is the second smallest of all sea turtle species and although it has the largest extant population size of all sea turtles (Witherington 2006), the olive ridley is classified as a threatened species (Abreu-Grobois & Plotkin 2008). The Eastern Pacific leatherback (Dermochelys coriacea) is listed as a Critically Endangered sea turtle species with its nesting population having been diminished in recent decades (Sarti 2000). Pacific green turtles (Chelonia mydas) and hawksbills (Eretmochelys imbricata), which are classified as Endangered and Critically Endangered, respectively (Mortimer & Donnelly 2008; Seminoff 2004), also nest at Playa Caletas.

The predation of sea turtles and nests by coyotes, skunks, raccoons, and crabs is one of the greatest threats at Playa Caletas. Although illegal, poachers also patrol the beach, collecting the eggs for food as well as selling them for a source of income. Another threat to turtles at Caletas is garbage. Plastics are not biodegradable in the marine environment or in the intestinal fluids of sea turtles (Mueller et al. 2011). From causing entanglement, severe lesions, digestive tract obstruction through ingestion, and interference with nesting and hatchling emergence, marine plastic pollution is causing deleterious effects to populations of endangered sea turtles worldwide (Bugoni et al. 2001; Carr 1987; Wabnitz & Nichols 2010).

For many years, observations of large amounts of garbage, specifically plastics, have concerned Pretoma workers at Caletas. Local people in San Francisco de Coyote, the nearest town to Caletas, report that many people dump and burn their trash right next to the Rio Bongo; after storms, it is easy to see the sediment and garbage from the river flowing towards the beach. The heavily- used road behind the beach at Caletas may be another local source of solid waste. Strong ocean currents and winds may also bring in trash from other areas.

Caletas is not unique. Worldwide generation of plastic waste reached 31 million tons in 2010, and only 8% was recycled (EPA 2012). The remaining plastic waste may easily wash into the waterways and water bodies if not properly disposed of or recycled. Up to 80% of the waste that ends up in the ocean to become marine debris consists of short-lived plastic products permanently discarded within one year of manufacture (Wabnitz & Nichols 2010).

This was the first shoreline man-made debris survey for Playa Caletas. In several marine debris studies within the past few years, plastic has been ranked as the most abundant material found on beaches within sea turtle nesting regions (Bravo et al. 2009; Claereboudt 2004; Nakashima et al. 2011; Zhou et al. 2011). Although aerial photography has become an innovative marine debris survey tool (Nakashima et al. 2011), most surveys are performed on the ground, within previously selected transects (Bravo et al. 2009; Claereboudt 2004). In one study done on several Chilean beaches, plastic debris was found most commonly (2 pieces/m) from the wrack line to the back beach barrier (Bravo et al. 2009). On the shores of the South China Sea, one study concluded that 95% of all beached marine debris observed came from land- based sources, with 42% of it being plastics (Zhou et al. 2011).

Due to the heightened concern of the effects of man-made marine debris pollution on turtle nesting at Playa Caletas, we addressed several research questions: (1) what are the characteristics of the garbage in terms of size (micro, macro, large) and type (bottles, wrappers, etc.), (2) how does the density of man-made debris relate to precipitation events, (3) how is the density of trash distributed spatially and (4) how can managers use this information to improve sea turtle habitat?

Location. Playa Caletas is an isolated, dark-sand beach 7 km away from the nearest town, San Francisco de Coyote. The beach is located on the Nicoya Peninsula in the Guanacaste region of Costa Rica (Fig. 1).

Figure 1. The sea turtle nesting beach at Playa Caletas, Guanacaste, Costa Rica.

Transect Surveys: man-made debris sampling and weather recording. We conducted surveys at low tide over 22 days between July 17 through August 29 during a portion of the 2011 olive ridley/ green turtle nesting season when weather and tides permitted. For Pretoma’s monitoring program, the beach is divided into 55 sectors identified by permanent marked signs approximately 50 - 100 m apart; beach width also varies (see Fig. 2). We sampled each of the 55 sectors with no repeats (Fig. 2), for a total coverage of 1,100 m of the 5,500 m shoreline. Each of the 55 sectors had 20 predefined transects 5-meters wide varying in length from the water’s edge to the first barrier (i.e., heavy vegetation, tree, or road). We chose four transects within each sector to survey from a random number table containing numbers 1-20. Between four and 52 transects were assessed each day of observation, resulting in total beach coverage of 20% (n = 220). The beach was divided into geographic areas by land use - north (sectors 1-17), middle (sectors 18-29), and south (sectors 30-55). The north zone consisted of a point and entrance to the beach, the middle zone was in front of the camp with the heaviest human traffic, and the south was a marshy area leading to the river mouth. In total we sampled 68 transects in the north, 48 in the middle, and 104 in the south zones of the beach. For each transect, we recorded garbage quantity, length at the longest dimension (cm), type, transect distance from the water’s edge to the first barrier, substrate type, land use, time, current weather, and description of the weather within the 12 hours prior to sampling.

