Katherine L. Mansfield

First satellite tracks of neonate sea turtles redefine the ‘lost years’ oceanic niche

Few at-sea behavioural data exist for oceanic-stage neonate sea turtles, a life-stage commonly referred to as the sea turtle ‘lost years’. Historically, the long-term tracking of small, fast-growing organisms in the open ocean was logistically or technologically impossible. Here, we provide the first long-term satellite tracks of neonate sea turtles. Loggerheads (Caretta caretta) were remotely tracked in the Atlantic Ocean using small solar-powered satellite transmitters. We show that oceanic-stage turtles (i) rarely travel in Continental Shelf waters, (ii) frequently depart the currents associated with the North Atlantic Subtropical Gyre, (iii) travel quickly when in Gyre currents, and (iv) select sea surface habitats that are likely to provide a thermal benefit or refuge to young sea turtles, supporting growth, foraging and survival. Our satellite tracks help define Atlantic loggerhead nursery grounds and early loggerhead habitat use, allowing us to re-examine sea turtle ‘lost years’ paradigms.

Determining origin in a migratory marine vertebrate: a novel method to integrate stable isotopes and satellite tracking

Stable isotope analysis is a useful tool to track animal movements in both terrestrial and marine environments. These intrinsic markers are assimilated through the diet and may exhibit spatial gradients as a result of biogeochemical processes at the base of the food web. In the marine environment, maps to predict the spatial distribution of stable isotopes are limited, and thus determining geographic origin has been reliant upon integrating satellite telemetry and stable isotope data. Migratory sea turtles regularly move between foraging and reproductive areas. Whereas most nesting populations can be easily accessed and regularly monitored, little is known about the demographic trends in foraging populations. The purpose of the present study was to examine migration patterns of loggerhead nesting aggregations in the Gulf of Mexico (GoM), where sea turtles have been historically understudied. Two methods of geographic assignment using stable isotope values in known-origin samples from satellite telemetry were compared: (1) a nominal approach through discriminant analysis and (2) a novel continuous-surface approach using bivariate carbon and nitrogen isoscapes (isotopic landscapes) developed for this study. Tissue samples for stable isotope analysis were obtained from 60 satellite-tracked individuals at five nesting beaches within the GoM. Both methodological approaches for assignment resulted in high accuracy of foraging area determination, though each has advantages and disadvantages. The nominal approach is more appropriate when defined boundaries are necessary, but up to 42% of the individuals could not be considered in this approach. All individuals can be included in the continuous-surface approach, and individual results can be aggregated to identify geographic hotspots of foraging area use, though the accuracy rate was lower than nominal assignment. The methodological validation provides a foundation for future sea turtle studies in the region to inexpensively determine geographic origin for large numbers of untracked individuals. Regular monitoring of sea turtle nesting aggregations with stable isotope sampling can be used to fill critical data gaps regarding habitat use and migration patterns. Probabilistic assignment to origin with isoscapes has not been previously used in the marine environment, but the methods presented here could also be applied to other migratory marine species.

Predicting the distribution of oceanic-stage Kemp's ridley sea turtles

The inaccessibility of open ocean habitat and the cryptic nature of small animals are fundamental problems when assessing the distribution of oceanic-stage sea turtles and other marine animals sharing similar life-history traits. Most methods that estimate patterns of abundance cannot be applied in situations that are extremely data limited. Here, we use a movement ecology framework to generate the first predicted distributions for the oceanic stage of the Kemps ridley sea turtle (Lepidochelys kempii). Our simulations of particle dispersal within ocean circulation models reveal substantial annual variation in distribution and survival among simulated cohorts. Such techniques can help prioritize areas for conservation, and supply inputs for more realistic demographic models attempting to characterize population trends.

