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Volume 2 | Issue 1 | Page 67-73 | Feb 2011
The Decline and Rise of a Sea Turtle: How Kemp’s Ridleys Are Recovering in the Gulf of Mexico
Adrienne McCracken
A Kemp's ridley hatchling on South Padre Island in Texas.
The Kemp’s ridley sea turtle (Lepidochelys kempii) has rebounded from the brink of extinction thanks to the cooperative efforts of citizens, scientists, and managers from two nations. This slow-growing species had dwindled from tens of thousands to a few hundred breeding females on a narrow beach in Tamaulipas, Mexico, in 1985, but has recovered remarkably since egg harvest halted in the 1970s and accidental deaths in trawl fisheries were reduced in the 1990s. With intensive egg protection and improved survival at sea, conservation efforts have boosted the current population to 7,000–8,000 breeding females nesting at multiple sites from Padre Island, Texas, to Veracruz in the southwestern Gulf of Mexico, with an average population growth rate of 15 percent per year. Hopefully, this growth has created enough momentum for the species to weather its most recent insult: the BP Deepwater Horizon oil spill, which directly or indirectly killed hundreds, and possibly thousands, of turtles. Projections from a simple population model suggest that recovery will likely slow, but there is cautious optimism for species resilience if high survival rates can be restored quickly. The Kemp’s ridley “success story” is unfinished but is an inspiring example of how intensive conservation efforts aimed at all facets of a species’ life cycle, from egg to adult, can lead to recovery of an endangered species. The future of this smallest of the sea turtle species will depend on continued cooperation by U.S. and Mexican authorities, conservation groups, fishermen, and local people who support the revival of this icon of the western Gulf of Mexico.
  • The Kemp’s ridley sea turtle came perilously close to extinction in the 1980s due to overexploitation and accidental mortality in fishing gear. The 800 remaining nests of this species were restricted to a single beach in Tamaulipas, Mexico, which had once harbored tens of thousands of nesting females.
  • Intensive conservation actions by researchers and volunteers in Mexico and the United States saved the species, with funding through government agencies, industry, and nonprofit groups.
  • Conservation was successful because it focused on stressors affecting all life stages of the turtles, from eggs to juveniles and adults at sea. Reductions in fishing mortality and increased productivity on nesting beaches have contributed to the observed 15 percent per year increase in nests and expansion of the population from southern Texas to Veracruz.
  • The future of the Kemp’s ridley is uncertain, however, due to potential impacts of disasters like the BP Deepwater Horizon spill. Continued support for conservation and research are essential to keep the species on its road to recovery.

Marine conservation has few success stories; it has been criticized for almost exclusively focusing on problems rather than solutions. And while it makes sense to fully understand a problem before framing possible solutions, scientists often focus on the problem description stage—writing the obituary for the oceans—rather than on working to forestall the declines. In the past, we have been particularly unsuccessful in recovering endangered species, leading some to argue that the Endangered Species Act and species recovery plans have been ineffective.1 Such failures have many reasons, including lack of basic biological information, population demography, and quantification and prioritization of threats—leading to “shopping lists” of potential management actions, which ultimately contribute to the failure of recovery plans.2,3 Often, we focus conservation efforts on only one aspect of a species’ life cycle, when the stressors that have led to endangerment are broad and cumulative across many life stages.4 With last year’s Deepwater Horizon catastrophe, we are reminded more than ever of the immediate need to move forward in recovery efforts for our ocean ecosystems and for threatened species and of the precarious position that newly recovered species may hold.

Recovery of sea turtles and other long-lived species is particularly challenging because their potential for population growth is limited by late age at first reproduction and relatively low fertility.5 Early on, sea turtle recovery efforts were limited to protection of nesting females and their nests in order to maximize reproductive output. However, an early loggerhead (Caretta caretta) population model6 suggested that relying solely on this strategy would only prolong time to extinction. To increase survival and population recovery, juvenile and older turtles at sea should be the primary focus of protection. In 1990, sea turtle declines in the northwestern Atlantic led to requirements for turtle excluder devices, or TEDs, in U.S. shrimp trawl fisheries and ultimately to TED implementation in other domestic and international fisheries that utilize bottom trawl gear. Initially, the approach looked promising,7 but successful protection was strongly tied to enforcement8 and it was later found that the TEDs initially required by the National Oceanic and Atmospheric Administration (NOAA) were too small to fully protect adult loggerheads.9 Loggerhead sea turtles that nest in the southeastern United States have proven especially difficult to recover—despite 30 years of concerted effort, the nesting populations are now at the lowest level since standardized surveys were established in the 1980s.10 The causes of the ongoing declines are many, requiring careful analyses to set priorities that can most effectively drive recovery.11,4

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Blair Witherington
In June 2010, this oiled Kemp's ridley was found dead in waters near the BP Macondo well, the source of the Deepwater Horizon oil spill, in the Gulf of Mexico.