Figure 2. The 55 sector boundaries of Playa Caletas, Guanacaste, Costa Rica (Google Earth).

Large trash was defined as any item longer than 30 cm. Man-made items longer than 30 cm were recorded on a separate sheet along with the status (sunken, stranded, etc.). Man-made macro debris was considered to be smaller or equal to 30 cm but greater than 1.0 cm in size. Finally, man-made micro debris items were defined as being from 5.0 mm up to 1.0 cm in length. A blind coin toss determined the locations of micro sampling within the same random transects used for macro and large sampling. We placed a 0.0625 m2 quadrat where the coin landed and after removing macro and large debris items, the top ~3 cm of sand was scooped up and then sieved through a screen with 5.0 mm mesh spacing. We then identified micro fragments that remained in the sieve using a magnifying glass and calipers. A lack of micro debris during the initial phase of the study resulted in reducing the frequency from daily to weekly sampling, or 11 sampling events over the study period.

Density. Man-made macro and large debris item densities (number of debris items/m2) per transect were calculated according to this formula:

Where N is the number of debris items observed; L is the recorded transect length, and W is the recorded transect width (5.0 m). Man-made micro debris item densities (number of debris items/m3) per transect were calculated using:

Where A is the quadrat sampling area 0.0625 m² and D is the sampled sand depth of 0.03 m.


Man-made Debris Type Categories. We chose the categories of garbage type based on the NOAA draft Marine Debris Shoreline Survey Field Guide (Opfer et al. 2012) and we added other categories as surveying proceeded. The main debris categories were divided into plastic, metal, rubber, paper/processed lumber, and cloth/fabric.

Sea Turtle Activity Observations. Turtle activity observations were recorded nightly by the primary author and Pretoma volunteers by walking the entire beach in pairs during two, two-hour shifts. Each individual turtle encountered was identified, measured, and tagged if needed. Unfortunately, not every emerging turtle could be processed due to the disproportional land-to-patroller ratio; however, for the entire beach all remnant activity observed was classified as a successful nesting event, aborted nest, false crawl, poached nest, or predated nest. All activities represented the potential for female turtles to encounter trash on the beach, and the successful nests also represented the potential for emerging hatchlings to encounter garbage. For Pretoma’s monitoring program the beach is also informally divided into three “zones” to describe how far the turtles crawl up the slope of the beach. The zones vary in beach width but are defined as being from the shoreline to mid-beach (Zone 1) from mid-beach to the wrack line (Zone 2) and from the wrack line through the vegetation (Zone 3). We recorded the species, tag number, morphometrics, activity, disturbances, zone number, the sector number, date, time, and tide for every turtle, track and nest observed.

Precipitation events related to garbage. In order to test the effect of precipitation on the deposition of solid waste, wet and dry samples were statistically compared. The sample was classified as affected by precipitation if there had been rain the night prior to assessment and dry if there was no rain during the previous 12 hours, and the sample was not included in the precipitation analysis if there was a mixture of weather or it was currently raining/drizzling.

Statistical Analysis. We used ANOVA tests to test for significant differences in mean garbage densities per geographic area (north, middle, and south). We also tested for differences in the mean frequencies of all sea turtle activities (i.e., false crawl, aborted nest, successful nest observances, etc.) and the mean frequencies of sea turtle nesting success per geographic area (<http://www.physics.csbsju.edu/stats/anova.html>). A two-tailed, unpaired Student’s T-test was run in Excel to test whether there was a significant difference between trash densities collected during dry and wet weather.

Average density of garbage in each size category. During our surveys, we found 6,116 pieces of trash comprised of 5,906 pieces of macro man-made debris, 209 pieces of large man-made debris, and only one piece of micro man-made debris. The mean macro debris density was 0.14 pieces/m² and large debris density was 0.01 pieces/m².

