Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR, USA.
Olive ridleys (Lepidochelys olivacea) are the most prolific species of sea turtle. My research focuses on the awe-inspiring synchronized mass-nesting behaviour exhibited by ridleys at a handful of beaches worldwide. During these events, called ‘arribadas’, thousands of turtles gather to nest during a few consecutive nights several times a year. The largest aggregations have hundreds of thousands of females nesting on beaches at La Escobilla, Mexico (Fig. 1); Ostional, Costa Rica; and Orissa, India.
Fig. 1. Females coming ashore to lay their nests. Most nesting usually occurs during the night, but this photo was taken in the late afternoon at the onset of the arribada. Photo by M. Ocana.
Olive ridleys’ omni-presence has led to their importance in cultures and diets throughout the globe. Despite being the most abundant species of sea turtle, they are listed as Vulnerable throughout their range by the IUCN Red List (IUCN 2007). Olive ridleys off Oaxaca, Mexico (the southern Pacific coast) endured an extensive industrial harvest in the 1960s-80s. A 1990 ban on turtle use in Mexico put a halt to this recent overexploitation. The population has since been rebounding, with over one million nests reached in 2000, up from approximately 55,730 in 1988 (IUCN 2007; Marquez et al., 1996). Despite this increase, the Pacific Mexico breeding population is still distinctly classified as Endangered by the U.S. Endangered Species Act (NMFS & USFWS, 2007). Turtle eggs are still frequently poached as a ready source of protein or devoured for their purported aphrodisiac qualities, often as shots in bars. During my first visit to Escobilla, many of the local people that I talked with referred to the time of industrial harvest, the fallout from overexploitation, and the continued economic difficulties faced by communities trying to ‘live peacefully’ with turtles.
Arribadas are believed to have evolved as a predator satiation strategy, whereby the sheer volume of prey is greater than the predation potential, ensuring that some will survive. In large olive ridley populations, arribadas often lead to high nest densities, which play a significant but as yet largely unknown role in subsequent hatchling production. The most visible potential result of high-density nesting is nest destruction (also known as nest superimposition, overlapping or covering). Females arriving later in the arribada often dig up earlier females’ eggs in the process of creating new nests (Fig. 2).
Fig. 2. Here is a clear example of nest destruction, where a female digs up previously laid eggs before they have completed their incubation period. Photo by M. Ocana.
The purpose of this research is to provide a greater understanding of arribada nesting dynamics and of density-dependent factors that affect hatchling production. This project pursues empirical estimates of productivity variables and considers relationships among various factors that affect hatchling production at Escobilla. The main study objective was to determine the pattern of nesting activity, nest densities in the most frequently used portion of the beach and probability of nest destruction by females. The overarching goal was to build on previous and current research activities to improve plans and methodologies for future conservation and management of La Escobilla for Mexico’s nesting turtles. This report is a preliminary discussion of my field season and a consideration of methodologies, given the restraints imposed by arribada field work.
I conducted a field study between August and November 2009, when there are fairly frequent arribadas and high nesting density at La Escobilla. In order to meet the demands of data collection associated with a project of this scope, I assembled a research team that included three volunteers from the U.S. and Ecuador, two Mexican students and roughly 15 hired local community members. I received logistical assistance and a wealth of site-specific knowledge from researchers at the Centro Mexicano de la Tortuga.
Arribada nesting beaches provide distinct challenges for field studies. There are restraints imposed by the nesting environs (e.g., geographic variability, local community resource needs, governmental regulation) and by the unique arribada behaviour (e.g., large volume of turtles in small temporal windows, large nesting areas with varying use). In light of this, how do we determine key variables for data collection and devise appropriate methodologies? The turtles’ biology, previous studies elsewhere, site-specific limitations and discussions with local researchers were all taken into account in developing the methods for the present study.
An ongoing challenge was devising the materials to use at the field site. The beach represents a vast quantity of shifting sand where equipment can easily be misplaced. Marked nests are often difficult to relocate. Sand turnover from thousands of nests being dug makes using expensive equipment in nests risky. With the propensity for lost materials, one has to think about the quantity of metal or plastic litter that will be abandoned to nature. As a result, invasive equipment is often discouraged at the site. One factor that we monitored to ensure that plots were comparable was temperature. Data loggers can record temperature at set time intervals – every 2 hours in this study with ibuttons (Maxim Integrated Products, CA). We did not want the sensors damaged or dislodged from their location at nest depth (40cm) so we covered each with a buried mesh cover. As an experiment, we also placed loggers into nests without covers. Sensors were inside plastic film containers to provide water resistance. This protective layer represents a trade-off, as it requires pre-testing to be sure that readings are still accurate.