Long-Term Trends in Loggerhead (Caretta caretta) Nesting and Reproductive Success at an Important Western Atlantic Rookery

Abstract The Archie Carr National Wildlife Refuge (ACNWR), located along the central east coast of Florida (USA) in the western North Atlantic, hosts one of the largest loggerhead (Caretta caretta) nesting assemblages in the western Hemisphere. Sea turtle nesting activity has been continuously monitored on this beach for > 31 yrs, representing one of the longest sea turtle reproductive data sets in the world. Between 1982 and 2012, an estimated 358,243 nests were deposited on the ACNWR with an estimated annual mean plus 95% confidence interval of 11,556 ± 1,129 nests. This constitutes 25.4% ± 0.8% of the mean annual Florida Index Nesting Beach Survey loggerhead complement. Mean annual clutch size was 113.9 ± 1.4, resulting in a 55.1% ± 4.0% mean annual hatching success rate and a mean emerging success rate of 53.3% ± 3.7%. The only egg-fate that was statistically correlated with emerging success were eggs washed out by erosion. The loss of eggs by erosion was significantly greater during storm and poststorm years, compared with nonstorm years. Among individual first-time nesting females that were measured, mean straight carapace length was 91.2 ± 0.15 cm and mean curved carapace length was 98.2 ± 0.15 cm. These data suggest that the ACNWR supports the greatest loggerhead nest density per kilometer in Florida, underscoring the importance of the ACNWR as one of the most important nesting habitats for loggerhead turtles in the Western Hemisphere.

Passive drift or active swimming in marine organisms

Predictions of organismal movements in a fluid require knowing the fluids velocity and potential contributions of the organisms behaviour (e.g. swimming or flying). While theoretical aspects of this work are reasonably well-developed, field-based validation is challenging. A much-needed study recently published by Briscoe and colleagues in Proceedings of the Royal Society B compared movements and distribution of satellite-tracked juvenile sea turtles to virtual particles released in a data-assimilating hindcast ocean circulation model. Substantial differences observed between turtles and particles were considered evidence for an important role of active swimming by turtles. However, the experimental design implicitly assumed that transport predictions were insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the model. Here, we show that the magnitude of variation in physical parameters between turtles and virtual particles can profoundly alter transport predictions, potentially sufficient to explain the reported differences without evoking swimming behaviour. We present a more robust method to derive the environmental contributions to individual movements, but caution that resolving the ocean velocities experienced by individual organisms remains a problem for assessing the role of behaviour in organismal movements and population distributions.

Foraging and recruitment hotspot dynamics for the largest Atlantic loggerhead turtle rookery

Determining patterns of migratory connectivity for highly-mobile, wide-ranging species, such as sea turtles, is challenging. Here, we combined satellite telemetry and stable isotope analysis to estimate foraging locations for 749 individual loggerheads nesting along the east central Florida (USA) coast, the largest rookery for the Northwest Atlantic population. We aggregated individual results by year, identified seven foraging hotspots and tracked these summaries to describe the dynamics of inter-annual contributions of these geographic areas to this rookery over a nine-year period. Using reproductive information for a subset of turtles (n = 513), we estimated hatchling yields associated with each hotspots. We found considerable inter-annual variability in the relative contribution of foraging areas to the nesting adults. Also reproductive success differed among foraging hotspots; females using southern foraging areas laid nests that produced more offspring in all but one year of the study. These analyses identified two high priority areas for future research and conservation efforts: the continental shelf adjacent to east central Florida and the Great Bahama Bank, which support higher numbers of foraging females that provide higher rates of hatchling production. The implementation of the continuous-surface approach to determine geographic origins of unknown migrants is applicable to other migratory species.