Frequency of garbage in each type category. Of all marine garbage assessed, 98.2% was classified as plastic. Combining the macro and large size categories, the top seven types most frequently found were: 1) plastic bottles, 2) plastic fragments, 3) Styrofoam/foam fragments, 4) plastic caps, 5) wrappers, 6) shoe/ shoe parts, and 7) bowls, containers and buckets (Fig. 3). Other notable categories with less than 100 pieces found are listed in Table 1.

Table 1. The total number of macro and large garbage pieces surveyed per category type during the study period.

Figure 3. The total number of macro and large garbage pieces surveyed per category type during the study period.

Precipitation events related to garbage. There were 11 days noted as being affected by precipitation and 9 were considered dry. There was an observed trend of increased density of solid waste on days affected by precipitation with a mean of 0.257 pieces/m², compared to the density of 0.169 pieces/m² observed during dry weather. One major storm resulted in the deposition of 853 pieces of macro man-made debris onto one transect alone, and the highest density transect observed (4.574/m²) occurred after a rain event. We expected that precipitation would have an effect on the density of garbage, but we did not find a significant increase in debris with wet weather (p = 0.431).

Spatial distribution of garbage. Trash density slightly decreased from north to south with a steady decline starting at sector 29, which corresponded with the beginning of a fence along the back of the beach to the river mouth (Fig. 4). Transect PC-Z_12-8_15 in Sector 26 had the largest macro garbage density at 4.574/m². Transect PC-F_13-8_19 in Sector 6 had the highest density of large solid waste with a density of 0.076 pieces/m2.

Figure 4. Combined macro and large garbage densities per sector.

Distribution of sea turtle activity. During the study, the greatest female sea turtle activity (successful nests, aborted nests, false crawls, poached, predated) occurred in sectors 26 - 43 (in the middle to south regions) as well as at the mouth of the river in sectors 52 - 55 (in the south region) (Fig. 5). In terms of distance from the shoreline, 74% of nests (during the study period) were found in zone 3 above the wrack line. Unfortunately, the sectors and zones were not measured for area so a density comparison is not possible.

Figure 5. The number of turtle activities (successful nests, aborted nests, false crawls, poached, predated) and successful nests only per sector.

Spatial analysis of observed turtle activity and garbage density between zones. We looked at the variability of mean trash density and mean turtle activity frequency between geographic areas (north, middle, and south). First we summed the number of observed turtle activities in each sector over the study period (frequency) and calculated the mean activity per sector for each geographic area. From this we extracted mean observed successful nests for each geographic area. We combined the macro and large item densities per transect, and calculated the mean garbage density per geographic area (Table 2).

Table 2. Mean frequency of observed turtle activity and nests, and garbage densities in each geographic area during the study period, with standard deviations.

The first ANOVA test suggests that the frequency of all sea turtle activities (indicating female presence on the beach) in the north area was significantly lower than both the middle and south areas (p < 0.05). However, the frequency of sea turtle activities was not significantly lower in the middle when compared to the south.

We also extracted the “successful nest” data (or potential hatchling presence on the beach) from all turtle activities and the frequency in the north area was again significantly lower than both the middle and the south areas (p < 0.05). The successful nest frequency, however, was not significantly lower in the north region when compared to the middle region.

A separate ANOVA test showed the density of man-made debris (macro and large combined) in the south area was significantly lower than the other two areas (p < 0.01). However, the garbage density in the north area was not significantly lower than density in the middle area.

Conclusions. The south area of Playa Caletas had significantly less trash than either the middle or north areas. Significantly more sea turtle activity occurred in the middle area, near the Pretoma camp, and in the south area, near the river mouth, when compared to the north area. However, the middle area had both the greatest number of successful turtle nests and the highest mean garbage density. We cannot directly correlate the sea turtle activity to man- made debris presence because the sampling areas and methods were not the same. In the future, measuring the areas of the turtle sectors that Pretoma monitors for sea turtle activity/nesting and analyzing sea turtle and solid waste patterns over several years would give a better picture of any physical or behavioral impacts to nesting sea turtles from garbage.

The abundance of bottles, caps, wrappers, etc. found in this study indicates that most beach trash at Playa Caletas is disposable plastic products. The most common size of the trash was 17.8 - 25.5 cm, which matches the most abundant trash category - personal beverage bottles. There is an urgent need worldwide to decrease the use of disposable plastic bottles for beverages, to increase the use of reusable beverage bottles, and to increase the recycling of plastic beverage bottles to lessen their presence in the environment and reduce the potential negative impacts of these plastic items on humans and wildlife.