The Kemp’s ridley (Lepidochelys kempii) is the world’s smallest sea turtle species and one that we nearly lost just three decades ago. The story we outline here is one of real international conservation success.12 And that success derives from the actions of a large number of dedicated researchers, managers, conservationists, and industry partners. But the Kemp’s recovery was not without its bumps, grinds, and missteps—and currently it is not complete, though the trajectory is very promising. Kemp’s ridleys reproduce at a much younger age than loggerhead turtles (about 12 years for Kemp’s ridleys, 25–30 years for loggerheads); this favors productivity and the potential for recovery once mortality stressors are reduced. But Kemp’s ridleys remain the most vulnerable sea turtle species due to the limited geographic range of the nesting beaches. Much of the current knowledge of the biology and conservation of Kemp’s ridley sea turtles was summarized in a recent volume, which we highly recommended to those who want to read more about it.13

A Short History of the Kemp’s Ridley Sea Turtle

Although Kemp’s ridley turtles were known to scientists prior to the 1960s, no one had identified where they nested. In the early 1960s, the mystery was solved when scientists unearthed a 1947 film made by a Mexican tourist, Andres Herrera.14 Herrera accessed the beach at Rancho Nuevo, Tamaulipas, by plane and filmed mass daytime nesting, known as an arribada, of Kemp’s ridleys on the Gulf coast of Mexico. The film showed tens of thousands of Kemp’s ridleys nesting in a single day.

Intensive commercial egg harvest occurred at Rancho Nuevo, and when the first scientists arrived on horseback to census the population in 1966 their initial estimate was just 6,000 nests in the entire season—about 2,000–3,000 nesting females at most.15 It also became clear that Kemp’s ridleys were highly restricted in their geographic range, with most of the remaining nesting population restricted to the Rancho Nuevo beach. Conservation efforts in Mexico that began in the 1960s were expanded in the mid-1970s, and in 1978 the United States joined cooperative efforts to protect the nesting area. These efforts doubled egg survival rates and hatchling production. Government agencies also initiated a captive rearing program, known as headstarting, for 1,000–2,000 hatchlings each year. This program moved eggs from Rancho Nuevo in an effort to expand the species’ range by starting a second nesting population at Padre Island National Seashore in Texas.

Despite 20 years of conservation efforts centered on the nesting beach, nest numbers reached their low in the mid-1980s—fewer than 800 nests per year, or about 300 nesting females.12 Protection of turtles at sea began in earnest in the1990s: bottom trawlers in the U.S. shrimp fishery were required by federal law to install TEDs in 1990 to protect juvenile and adult sea turtles; in 1995, TEDs became required in Mexico. Although compliance has been spotty, TEDs have proven effective when used properly.8

Back from the Brink: A Binational Effort

The combination of TEDs, reductions in shrimping, and nest protection on Mexican beaches has resulted in an unusually rapid recovery for a long-lived, slow growing vertebrate. Beginning in the mid-1990s, Kemp’s ridley nest counts have steadily increased to an annual growth rate of 15 percent.12 In 2009, more than 21,000 nests were counted (including 16,000 at Rancho Nuevo), and these nests produced more than one million hatchlings.16 Many of the key recovery actions instituted from 1966 to the mid-1980s required significant resources from both Mexico and the United States. By 1978, nest protection had doubled hatchling output, many of these hatchlings would return as nesting females as little as a decade later.17 The nest monitoring and protection camps in Tamaulipas and Veracruz have been operating since 1981 through a partnership between the Mexican government, the U.S. Fish and Wildlife Service, and the Gladys Porter Zoo in Brownsville, Texas. Financial support has been provided by government agencies, the Texas Shrimpers Association, and donations from conservation organizations and individuals.