Many nesting beaches are frequently used by those living in neighbouring communities, and La Escobilla is no exception. While it retains sanctuary status, the beach still sees a lot of use from local residents who fish and swim, marines who patrol to prevent poaching, people conducting research and tourists who come to view the nesting turtles. With the potential flurry of activity, theft and property damage is another concern. We selected the most unattractive wooden posts for this study, to lessen the temptation for them to be stolen. Even with this precaution, we lost posts to vandalism, apparently by poachers.
The next methodological quandary was how to promote natural nest laying behaviour in a structured manner. Other studies have created artificial density plots by relocating nests, which in this case, of course, defeats the purpose of estimating naturally occurring nest densities at La Escobilla. Study plots would have to be sturdy and either inhibit or facilitate normal nesting activities, depending on our study requirements. In the end, we constructed 3x3 metre plots consisting of four corner wood posts with partially buried metal fencing that could be rolled open and closed (Fig. 3). To inhibit damaging nests with fence construction, we discouraged nest laying along plot perimeters with netting anchored underground at each corner, as this texture is unsuitable to nest on. Forty plots, constructed in pairs to allow for comparison between conditions, were located along the most frequently used area of beach to increase the probability that turtles would arrive in sufficient numbers within each of the plots.
Fig. 3. Study plots were built across the middle zone of the beach where nesting is frequent. Photo by M. Ocana.
With these plots in place, the study commenced on the first night of the first study arribada and concluded at the end of the incubation period of the second arribada. Nesting behaviour was monitored each half an hour all night within the plots during the multi-day arribada events. Following arribada convention, the team began recording data as soon as 1000 females were present on the beach and stopped upon the marked drop in females that signals the end of the event. During an arribada, the team recorded turtle behaviour: digging, laying eggs, covering nests and (probable) destroying of nests. Nest destruction is defined as observing a female unearthing another female’s eggs and laying or covering her nest amidst freshly broken eggs. Since the entire nesting process takes about 45 minutes, the team observed every turtle present. To avoid double-counting, females were marked with chalk and maps were drawn.
After the first arribada, plots were fenced to hinder unobserved solitary female entrance that would change the nest density. Large links allowed hatchlings from prior nesting to exit; however, there were effectively no nests in the ground at the start of the study. Plots were re-opened during the second arribada and then re-blocked after its end.
The sheer volume of turtles has long made meaningful estimation during arribadas difficult. Researchers should start off investigations by considering the most meaningful way to represent variables of interest. In order to answer questions about nest density or destruction, we first had to interpret behavioural observations.
Nest destruction can be interpreted in a number of ways. Ideally, we would be able to follow what happens to each individual egg. In doing so, we would be able to determine the exact impact of destruction if we could estimate the percentage of eggs in each clutch being destroyed. Unfortunately, it is very difficult to observationally follow destruction of individual eggs. Other studies echo the difficulty in empirically obtaining more specific egg level destruction estimates: “the proportion of the eggs destroyed for an excavated nest is impossible to estimate directly” (Girondot et al., 2002). The next option could be defining destruction at the nest level. What percentage of nests laid are later damaged by females? One classic study placed ribbons inside marked nests and categorized the nests as destroyed if the ribbon appeared above ground (Cornelius et al., 1991). Given the volume of sand upheaval, as well as turtle and human activity across the Escobilla beach, I concluded that this method would be insufficiently exact. The other tricky aspect is that turtles may be digging up eggs from multiple nests. In order to thoroughly evaluate at the nest level, one needs to have a way to distinguish eggs from different nests, such as by a controlled experiment where eggs are pre-labelled. Otherwise, one needs to excavate the nests immediately to attempt to determine egg origins, which may negatively impact on subsequent hatching success, or assume that eggs destroyed reflect at least one impacted nest and leave it at that. The representation we ultimately opted for looks at destruction at the turtle level. What percentage of turtles observed in study plots dig up eggs? Since individual eggs from nests are destroyed, not entire nests, we did not interpret destruction to indicate a net loss of a nest and therefore opted not to change our estimates of nest density with each destruction event.
Turtles nested in 26 plots over two study arribadas, from August 14th to18th (1st Arribada) and from August 27th to September 4th, 2009 (2nd Arribada). These 26 plots were between station markers 8 and 25.5 on the beach, representing a lateral distance of roughly 1.75km. A total of 1293 turtles were observed over all the plots for both arribadas (Table 1).
| || 1st Arribada||2nd Arribada |
|Total # Turtles observed||566||727|
|Total # Turtles / 9m2 plot|| || |
|Mean, (Standard Deviation)||21,(8)||28, (11)|
Table 1. Total number of turtles observed over two arribadas studied. The second arribada was slightly larger than the first, as is frequently the case at that time of year.
|Temperature per plot|| 1st Arribada||2nd Arribada |
|Mean (Standard Deviation)||34.6°C (.68)||35.0°C (.69)|
| Mean Minimum (SD)||31°C (1.2)||31.5°C (1.4)|
|Mean Maximum (SD)||37°C (.92)||37.1°C (1.0)|
Table 2. Temperature at nest depth measured with ibutton loggers (n = 22), one per plot. These values represent estimates for the 45-day incubation period, as nests were laid over a period of a few days. For the first arribada, sensors were not put in until a few days after the start of the incubation period.
