Pacific Leatherback Turtles
New tracking technologies have allowed researchers to examine the movements of the critically endangered Pacific Leatherback turtle. Several years of tracking have revealed a consistent foraging area for leatherback turtles in the South Pacific Gyre.
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Studying pelagic species on the high seas has traditionally been difficult. The large distances from shore coupled with the highly mobile nature of the organisms have precluded direct observation. Recent technological advances have permitted researchers to track highly migratory pelagic species by allowing data collection and transmission remotely (Eckert, 2006). These novel electronic tags have been particularly useful for studies involving air-breathing animals in the open ocean, as frequent surfacing allows for direct uplinks to satellites, and animals can therefore be tracked in near real time. While the data these tags have returned is invaluable in shedding light on the basic biology of pelagic species, they gain even more importance when addressing questions pertaining to conservation of severely threatened and endangered species. A prime example of this is the recent electronic tracking conducted on leatherback turtles in the eastern Pacific Ocean.
Like many marine turtle species, the slow growth and low reproductive potential of leatherback turtles makes them particularly sensitive to excessive mortality of adult life stages. Leatherbacks in the eastern Pacific Ocean have suffered through illegal poaching and egg collecting on the nesting beaches, resulting in severe population declines. Surveys at nesting beaches in Costa Rica and southern Mexico have seen numbers decline by over 90% in the last 20 years and these populations are now recognized as critically endangered (Spotila et al. 2000). Ongoing conservation efforts have decreased land based mortality, yet little was known regarding threats to the turtles in the open ocean. Long-term electronic tagging of the Costa Rica population was initiated in 2004 and in the following three years, 46 mature female leatherbacks were tagged (Shillinger et al. 2008). After leaving the nesting beaches, tagged turtles travelled to the south with most following the same migration corridor across the Equator and into the South Pacific. Although the Equatorial region of the eastern Pacific shows consistent divergence induced upwelling, resulting in localized peaks in primary productivity, turtles spent little time in this region and instead showed increased swim speeds across the Equator (figure 1). Upon arrival into the south Pacific, swimming speeds slowed and leatherbacks exhibited a more meandering swimming pattern, indicative of foraging behavior.
Figure 1: Reproduced from Shillinger et al. 2008. Panel A: daily swimming speed of tracked turtles as they moved to the south from the tagging area in Costa Rica. Panel B: Tracks of tagged turtles plotted over eddy kinetic energy, red indicates high energy regions and white indicates low energy regions. Panel C: Turtle tracks plotted over chlorophyll concentration, green areas indicating highest chlorophyll levels.
At first glance, this portion of the South Pacific Gyre appears unremarkable in almost any measureable oceanographic variable. Primary productivity levels are among the lowest in the ocean, there is little bathymetric structure, temperatures are mainly uniform, providing little in the way of thermal breaks and edges that usually aggregate prey and current patterns and eddies are particularly weak. However, conditions that are commonly perceived to aggregate pelagic fish species (high productivity, thermal and bathymetric structure and complex eddies) may not be the same conditions that are most beneficial for leatherback turtle foraging. While predatory pelagic fish species feed primarily on smaller fish, leatherback turtles forage almost exclusively on gelatinous zooplankton such as jellyfish (Shoop and Kenney, 1992). Passively drifting or weakly swimming prey items such as jellyfish may tend to accumulate in areas due to broad scale current patterns that can aggregate these species over time. This purely physical effect may, in some cases, outweigh the benefits of increased productivity. In addition, clear, nutrient poor water and low oceanic current may serve to maximize foraging efficiency by enhancing prey detection and minimizing swimming effort, respectively (Schillinger et al. 2008). In addition to defining oceanography statically, temporal patterns may also play an important role in defining a given region. The South Pacific Gyre is also a region of incredibly low levels of seasonal or inter-annual variability in both temperature and primary productivity. This region, as well as other Pacific leatherback high-use areas in the central north Pacific, sees some of the lowest levels of temperature variability anywhere in the world’s oceans. Selecting foraging areas in inherently stable regions may act to minimizing uncertainty in what conditions an animal will find upon arrival. This may be of added importance to highly migratory species, given the large distances covered between nesting/spawning areas and foraging grounds
How the area of importance for threatened, endangered or declining species and/or habitats was identified
Shillinger et al (2008) noticed that tracked leatherbacks showed strongly directed movement during the northern portion of their migration and then more meandering tracks once they arrived in the South Pacific Gyre. These movements were analyzed in relation to four oceanographic and physiographic features: Ocean currents, chlorophyll concentration, bathymetry and geomagnetism. Turtles did not appear to travel in relation to bathymetry or geomagnetism. Current patterns affected turtles in two ways, firstly through a direct physical influence on swimming. As turtles migrated south from the nesting beaches they encountered strong current patterns that caused them to veer off course, even if they maintained a constant heading. To compensate for this effect, turtles increased swimming speed when moving through these high current regions. Secondly, currents and eddies influenced preferred habitat of the turtles after traveling through the Equatorial region. Tagged turtles spent more time in and showed more meandering tracks while in areas of lower eddy kinetic energy, suggesting that these areas are important foraging areas for Pacific Leatherbacks. Likewise, turtles showed slower swimming and less directed paths while in areas of lower productivity, indicating a preference for these conditions. Using this information, Shillinger et al (1998) were able to identify a region of low eddy kinetic energy and phytoplankton concentration (figure 2).
Figure 2: Reproduced from Shillinger et al. 2008. Colors from red to yellow show the density utilization distribution of tracked leatherback turtles with red areas being the regions with the highest utilization. The green outline highlights the region identified as having particularly low primary productivity and eddy kinetic energy.
Sources of Data
Turtle track data were obtained by tagging turtles with either Sea Mammal Research Unit (SMRU) Satellite Relay Data Logger (SRDL) tags or Wildlife Computer, Smart Position Only (SPOT) tags. Both these tag types uplink to ARGOS satellites when the turtles surface, providing information on the position of the animal. The SRDL tags also collect and transmit temperature, dive data, and tag diagnostic information. Oceanographic and physiographic data were obtained through freely available sources. Chlorophyll data were obtained from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite, the ocean current data through the Archiving, Validation, and Interpretation of Satellite Oceanographic data (Aviso) project satellites, bathymetry came from the Smith and Sandwell (1997) global bathymetry dataset and geomagnetic data were obtained through the International Geomagnetic Reference Field (IGRF-10) dataset.
When trying to identify ecologically and biologically significant areas based on marine animal tracking data, it is important to delineate different behavior patterns throughout the track. Some animals show a distinct migratory phase consisting of highly directional travel and a foraging phase that shows more circuitous movement paths. In the case of Pacific leatherbacks, migration corridors were found in the region between the nesting beaches and the Equator. This corridor was limited in both its spatial extent as well as the timing of migration. In addition, it was possible to identify a specific region of the South Pacific Gyre as an important foraging region for these turtles. Although the South Pacific Gyre appears unremarkable based on the criteria we normally choose to define ecologically and biologically significant areas (high productivity, complex bathymetry, and major current patterns), migrations over three tagging years showed to be a consistent foraging area for leatherback turtle populations in the eastern Pacific. The high number of turtles tracked to this region over multiple years can give us confidence that this region is vital to post nesting leatherback turtles. These findings show that, by letting animals of interest show us what regions are important to them, it is possible to define EBSAs even in regions that we may not have originally thought were significant.