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Selina Heppell and Richard Morin/Solutions
Potential effects of the Deepwater Horizon oil spill on the expected numbers of Kemp’s ridley sea turtles nesting in Tamaulipas, Mexico. “Impact” is the proportion of turtles killed in 2010, which is unknown and is modeled as an effect on different life stages of the turtles. (Left) Percentage reduction in the expected number of nesting females in 2030 when the impact lasts only one year and affects hatchlings only; hatchlings and small juveniles (age 0–5); all life stages but with large juveniles and adults suffering half of the modeled impact level; or with all life stages affected equally. (Right) Percentage reduction in the expected number of nesting females when the impact affects all life stages and decays gradually over a 20-year period. Higher decay rates mean that the mortality of turtles decreases quickly, resulting in lower overall impact. Each line represents a projection of an age-structured, deterministic model fit to known hatchling production and nests for years 1978–2003 (12 years to maturity model, adapted from Heppell et al. 2004).

The twin goals of the headstarting program were to enhance population numbers and to establish a second nesting population in the United States. Although the net benefits of headstarting as a tool to enhance recovery have been debated, mostly due to the lack of an intense monitoring program to follow the success of the captive-reared turtles, some turtles from the headstarting effort have returned to Padre Island and the population is increasing.18,19 The implementation of TEDs by the shrimp trawl fishery in the Gulf of Mexico very likely reduced mortality of the in-water juveniles and adults—previous estimates put annual takes by U.S. shrimp fisheries, alone, in the thousands.20

The Kemp’s ridley owes its salvation to the dedication of hundreds of individuals and organizations, including the first scientists who journeyed to the nesting beach and prevented removal of the remaining nests, the fishing industry that supplied equipment and funding for nesting beach monitoring and capacity building in the isolated town of Rancho Nuevo, the agency personnel and biologists who have devoted their careers to the study and protection of the species, and the volunteers who have spent long days on beaches and in headstarting warehouses to protect and raise hatchling turtles.

Deciding on Recovery Goals and Assessing Success

The Kemp’s ridley’s recovery has been guided in part by population assessments carried out by scientists and managers.12 Mathematical models can be used to assess sea turtle population status and trends and to incorporate multiple threats that act on different life stages with different magnitudes.6,11 But models also allow scientists and managers to evaluate how the population might change with alternative management approaches that address different turtle life history stages in different habitats.

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Adrienne McCracken
Kemp's ridley hatchlings are released on South Padre Island.

As part of an early Turtle Expert Working Group (TEWG),21 Heppell and colleagues22 assembled the first Kemp’s ridley population model by fitting an age-structured population model to the 25-year time series of hatchling output, nesting numbers, and population trends. This model suggested that active egg protection after 1978 could have stabilized the population but that the observed increase in nesting turtles since the mid-1980s required a concurrent increase in survival of the in-water population.21 The model indicates that more turtles have survived to maturity, likely in response to in-water management efforts of the 1990s (TED requirements) and recent reductions in shrimping effort.23 New information on growth rates and additional years of census data from Mexico have allowed researchers to update the age-structured model and apply it as a tool in recovery planning. Unfortunately, there has not been a coordinated effort to measure demographic rates for Kemp’s ridleys away from the nesting beaches, and this creates uncertainty in the model’s rates (i.e., vital rates, including survival, growth, and frequency of reproduction as well as mortality rates from fisheries and other stressors). Nevertheless, the Kemp’s Ridley Recovery Team has used the model to project likely recovery times and reproductive values of different turtle life stages for assessing impacts of anthropogenic stressors.24

The numerical recovery goal for downlisting the species from “endangered” to “threatened” is expected to be reached by 2013 if the population’s trajectory and estimated survival rates remain constant. Even as space and manpower limitations have essentially capped the number of nests moved to protective corrals, and many nests are left in situ, the model projects that the population will continue to increase exponentially for many years due to the momentum of increasing hatchling production through the past two decades. While population projections and response to management from a simple, deterministic model like this one are not suitable for precise predictions, they can provide information to improve turtle recovery planning and monitoring.25

The Potential Impact of the BP Deepwater Horizon Oil Spill

On April 20, 2010, the Deepwater Horizon drilling rig exploded and later sank, leading to the largest oil spill in U.S. history. While the spill did not reach the nesting beaches in Mexico, some Kemp’s ridleys were undoubtedly affected both through direct mortality caused by oil and dispersants and by indirect mortality during unmonitored fishing activities just prior to area closures.26 It is very likely that hatchling, juvenile, and adult Kemp’s ridleys have entered the oil impact zone in large numbers. Adults are known to migrate from Rancho Nuevo through the northern Gulf,27 juveniles use shallow water and inshore habitats between Texas and Florida,28 and hatchlings are thought to seek out open ocean currents and patches of floating seaweed (Sargassum) in the northern and central Gulf of Mexico.29

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Adrienne McCracken
A Kemp's ridley leaving its nest on South Padre Island.