Additional Observations. Most garbage that we found was located on the surface of the sand and the most common size of the objects was > 1.0 cm in length. The lack of broken down and buried pieces of man-made marine debris provides evidence that Playa Caletas may just be the first of many stops on the journey of these plastics around the oceans. The proximity of the Rio Bongo as well as the strong Pacific Ocean currents (capable of transporting whole trees on and off the beach within one day) are among the reasons that we think that the garbage at Playa Caletas is both new and temporary.

Melted man-made macro debris is an indication of trash burning, which is a common practice in places with no trash collection service; this is the situation at Playa Caletas. There were 15 pieces of melted debris found after storm activities compared to five pieces of melted plastic found during dry weather, perhaps indicating that the river brought burned trash to the beach during the storms.

We observed that almost all man-made macro debris found was located at or above the wrack line (Pretoma zone 3); this was found in other studies as well (Bravo et al. 2009; Gregory 2009). At Caletas, zone 3 is also the most common olive ridley nesting area. Surveyors in zone 3 observed a high density of garbage at the interface of the beach and the vegetation line. Therefore, shoreline survey protocols should focus on this ‘edge effect,’ where we found both high trash density and turtle activity.

Suggestions for improving the survey. There were some challenges presented while surveying Playa Caletas for man-made debris. First, the sampled garbage was not removed from the beach because no system of trash collection exists for Playa Caletas or the community of San Francisco de Coyote. Inevitably, counting previously surveyed trash that may have moved into adjacent transects may have occurred, possibly introducing error. Ideally, the garbage should be removed after being surveyed, but future surveys should be carefully planned to minimize introduced errors from beach clean ups when attempting to accurately quantify the natural densities and accumulation rates of solid waste.

We recommend continuing this garbage survey at Playa Caletas following a protocol of repeated, random transect measurements within distinct, 100-meter study zones within different geographic areas along the beach, similar to the methods outlined in the NOAA Draft Marine Debris Shoreline Survey Field Guide (Opfer et al. 2012). Trash surveys over an entire nesting/hatching season and over several years would allow a comparison between multiple seasons and years, as well as with other beaches. Many studies over the past four to five decades have shown an increased presence of man-made micro debris on shorelines (Barnes et al. 2009), but the irregularity of sampling, lack of stable study sites, and differing protocols (including this study), make accurate quantification of micro debris difficult. Therefore, we suggest doing consistent micro debris surveys coupled with the macro and man-made large debris transects to gain a more accurate assessment of micro debris presence and characteristics at Playa Caletas.

Conservation Recommendations. This study demonstrated how a desolate sea turtle beach can be affected by man-made debris, mostly plastics. We found an alarmingly high density of garbage (0.14 pieces per m²), which is similar to densities in heavily populated/tourist areas like Armação dos Búzios, a beach near Rio De Janeiro, Brazil (0.14 pieces per m²) (Oigman-Pszczol & Creed 2007). The large amount of trash at Playa Caletas has the potential to affect sea turtles by causing entanglement of nesting females, blocking nesting areas, increasing sand compaction, entangling emerging hatchlings, and blocking/entangling hatchlings from reaching the sea. If solid waste is blown or washed out to sea it may also become ingested and/or cause further entanglement (Bugoni et al. 2001; Carr 1987; Wabnitz & Nichols 2010).

We recommend involving local community members as much as possible in sea turtle and garbage surveys and cleanups for both educational purposes and to increase support for the continuation of the programs. Apart from the opportunity for extensive sampling, community participation in beach cleanups has been considered an important strategy of increasing public awareness and motivating community action (Bravo et al. 2009; Marcovaldi & Marcovaldi 1999). Costa Rica in particular, with an economy that depends on eco-tourism, and a local population that is environmentally- conscious and educated, could become a pioneer in developing solutions to the plastic problem.

Acknowledgements. Many thanks to the volunteers at the Pretoma sea turtle camp at Playa Caletas who helped with field data collection, the MTN reviewers, E. Whitman for earlier reviews of the manuscript, and R. Lewison at San Diego State University for issuing undergraduate biology credit for this internship. During this research the author (J. Ramos) was enrolled in The Science Exchange Sea Turtle Research Internship Program (www.thescienceexchange. org) through San Diego State University.

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