Molecular evolution of fibropapilloma-associated herpesviruses infecting juvenile green and loggerhead sea turtles

Chelonid Alphaherpesvirus 5 (ChHV5) has long been associated with fibropapillomatosis (FP) tumor disease in marine turtles. Presenting primarily in juvenile animals, FP results in fibromas of the skin, connective tissue, and internal organs, which may indirectly affect fitness by obstructing normal turtle processes. ChHV5 is near-universally present in tumorous tissues taken from affected animals, often at very high concentrations. However, there is also considerable asymptomatic carriage amongst healthy marine turtles, suggesting that asymptomatic hosts play an important role in disease ecology. Currently, there is a paucity of studies investigating variation in viral genetics between diseased and asymptomatic hosts, which could potentially explain why only some ChHV5 infections lead to tumor formation. Here, we generated a database containing DNA from over 400 tissue samples taken from green and loggerhead marine turtles, including multiple tissue types, a twenty year time span, and both diseased and asymptomatic animals. We used two molecular detection techniques, quantitative (q)PCR and nested PCR, to characterize the presence and genetic lineage of ChHV5 in each sample. We found that nested PCR across multiple loci out-performed qPCR and is a more powerful technique for determining infection status. Phylogenetic reconstruction of three viral loci from all ChHV5-positive samples indicated widespread panmixia of viral lineages, with samples taken across decades, species, disease states, and tissues all falling within the same evolutionary lineages. Haplotype networks produced similar results in that viral haplotypes were shared across species, tissue types and disease states with no evidence that viral lineages associated significantly with disease dynamics. Additionally, tests of selection on viral gene trees indicated signals of selection dividing major clades, though this selection did not divide sample categories. Based on these data, neither the presence of ChHV5 infection nor neutral genetic divergence between viral lineages infecting a juvenile marine turtle is sufficient to explain the development of FP within an individual.

Did Declining Carrying Capacity for the Kemp's Ridley Sea Turtle Population Within the Gulf of Mexico Contribute to the Nesting Setback in 2010−2017?

Abstract The Kemps ridley (Lepidochelys kempii) is the most endangered sea turtle species. During 1966–2017, an annual count of nests (i.e., clutches of eggs laid) has served as an annual index of Kemps ridley nesting female abundance on the Gulf of Mexico (GoM) index beach in Tamaulipas, Mexico. This index was increasing exponentially at 19% per year in 2009, but it dropped unexpectedly by more than a third in 2010 and through 2017 remained well below levels predicted. We hypothesize that pre-2010 declining carrying capacity for the Kemps ridley population within the GoM contributed to this nesting setback. We discuss pre-2010 factors that may have caused carrying capacity to decline, including degradation of the GoM ecosystem, the exponentially increasing Kemps ridley population, and declining per capita availability of neritic (i.e., postpelagic) Kemps ridley food, including natural prey and scavenged discarded bycatch from shrimp trawling. We encourage evaluations (especially those within a robust modeling framework) of this hypothesis and others put forth to explain the nesting setback to provide information needed to guide restoration of the populations progress toward recovery.

Supplement 1. Raster format of loggerhead δ13C and δ15N isoscapes and associated standard deviation.

File List loggerhead_d13C_isoscape.txt (md5: 75d5be745e2018c1fbf639224e598736) loggerhead_d15N_isoscape.txt (md5: d76570667098245e4102e8380b588324) loggerhead_d13C_error.txt (md5: 7f8bb3cc91d34eb8ad033942bd2f93cf) loggerhead_d15N_error.txt (md5:995ce245c4e066001d6f8322e33038ef ) Description loggerhead_d13C_isoscape.txt – krigged surface of δ 13 C values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d15N_isoscape.txt – krigged surface of δ 15 N values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d13C_error.txt – standard deviation of the kriging prediction of δ 13 C values loggerhead_d15N_error.txt – standard deviation of the kriging prediction of δ 15 N values These four files are the δ 13 C and δ 15 N isoscapes and standard deviation rasters that are depicted in Fig. 1 of the main text. They are ASCII-formatted text files and contain a header to identify the structure of the file. They can be opened in ArcGIS by using the ASCII to Raster conversion tool or in R with the raster() function using the Raster package and then assigned a spatial reference within the program used. Resolution of each of the files is 0.0572 degrees.