While the precise number of turtles affected by the oil spill will never be known, we can use the Turtle Expert Working Group's model to compare possible impacts of the disaster on the Kemp’s ridley population and its recovery. For details on model formulation and structure, refer to TEWG (2000)—for all of the projections shown here, we conservatively set the capacity in protective corrals to 12,000 nests with an average survival rate of 70 percent per year, and gave the remaining nests in situ a survival rate of 35 percent per year. The approach is purely heuristic—a “what if?” analysis that can show how the magnitude of impact could affect the population, depending on several factors. The intensity of the impact can be modeled as the proportion of turtles removed from the population and the life stages of turtles that are affected (hatchlings, small juveniles age 2–6, large juveniles age 7–11, and adults age 12+). Importantly, we must specify whether the impact is a “pulse” perturbation (e.g., one year of additional mortality) or a “press” perturbation, in which the additional mortality may persist for several years while gradually decaying in magnitude.

If the spill impact is modeled as a single-year impact, large reductions in population size (shown as the model’s expected number of nesting females in 2030) or recovery potential are not expected unless the spill affected large juveniles and adults, or the impact was very large. One-year impacts affecting large juveniles and adults could increase the time to the first recovery benchmark (10,000 nesting females) by a relatively modest 5–10 years, primarily because of “population momentum” driven by two decades of increased productivity at the nesting beach. The model suggests that a pulse impact could be overcome by population growth, but only if the population continues to have high survival and fertility rates.

If the oil spill causes a longer-term impact through habitat degradation or contamination of the food web (which is a likely outcome), the press perturbation effect depends on how quickly the impact decreases over time (modeled as a 20-year period). While it is not surprising that larger impacts over a longer time period lead to smaller population size in 2030 and greater delays in population recovery, the model simulations show that the population response is strongly driven by how the impact decays over time. Rapid identification and cleanup of oil and dispersants that reduce turtle survival, growth, or reproduction are key to keeping the Kemp’s ridley on its road to recovery, assuming that the population can regain its pre-spill survival rates and that strong conservation efforts will continue.

What Is the Future for the Kemp’s Ridley?

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Adrienne McCracken
A nesting Kemp's ridley on South Padre Island

International cooperation has increased Kemp’s ridley nest numbers in Tamaulipas, Mexico, from a low of 800 to more than 21,000 in just 25 years. This heroic effort has enabled a population on the brink of extinction to increase at about 15 percent per year and to be on track to reach recovery goals. This rate of recovery was unimaginable to scientists and managers who struggled through the 1970s and ’80s to keep this program alive, and it may be the key to resilience and recovery from the Deepwater Horizon spill. Importantly, conservation plans reduced mortality for multiple life stages—eggs, juveniles, and adults—ensuring that the stressors that led to endangerment were reduced or mitigated. This shows that even the most critically endangered species can be pulled out of a death spiral if people come together and mount a concerted and wise effort to reach a sustainable solution.

Only time will tell how the BP Deepwater Horizon disaster will affect the Kemp’s ridley population. We hope that conservation efforts of the past and future will enable the population to overcome this setback, even if the rate of increase in nest numbers is reduced over the next few years to decades. The future of the Kemp’s ridley is still uncertain and ultimately still in our hands. Now, as much as ever, we need to continue to increase conservation and recovery efforts to ensure that the Kemp’s ridley and its nesting beaches and marine habitats are resilient in the face of large environmental disasters such as the Deepwater Horizon spill.

References

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Larry Crowder Principal investigator for the South Atlantic Bight Recruitment Experiment, OBIS SEAMAP, and Project GLOBAL
Selina Heppell Associate professor and marine fisheries ecologist at Oregon State University.
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