Shaver, D.J.

THOUSANDS OF SINGLE NUCLEOTIDE POLYMORPHISMS IN THE CRITICALLY EN- DANGERED KEMP’S RIDLEY SEA TURTLE (LEPIDOCHELYS KEMPII) REVEALED BY DOU- BLE-DIGEST RESTRICTION-ASSOCIATED DNA SEQUENCING: OPPORTUNITIES FOR PREVIOUSLY ELUSIVE CONSERVATION GE- NETICS RESEARCH.—Among sea turtles, the Kemp’s ridley is the most endangered and geographically restricted, with its distribution mostly confined to the Gulf of Mexico (NMFS and USFWS, 2015). After experiencing a severe and sustained bottleneck that put this species on the verge of extinction, it appeared to be rebounding successfully, as evidenced by an exponential growth in the number of nests observed per nesting season, following decades of Mexico–United States bi-national efforts aimed at its recovery (Heppell et al., 2007). Unfortunately, nesting was severely reduced by ~35% during 2010 (the year of the BP Deepwa- ter Horizon oil spill in the Gulf of Mexico), as compared to nesting rates in 2009 (NMFS and USFWS, 2015). Although nesting rebounded during 2011 and 2012 to levels similar to that of 2009, nesting declined drastically again during 2013 and experienced a further drop during 2014 (NMFS and USFWS, 2015; Shaver et al., 2016). The number of nests in 2014 represents a 46% decrease from 2012, which was the year with the highest recorded number of nests since 1965 (Sarti, 2014). Should nesting continue to de- cline, long-term species recovery efforts will be compromised. Therefore, there is deep concern about the future of the Kemp’s ridley, and data to inform and assess bi-national management and conservation measures are urgently needed (Plotkin and Bernardo, 2014). Population ge- netics information crucial to the long-term conservation of the Kemp’s ridley, including baseline data required for monitoring its future status, is lacking. This includes estimations of genomic diversity, effective population size, and number of breeders; assessment of levels of population differentiation; and detection of genomic signatures of bottlenecks. 

Coincident with the 2010 Deepwater Horizon oil spill, unprecedented numbers of Kemp’s ridley sea turtles (Lepidochelys kempii) stranded on northern Gulf of Mexico beaches and the number of nests recorded on the primary nesting beaches plummeted far below expected levels. High levels of strandings have continued since 2010 and the number of nests recovered to approximately 2009 levels in 2011, and improved slightly in 2012. A stock assessment conducted in 2012 indicated that a mortality event occurred in 2010, and that the number of nests should once more exhibit an increasing trend from 2013 and beyond. This has not happened; rather, the number of nests declined sharply in 2013. We conducted a new stock assessment to evaluate additional scenarios, including 1) three stock-recruitment options; 2) the potential that a new source of ongoing mortality is present; and 3) the potential that the number of nests- per-adult-female is dependent on the size of the age-2þ benthic population. The latter model provided the best fit to the data. Further, the preliminary estimate of actual nesting in 2014 is consistent with model projections. The reduction in reproductive output could be due to the combination of a large population and reduced prey levels. Together these may have increased the remigration interval or reduced the number of nests per female. However, research is needed to evaluate this and other plausible hypotheses. Nesting may be highly variable in the future depending on feeding conditions on the foraging grounds. 

Kemp’s ridley (Lepidochelys kempii) is the world’s most endangered sea turtle species, and nests primarily on the Gulf of Mexico coast in Mexico. In 1978, a binational project was initiated to form a secondary nesting colony of this species in south Texas at Padre Island National Seashore (PAIS), as a safeguard against extinction. During 1978–2014, we documented 1,667 Kemp’s ridley nests in Texas, with 56% found at PAIS. Most nests (89%) found in south Texas were from wild-stock turtles; south Texas is the northern extent of the documented historic nesting range for the species. We documented nesting in north Texas starting in 2002, and most nests (53%) found there were from turtles that had been head-started (reared in captivity for 9–11 mo), and released off the Texas coast as yearlings. Kemp’s ridley nesting increased in Texas during the mid-1990s through 2009, before annual nest numbers dropped in 2010, rebounded and plateaued in 2011 and 2012, and then decreased again in 2013 and 2014. Annual numbers of nests found in Texas and Mexico followed similar trends and were correlated (R2 1⁄4 0.95). We examined nesting turtles for presence of tags at 55% of the nests located in Texas. Of the Kemp’s ridleys we examined during 2000–14, the annual percentage of apparent neophytes decreased and the annual percentage of remigrants increased over time. Mean annual remigration intervals of Kemp’s ridleys increased steadily from 1.9 yr in 2008 to 3.3 yr in 2014. These changes in demographic parameters are critical to understanding the recent fluctuation in the number of nesting Kemps ridleys and will be used in population models to investigate possible causes of the recent and sudden decline of nesting Kemp’s ridleys in Texas and Mexico. 

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.