The beach in Mayumba National Park, located on the coast of Gabon, has one of the highest nesting densities of sea turtles in Africa. Along with the leatherback turtle, Mayumba is also a nesting ground for the olive ridley, a smaller marine turtle. Named for its olive colored shell, the olive ridley sea turtle is not well understood; however a new paper published by Sara Maxwell et al. helps shed light on the internesting movements of this pelagic animal.
In this author spotlight, Sara Maxwell, a Postdoctoral Fellow with Marine Conservation Institute and the University of California, Santa Cruz, answers questions about her background, her research and her PLOS ONE manuscript, Using Satellite Tracking to Optimize Protection of Long-Lived Marine Species: Olive Ridley Sea Turtle Conservation in Central Africa.
First, a bit of background – how did you become interested in studying migratory patterns of large pelagic animals?
My interests have always centered around conservation primarily. In my mid-20s as I was considering going back to graduate school, I knew I wanted to study a system that would interest me over the course of an entire career – and a system where my science could make a difference. The migratory patterns of marine mammals fit the bill well, and turtles in particular have always been particularly special to me. During my undergraduate career, I spent a summer working with sea turtles at Duke University’s Marine Laboratory and it was ultimately the turtles that drew me into marine biology and conservation. When the opportunity to work with turtles in Gabon arose, I didn’t think twice!
In the manuscript, it mentions that sea turtles are “excellent candidates and models for protecting vulnerable, discrete breeding areas.” Why is this so?
Sea turtles are a highly migratory and dispersed species throughout most of their lives, but during the breeding and nesting season, they come together in large concentrations – and for fairly long stretches of time. Breeding and nesting areas are relatively close together. Olive ridley sea turtles, for example, nest every year to three years. Female turtles stay in the breeding-nesting grounds for several months while they mate and lay up to three clutches of eggs, and breeding males are found in the same areas. Breeding and mating areas, however, are close to shore – so closer to humans, fishing and other threats – but because they are in a small area for a specific period of the year and during an important life history stage, it makes protecting them easier and more politically and socio-economically feasible.
Mayumba National Park is a marine protected area just north of the border of the Republic of Congo. The area is home to leatherback and olive ridley sea turtles but of the two turtles, olive ridleys make up 59 to 95 percent of the dead turtles that wash up on shore. Is there a reason for this and how does tracking via satellite help to minimize the number of stranded olive ridley sea turtles?
We understand the situation in Mayumba a lot better than we did before this study but it is still a bit of a thousands of leatherbacks sea turtles nesting in Mayumba every year but only a few hundred olive ridleys – yet the olive ridleys are dying in much greater numbers. We used satellite tracking to determine where the turtles were going in relation to the park that was created to protect them. We found that they are spending a lot of time outside the park boundaries where they are exposed to fishing nets and drowning. Leatherback movement patterns are somewhat similar; however we don’t see the same number of dead animals washing ashore. This could be because leatherbacks are heavier and their bodies sink instead of washing ashore or they don’t encounter fishing nets as frequently because they are in deeper waters or have different diving behavior. There are lots of things we still don’t know, but what we do know is that creating a larger park – one that is in both Gabon and the Republic of Congo – will protect both species better more from fishing nets. Satellite tracking has been the key tool that brought us to that conclusion, through this study and another great study by Matthew Witt and colleagues published in Oryx in 2008.
One of the three elements you mention, as being necessary to provide protection to breeding females, is the level of confidence in tracks given the limitations of satellite telemetry. How accurate is satellite telemetry and what was done to eliminate location error of internesting movements?
Satellite tracking is an incredibly powerful technology – but it can be tricky too! We were focused on a fairly small area of the world in this study (approximately 1500 km2), and like all measurements, satellite telemetry locations have error associated with them. In the case of telemetry, however, this error can be tens of kilometers which could have a significant impact on the conclusions we were making about where turtles are found in relation to park boundaries. To help counter this, we used a technique called state-space modeling that has been fairly recently applied to animal movement. State-space models allowed us to assign the Bayesian equivalent of confidence intervals to every track location. By doing this, we could calculate error estimates for how animals are using park boundaries – and recommend changes in the park boundaries that take this uncertainty into account.
You have included a few figures in the manuscript. Of them, which one is the most important and why?
This is a difficult question! I think Figure 3, (above figure) which shows the key areas that the olive ridleys are using along with the outer bounds for our error estimation, is critical in showing what we learned from the study – and what we are still uncertain about due to satellite telemetry error. Figure 4 is important as well because it shows the olive ridley and leatherback sea turtle distributions together. Though the analytical techniques were different, this figure shows that the proposed park boundaries (which extend into both Gabon and Congo as well as further offshore) are really on target to protect BOTH of these species – this was a really exciting finding, and one in which is already helping to guide management in the area.
Did you find anything surprising or unexpected while conducting your research for this paper?
One of the most critical things that we learned from conducting this research is not in this paper – and is critical to protecting the olive ridley population. There is a substantial nest monitoring effort that goes on in Gabon and in Mayumba National Park but most of the monitoring has focused on leatherback sea turtles. Olive ridleys, however, nest earlier in the year, and in the most remote part of the park. We were on this remote part of the beach earlier in the year, both to deploy our satellite transmitters and to monitor the nests. As a result we discovered that almost 100% of the nests being laid by olive ridleys were being predated upon by natural predators such as crabs and mongooses. Olive ridley nests are not as deep in the sand as leatherbacks, so predators could reach the eggs and none of the eggs being laid were hatching. This obviously will have a huge impact on the population. As a result of what we learned, however, over the last three years, with support from the US Marine Turtle Conservation Fund, Mayumba National Park has started a hatchery where they incubate the eggs in a protected area of the beach to ensure that the eggs are safe and hatchlings are born.
What’s next? Where do you hope to go from here?
The directions are endless! But there are several directions we are actively working towards. We are working to combine not just the satellite tracks from olive ridley and leatherback sea turtle data but also humpback whales. The goal is to have a comprehensive understanding of how key species in the region are moving in relation to both park boundaries and also other human activities such as fishing and oil and gas development. We have begun modeling the at-sea habitat olive ridleys use during the nesting season in relation to the physical and oceanographic environment. The goal is predicting where ridleys are likely to be found in relation to other nesting beaches where we weren’t able to deploy satellite tags. Finally, we are also collaborating with scientists from Pendoley Environmental who have tracked olive ridleys from another nesting beach in Angola. We are finding that the turtles have similar migratory patterns and are focusing on the same foraging grounds and oceanographic features. All of these efforts are focused on how can we better understand and protect the turtles.
To read more about Sara’s experience in the field, please see her posts on Turtling in Africa in the On TOPP of the World Blog and to view the satellite tracks online, visit the SEATURTLE.ORG tracking page. The paper Using Satellite Tracking to Optimize Protection of Long-Lived Marine Species: Olive Ridley Sea Turtle Conservation in Central Africa is freely available to rate, comment on, and share.