Tracking Juvenile Loggerhead Sea Turtles (Caretta caretta)
Juvenile loggerhead sea turtles from the east coast of the United States are shown to often take long migratory journeys through open-ocean waters. Using telemetry data from tagged loggerhead sea turtles, we identified an area of special importance based on a kernel home range of the turtles from the Gulf Stream to the Azores.
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The loggerhead sea turtle (Caretta caretta), listed as endangered on the World Conservation Union Red List, is known as one of the greatest migratory species on the Earth. Studies using genetic markers or satellite telemetry data have confirmed extraordinary transoceanic migrations in both the Atlantic and Pacific. As hatchlings these migrations consist of leaving the beach and immediately making their way to open ocean gyres. After spending several years or more in open oceans, the now larger juveniles or subadults enter coastal waters where they feed on benthic organisms and prepare for their adult migration back and forth to reproduce near (or in some cases, on) the very beach they where they were born.
However, some individuals remain in open oceans and even some adults that have ‘settled’ into near-shore areas are sometimes found to migrate back to oceanic areas to forage. Thus, areas of the open oceans are regarded as important nursing and foraging areas for early juveniles and some adult loggerheads. As successful migration is critical for reproduction and the survival of the species, especially in the juvenile stage at which the mortality is highest, identifying migratory corridors is imperative to applying the EBSA criterion “special importance for life-history stages of species.” Similarly, the identification of terminal foraging areas the turtles are migrating to is also extremely important.
How the area of special importance for life history stages of loggerheads was identified
Several methods of computing an animal’s home range have been implemented, and the most popular and robust are kernel home range estimators. Kernel estimators calculate a utility distribution, allowing us to see one or more areas of high use – and therefore high probability of finding the animal.
Here we used the Home Range Tools (HRT) for ArcGIS to produce a raster layer of the estimated density for the tagged turtles. First, the telemetry data of 11 turtles were selected from a dataset (see “Sources of data” for details). To remove extreme points, a speed-distance-angle filter was applied. As our interest is in open waters, points within 200nm buffers from coasts, including those on land, were also removed. To standardize the locations, the track was interpolated to generate one location per day. HRT was run on these data for each of the 11 turtles. The resulting density layers were reclassified and overlaid each other to generate a single estimated density layer for all the 11 turtles combined. This layer was then smoothed by taking neighborhood statistics to better represent high density areas. Finally, a contour line encompassing a threshold density was delineated as an EBSA for loggerheads in the North Atlantic (Figure 1).
Other methods exist for estimating animal home ranges. The most basic and widely used is the Minimum Convex Polygon (MCP) which estimates the entire range of space use by drawing a polygon around the outermost locations, which can encompass a very large area. Some newer methods are becoming popular, including the local convex hull (LoCoH) nonparametric kernel method. In addition to home range estimates, there are other methods for predicting where a pelagic species is likely to be found. For example, by employing habitat models we can estimate sea turtle habitat distribution using remotely sensed data such as sea surface temperature, chlorophyll concentration and geostrophic currents. The key advantage of habitat modeling is that it can include areas where tagging or observation studies have not yet occurred; however, modeling does add another layer of uncertainty, and tends to have many false positives (i.e. areas that are predicted as habitat where the animals in fact do not occur). Hence, although not comprehensive, using tagging studies is a very conservative approach since the animals are known to frequent the identified areas.
Sources of data
The loggerhead telemetry data were downloaded from OBIS-SEAMAP (Ocean Biogeographic Information System Spatial Ecological Analysis of Megavertebrate Populations ) on July 28, 2009. All the telemetry data used in this illustration originate from the Satellite Tracking and Analysis Tool (STAT) and are incorporated into the OBIS-SEAMAP archives at a scheduled interval. Using the OBIS-SEAMAP mapping interface, we identified the “Duke North Atlantic Turtle Tracking” dataset that recorded loggerhead telemetry data in the oceanic areas of the North Atlantic. While the dataset contains data on 43 green and loggerhead sea turtles, we subdivided out 11 loggerhead turtles that traveled into open oceans for use in this study. The use of the data is courtesy to Catherine McClellan, Duke University Marine Lab.
OBIS-SEAMAP is a spatially referenced online data warehouse, aggregating data on marine megavertebrates (i.e.. marine mammals, seabirds and sea turtles). The telemetry datasets held in OBIS-SEAMAP are ideal for home range studies and habitat models like that presented in this illustration. Larger synthetic analyses of the tracking data held in OBIS-SEAMAP and other data warehouses could be employed to derive more robust estimates of population home ranges and habitat preferences, though standardization of these datasets is a challenge.
All the telemetry data available in OBIS-SEAMAP are cascaded up to OBIS (Ocean Biogeographic Information System). OBIS, a project of Census of Marine Life, is a data center for the Census projects and a parent node of OBIS-SEAMAP. It collects data for all marine life, and currently holds more than 19 million records for 100,000 species (as of August 1st, 2009).
This illustration used telemetry data for 11 individual loggerhead turtles. This should not be presumed to represent the entire home range, or foraging areas, of the loggerhead in the North Atlantic. While the potential EBSA based on home range data identified in this illustration generally agrees with the known migration routes that circumnavigate the edge of Sargasso Sea, as more loggerheads are tagged and data becomes more available the accuracy of the home range estimates will be improved. Further, The 11 tracks available for this illustration are insufficient to allow for division of the study into seasonal components, thus the seasonal importance of different parts of the home range is unknown. Proper identification of EBSAs using this technique should incorporate a temporal component into the analysis.
Another consideration when using tagging data is the fact that some of the tagged animals may have been captive or relocated before they were released to the oceans. These turtles may behave differently than wild turtles (McClellan, personal communication). Thus, the effect of the capture mechanism and the duration of captivity should be considered when contemplating what data should be used or discarded.