Plotkin, P.T.

We developed a Kemp’s ridley (Lepidochelys kempii) stock assessment model to evaluate the relative contributions of conservation efforts and other factors toward this critically endangered species’ recovery. The Kemp’s ridley demographic model developed by the Turtle Expert Working Group (TEWG) in 1998 and 2000 and updated for the binational recovery plan in 2011 was modified for use as our base model. The TEWG model uses indices of the annual reproductive population (number of nests) and hatchling recruitment to predict future annual numbers of nests on the basis of a series of assumptions regarding age and maturity, remigration interval, sex ratios, nests per female, juvenile mortality, and a putative ‘‘turtle excluder device effect’’ multiplier starting in 1990. This multiplier was necessary to fit the number of nests observed in 1990 and later. We added the effects of shrimping effort directly, modified by habitat weightings, as a proxy for all sources of anthropogenic mortality. Additional data included in our model were incremental growth of Kemp’s ridleys marked and recaptured in the Gulf of Mexico, and the length frequency of stranded Kemp’s ridleys. We also added a 2010 mortality factor that was necessary to fit the number of nests for 2010 and later (2011 and 2012). Last, we used an empirical basis for estimating natural mortality, on the basis of a Lorenzen mortality curve and growth estimates. Although our model generated reasonable estimates of annual total turtle deaths attributable to shrimp trawling, as well as additional deaths due to undetermined anthropogenic causes in 2010, we were unable to provide a clear explanation for the observed increase in the number of stranded Kemp’s ridleys in recent years, and subsequent disruption of the species’ exponential growth since the 2009 nesting season. Our consensus is that expanded data collection at the nesting beaches is needed and of high priority, and that 2015 be targeted for the next stock assessment to evaluate the 2010 event using more recent nesting and in-water data. 

The Kemp’s ridley sea turtle (Lepidochelys kempii) is an iconic Gulf of Mexico species and an enduring symbol of restoration and recovery. Its population declined dramatically during the 1960s and 1970s after decades of egg harvest and incidental capture in fisheries. By the mid-1980s only a few hundred female Kemp’s ridleys came ashore to lay eggs on their nesting beaches in the Mexican state of Tam- aulipas. Conservation practices implemented to reduce mortality and increase recruitment saved the species from extinction and led to exponential growth in the number of adult female Kemp’s ridleys from the few hundred turtles counted in 1985 to nearly 10,000 turtles by 2009. This outcome is one of the greatest wildlife conservation successes of our time.

One valuable lesson learned is that sea turtle restoration is slow, but possible, if threats are reduced or removed, and recruitment into the population is sustained at high levels. Key to the success of the initial Kemp’s ridley recovery were the intensive conservation actions and coopera- tion of two nations; federal, state, and local resource agencies; nongovernmental organiza- tions; industry; hundreds of volunteers; and the funding made available for recovery.

There are many key events that contributed to saving the Kemp’s ridley from extinction (Hep- pell et al., 2007). Most notable, however, is the combination of long-term protection of nesting beaches, requiring the use of turtle excluder devices on shrimp fishing vessels in U.S. and Mexican waters, seasonal and spatial closures to shrimp fishing in critical habitat, and the reduction in shrimp fishing effort in the Gulf of Mexico. Collectively, these actions led to a increase in the number of Kemp’s ridley nests in Mexico and Texas, an expansion of their nesting range in the Gulf of Mexico, and the impressive population growth observed through 2009 (Hep- pell et al., 2007; Crowder and Heppell, 2011).

Unfortunately, the recovery of the Kemp’s ridley slowed substantially after 2009 and corre- lated spatially and temporally with multiple natural and anthropogenic stressors in the Gulf of Mexico. In response to concerns about the species and its status, Texas Sea Grant and the Gladys Porter Zoo cohosted the Second Interna- tional Kemp’s Ridley Sea Turtle Symposium in November 2014 to provide a timely forum for the presentation and discussion of recent advances in the science, conservation, and management of this endangered species. The papers in this special issue were presented at the symposium and greatly advance our knowledge of the biology of the species, the history of conservation efforts that saved it from extinction, and the impacts of recent stressors in the Gulf of Mexico.