Hayden Jensen (Surg Tech Bio 1615 class)
Title: Description of Leatherback Turtle Migration Affecting Conservation Efforts, 2008
Introduction: Leatherback turtles (Dermochelys coriacea) have shown a big population decline
in the last two decades. This decrease in leatherback turtle population is majorly due to
unsustainable egg harvest and fisheries bycatch mortality. A better enhanced education of
movements of the leatherbacks can improve and better educate conservation efforts. This
species has shown a persistent pattern in their post-nesting migration. The ETPS (a protected
marine life area) provides a protected area to record and receive data accurately. This area is a
framework for which movement data and habitat use can be converted into realistic
conservation actions. The data received provides vital information to ensure the survival of this
species in the Eastern Pacific.
Reason for research: The data that this experiment gave enabled researchers to describe the
distribution and movements in the Eastern Pacific, study the influence of oceanic currents on
the leatherback’s migration, determine high use habitats, and view movements of leatherbacks
into the South Pacific. The goal of this experiment is to identify effects on leatherback turtle
migration in order to better conservation efforts.
Materials and Methods: Leatherback sea turtles were tagged with Satellite Relay Data Logger
(SRDL) tags during the years of 2004, 2005, and 2007. SRDL tags are programmed to collect and
record position, temperature, dive data, and tag diagnostic information. In the year of 2004, ten
extra turtles were tagged with Wildlife Computer Smart Position Only (SPOT) Tags. Data from
each set of tags was transmitted through the ARGOS satellite system. State-space models were
applied to the unfiltered satellite data in order to improve accuracy of the estimates of the
turtle’s final position. The observed path of an animal is the result of that animal’s movement
or swimming in addition to the displacement occurred by oceanic currents. To obtain the real
behavior of the turtles, the influence of currents on the animal’s direction was removed.
Distribution of phytoplankton is a known useful indicator of biogeography and the structure of
an ecosystem. Near surface CHL concentration is a surrogate of the phytoplankton standing
stock, and was obtained from the Sea-viewing Wide Field-of-view Sensor. (SeaWiFS) The
relationship between the turtle’s average daily speed and concentration of near surface CHL
was investigated in order to support or invalidate a cause for migration movements including
phytoplankton.
Results: CHL (chlorophyll-a) concentrations were the highest in the equatorial region, and the
lowest concentrations were in the South Pacific Gyre. Swimming speed of the leatherbacks
were significantly higher in regions of high concentration like the equatorial region. Swimming
speeds lowered in regions of low concentration. Energetics of the ocean currents had a
significant impact on the turtle’s migration route. Shifts in direction and magnitude of the
turtle’s migration were all a result of the energetics of the ocean currents. The turtles showed a
pattern in following areas of low kinetic energy within the water. This could possibly due to
turtles need to forage and find food sources that exist in these areas. Further study of the
comparison of turtle meridional velocity to current zonal velocity within the 12 degrees N and 5
degrees South region, uncovered that in zones of strong ocean currents, the leatherbacks
responded by increasing their southward speed, no matter the direction of the currents flow.
After the displacement of the currents were removed, it showed a much straighter pathway
heading southward.
Discussion: The data collected from this study shows a persistent migration pattern for
leatherbacks in the area(s) between the Pacific Coast near Central America, and the South
Pacific. After examination of ocean currents, they were proven to highly affect the turtle’s
migration pathway, but not direction. Response to zonal currents is shown by their increase in
southward velocity, which is used to avoid being pushed out of range of their destination in the
South Pacific. This is why the turtles increase their southward velocity in areas of strong
currents. Meandering movement of the turtles in the South Pacific is largely due to foraging
after the physically demanding energetic process of egg production. Post-nesting leatherbacks
largely occupy oceanic regions rather than coastal regions due to food sources, high water
clarity for hunting, and minimal effort involved in swimming.
Conclusion: Consistent monitoring of leatherback bycatch should be improved by fisheries in
regions of the EP. Time-area closures would provide conservation benefits for other species
whose migratory movements are similar. Time-area closures have been proven to significantly
reduce species contact with fisheries. These are a few conservation suggestions that could be
improved given the data from this experiment.
Bibliography: Shillinger GL, Palacios DM, Bailey H, Bograd SJ, Swithenbank AM, Gaspar P, et al.
(2008) Persistent Leatherback Turtle Migrations Present Opportunities for Conservation. PLoS
Biol 6(7): e171. doi:10.1371/journal.pbio.0060171
http://journals.plos.org/ploscollections/article?id=10.1371/journal.pbio.0060171