Download Juvenile green sea turtle (Chelonia mydas) foraging ecology

Juvenile green sea turtle (Chelonia mydas) foraging ecology
Annabelle Brooks, Cape Eleuthera Institute
Sea turtles are long-lived marine reptiles that spend the majority of their lives at sea. They have survived
for millions of years but recently they have succumbed to anthropogenic threats that could lead to their
eventual extinction (Wallace et al. 2001, Hamann et al. 2010). The history of sea turtles in the Caribbean
dates back centuries and four of the world’s seven sea turtle species are found in The Bahamas.
Traditionally harvested by local fishermen for their rich meat, sea turtles provided a valuable source of
income to fishermen, and an important food source to island people. Overexploitation and habitat loss
has caused all four species to be recognized as endangered or critically endangered, and subsequently
listed on the International Union for the Conservation of Nature (IUCN) Red List. In response to these
severe declines in population numbers, the Bahamas Ministry of Agriculture and Marine Resources
passed legislation that gave full protection to all sea turtles found in Bahamian waters in 2009. It is now
illegal to harvest, buy or sell any marine turtle products, yet sea turtles are still in danger of losing
important nesting grounds and foraging habitats due to coastal development. The protection of
individuals in Bahamian waters allows for the investigation of information that is vital for the effective
management and conservation of sea turtles populations throughout their early life stages, as harvest or
other anthropogenic disturbance could reduce the reproductive potential of the entire population.
The shallow banks environment of The Bahamas is an important foraging ground for green sea turtles
during the juvenile and sub-adult phases, when they frequent coastal habitats such as tidal mangrove
creeks, seagrass beds and coral reefs where they primarily feed on seagrass (Aragones et al. 2006).
These sites vary greatly in biotic and abiotic structure (e.g. in size, abundance of seagrass, predation
threat) which affects their quality as a feeding ground. Variation in resources, as well as resource use,
will occur throughout the home range of an individual as it occupies different habitats. Predation
pressure within foraging sites is a biotic factor that may influence the movements and distribution of sea
turtles both within and between foraging sites. High predation risk may cause sea turtles to spend time
in sub-optimal foraging areas (Heithaus et al. 2007) or alter their behaviors (e.g. swimming, feeding,
resting etc.). These can have knock-on effects throughout an ecosystem such as changing structure and
productivity of seagrass pastures (Moran & Bjorndal 2005). The spatial and temporal structure of finescale movements and activities conducted by individuals, including entry and exit of the site, take place
on shorter timescales (e.g. daily) and can be caused by a number of physical, biological and
environmental factors (Patterson et al. 2008). Identifying these fine-scale patterns is vital to a better
understanding of habitat use within discrete aggregations of foraging sea turtles and will also identify
essential habitat. As anthropogenic impacts on natural systems become more widespread, this
information may potentially allow mitigation of disruption to these endangered species.
Eleuthera provides an interesting landscape to investigate the spatial dynamics of immature green
turtles within foraging grounds. The goal of the research conducted by the Cape Eleuthera Institute sea
turtle research team is to elucidate the processes of site selection, movements and site fidelity, resource
use, and interactions between individuals within foraging grounds. Students joining this team can
investigate the relative abundance and density of sea turtles to elucidate what factors may influence
foraging site selection. Predator-focused projects can investigate the diversity and relative abundance of
predators across foraging sites. Benthic habitat projects can map the variability in resources across and
within foraging sites, and subsequently compare this to relative abundance patterns in sea turtles.
Morphomeric data of sea turtle individuals could also be explored. Students will be fully involved in all
aspects of data collection.
Suggested Reading:
Aragones LV, Lawler IR, Foley WJ , Marsh H. 2006. Dugong grazing and turtle cropping: grazing
optimization in tropical seagrass systems? Oecologia. 149:635–647
Bjorndal KA, Bolten AB, Chaloupka MY. 2000. Green turtle somatic growth model: evidence for density
dependence. Ecological Applications 10:269-282
Brooks EJ, Sloman KA, Sims DW, Danylchuk AJ. 2011. Validating the use of baited remote underwater
video surveys for assessing the diversity, distribution and abundance of sharks in the Bahamas.
Endangered Species Research. 13: 231–243
Hamann M, Godfrey MH, Seminoff JA, Arthur K, Barata PCR, Bjorndal KA, Bolten AB, Broderick AC,
Campbell LM, Carreras C, Casale P, Chaloupka M, Chan SKF, Coyne MS, Crowder LB, Diez CE, Dutton
PH, Epperly SP, FitzSimmons NN, Formia A, Girondot M, Hays GC, Cheng IJ, Kaska Y, Lewison R,
Mortimer JA, Nichols WJ, Reina RD, Shanker K, Spotila JR, Tomás J, Wallace BP, Work TM, Zbinden J,
Godley BJ. 2010. Global research priorities for sea turtles: informing management and conservation in
the 21st century. Endangered Species Research. 11: 245–269
Heithaus MR, Frid A, Wirsing AJ, Dill LM, Fourqurean JW, Burkholder D, Thomson J, Bejder OL. 2007.
State-dependent risk-taking by green sea turtles mediates top-down effects of tiger shark intimidation in
a marine ecosystem. Journal of Animal Ecology. 76(5)837-844
Kubis S, Chaloupka M, Ehrhart L, Bresette M. 2009. Growth rates of juvenile green turtles Chelonia
mydas from three ecologically distinct foraging habitats along the east central coast of Florida, USA.
Marine Ecology Progress Series. 389:257–269
Moran KL, Bjorndal KA. 2005. Simulated green turtle grazing affects structure and productivity of
seagrass pastures. Marine Ecology Progress Series. 305: 235–247
Wallace BP, DiMatteo AD, Bolten AB, Chaloupka MY, Hutchinson BJ, Abreu-Grobois A, Mortimer JA,
Seminoff JA, Amorocho D, Bjorndal KA, Bourjea J, Bowen BW, Briseno Duenas R, Casale P, Choudhury
BC, Costa A, Dutton PH, Fallabrino A, Finkbeiner EM, Girard A, Girondot M, Hamann M, Hurley BJ,
Lopez-Mendilaharsu M, Marcovaldi MA, Musick JA, Nel R, Pilcher NJ, Troeng S, Witherington B, Mast
RB. 2011. Global Conservation Priorities for Marine Turtles. PLoS ONE 6(9): e24510.
doi:10.1371/journal.pone.0024510