Date Published: March 23, 2017
Publisher: Public Library of Science
Author(s): Larisa Avens, Lisa R. Goshe, Lewis Coggins, Donna J. Shaver, Ben Higgins, Andre M. Landry, Rhonda Bailey, Mariana M. P. B. Fuentes.
Effective management of protected sea turtle populations requires knowledge not only of mean values for demographic and life-history parameters, but also temporal and spatial trends, variability, and underlying causes. For endangered Kemp’s ridley sea turtles (Lepidochelys kempii), the need for baseline information of this type has been emphasized during attempts to understand causes underlying the recent truncation in the recovery trajectory for nesting females. To provide insight into variability in age and size at sexual maturation (ASM and SSM) and long-term growth patterns likely to influence population trends, we conducted skeletochronological analysis of humerus bones from 333 Kemp’s ridleys stranded throughout the Gulf of Mexico (GOM) from 1993 to 2010. Ranges of possible ASMs (6.8 to 21.8 yr) and SSMs (53.3 to 68.3 cm straightline carapace length (SCL)) estimated using the “rapprochement” skeletal growth mark associated with maturation were broad, supporting incorporation of a maturation schedule in Kemp’s ridley population models. Mean ASMs estimated from rapprochement and by fitting logistic, generalized additive mixed, and von Bertalanffy growth models to age and growth data ranged from 11 to 13 yr; confidence intervals for the logistic model predicted maturation of 95% of the population between 11.9 and 14.8 yr. Early juvenile somatic growth rates in the GOM were greater than those previously reported for the Atlantic, indicating potential for differences in maturation trajectories between regions. Finally, long-term, significant decreases in somatic growth response were found for both juveniles and adults, which could influence recruitment to the reproductive population and observed nesting population trends.
The conservation history of the endangered Kemp’s ridley sea turtle (Lepidochelys kempii) has been one of extremes. Although historically individual Kemp’s ridleys were occasionally reported from the Gulf of Mexico (GOM) and western North Atlantic, their origins were obscure and the location of the constituent breeding areas remained an enigma for decades [1, 2]. Then in 1961, film footage obtained in 1947 by Andres Herrera came to light, showing thousands of female Kemp’s ridleys nesting simultaneously at a single location in Rancho Nuevo, Tamaulipas, Mexico, highlighting the extremely limited spatial scope of reproductive habitat for the species [2, 3]. However, by the time this information was revealed, the nesting population had dwindled to several thousand and, by the 1980s, only a few hundred nesting females remained, presumably due to a combination of poaching and fishery bycatch . Following extensive, bi-national conservation efforts by the USA and Mexico, the number of nesting females began to increase exponentially, offering encouragement for population recovery and the possibility that the species might even be downlisted under the US Endangered Species Act within the foreseeable future [5, 6]. Then in 2010, coincident with the massive Deepwater Horizon (DWH) oil spill in the northern GOM , nest numbers suddenly decreased by more than 30% relative to 2009 levels . While nest numbers subsequently rebounded to some extent, exponential growth has not resumed and a further decrease in 2014 raised concerns regarding future recovery prospects for the species . Recent estimates indicate that the nesting population remains reduced by 90% relative to historic levels .
Despite an extended period of increasing Kemp’s ridley sea turtle nesting trends from 1990 to 2009 that suggested recovery of this endangered species might be a near-term possibility, recent fluctuations and overall decreases in nest numbers 2010 to present have raised concerns regarding population status and future trends. As a result, in the most recent status review for the species, the US National Marine Fisheries Service (NMFS) elevated the recovery priority number for Kemp’s ridleys to the highest level, defining the species as one:
Results of the current study indicate substantial variability in Kemp’s ridley SSM and ASM, as well as long-term decreases in somatic growth rates, which have the potential to influence recruitment to the reproductive population. Recent tag return data from Texas indicate that the proportion of nesting females identified as neophytes has decreased over the last decade  and while a reduction in neophytes might be indicative of nesting population stability , lack of recruitment may contribute to observed fluctuations in nesting trends. Reproductive longevity was also variable and relatively short compared to larger sea turtle species, which has implications for lifetime reproductive output if resource limitations alter nesting frequency. Incorporation of this information into updated population models is needed to ascertain its potential influence on population dynamics. While this study mainly focused on the GOM prior to 2010, analysis of post-DWH oil spill GOM samples is needed to increase understanding of possible, subsequent changes in these population parameters. In addition, as samples from Texas and Florida predominated in the current study, more data from the northern GOM are needed to evaluate regional differences within this area. Furthermore, given that large numbers of juvenile Kemp’s ridleys are also present in foraging areas along the Atlantic US coast , there is a need to update age and growth data for juveniles in this area, to allow current comparison with the GOM, both prior and subsequent to the DWH oil spill. Finally, application of recent advances in integrating skeletochronology with stable isotope and trace element analyses  should be applied to increase understanding of long-term changes in trophic ecology for GOM Kemp’s ridleys, as well as movements between Atlantic and GOM foraging habitats and the relative contributions of these regions to the reproductive population .