Mexico’s waters are home to six of the seven sea turtle species, making it an important case study for turtle use. As one of the largest arribada nesting beaches, La Escobilla, Mexico is an ideal location to consider the intricacies of arribada dynamics and factors affecting hatchling production. This research project succeeded in testing methodologies and highlighting important productivity factors for future research.
There are a number of ways in which destruction might be defined. Our interpretation of destruction in the present study, namely, as the percent of females destroying eggs, is perhaps the simplest way to quantify this factor. Hopefully, it can be incorporated into existing methods to estimate the number of nesting females. Future studies should attempt to estimate destruction as a fraction of nesting activity or as a probability estimate, instead of relying on current methods that simply estimate the absolute number of turtles destroying, to allow future projection from numbers of turtles or nest density. Nest destruction is clearly an important factor at La Escobilla; however, its visibility may afford it disproportionate attention. In discussions with local researchers, some hypothesized that destruction is an evolved strategy of self-limitation of the population. Arribada beaches are believed to undergo natural cycles based on beach characteristics or nesting population fluxes (e.g., Valverde et al., 1998). Nest destruction could be a part of this cycle. Regardless of the importance of destruction, it is clear that microclimatic conditions can trump other potential impacts, as temperature is likely to determine nest success in the present study. My results indicate that future studies should obtain a detailed temperature profile for different areas of the beach.
The rebound of La Escobilla’s olive ridley population has provided the opportunity to study ‘healthy’ arribada nesting behaviours, which in turn helps ensure effective future protection methods. This study’s methodological explorations provided interesting ideas for future monitoring. I hope that future studies will estimate hatchling production as a function of nest density and continue to investigate factors that determine nest success at La Escobilla. Working with a diverse research team was personally enriching, and I hope future research activities can similarly incorporate integrated teams of local and international student volunteers and community members. Integrating community members into research efforts is one important way to solidify local investment in conservation efforts.
Thank you to the British Chelonia Group and the Aquafish CRSP (Collaborative Research Support Program) through Oregon State University for supporting this project. This research was the result of dedicated work by the field team of U.S. and Mexican volunteers and local community members, with support from government researchers at the Mexican Sea Turtle Center (especially Martha Harfush Melendez, Ernesto Albavera, Cuauhtemoc Penaflores and Erika Peralta Buendia). Thanks to my advisor, Selina Heppell, my fellow Heppell lab members, and to Oregon State University for supporting my graduate studies. Thanks to Ezra, for everything, including comments on this article.
Cornelius, S., Ulloa, M., Castro, J., Mata del Valle, M. & Robinson, D. (1991). Management of olive ridley sea turtles (Lepidochelys olivacea) nesting at Playas Nancite and Ostional, Costa Rica. In: Neotropical Wildlife Use and Conservation, eds J. Robinson & K. Redford, University of Chicago Press, USA, pp. 111-35.
Girondot, M., Tucker, A.D., Rivalan, P., Godfrey, M.H. & Chevalier, J. (2002). Density- dependent nest destruction and population fluctuations of Guianan leatherback turtles. Animal Conservation 5: 75-84.
IUCN Marine Turtle Specialist Group (2007). Lepidochelys olivacea Red List Assessment.
Marquez, R., Penaflores, C. & Vasconcelos, J. (1996). Olive Ridley Turtles (Lepidochelys olivacea) show signs of recovery at La Escobilla, Oaxaca. Marine Turtle Newsletter 73: 5-7.
National Marine Fisheries Service and U.S. Fish and Wildlife Service (2007). Olive Ridley Sea Turtle (Lepidochelys olivacea), 5-Year Review: Summary and Evaluation.
Tordoir, M.T., Gómez, F., Wingard, S., Orrego, C.M. & Valverde, R.A. (in press.) Lethal effect of elevated incubation temperature on olive ridley sea turtle (Lepidochelys olivacea) embryos at Ostional Beach, Costa Rica, a mass nesting rookery.
Valverde, R.A., Cornelius, S.E. & Mo, C.L. (1998). Decline of the olive ridley sea turtle (Lepidochelys olivacea) nesting assemblage at Nancite beach, Santa Rosa National Park, Costa Rica. Chelonian Conservation and Biology 3: 58-63.
Tustudo Volume 7 Number Two 2010