East Pacific Green Sea Turtles (Chelonia mydas)
The Mexican nesting stock of East Pacific green sea turtles is currently designated as endangered due to the decline in nesting in recent decades, as well as intense poaching and harvesting during the middle of the century. This stock of green turtles ranges from South America to southern California, but the vast majority are born on sandy beaches in southern Mexico, primarily in the state of Michoacán, before beginning their juvenile stage of life (see region shown in map below).
The oceanic juvenile stage of East Pacific green turtles remains one of the least understood stages in the life history of this endangered species population. In the last decade, progress has been made on elucidating the lost years oceanic juvenile state of marine turtles (Reich et al. 2007, Arthur et al. 2008, Cardona et al. 2010, Snover et al. 2010). However, the duration of the juvenile stage of this endangered stock of green turtles remains to be known and is current research priority for the management of this slowly recovering population of turtles.
This oceanic juvenile stage is currently estimated to last from two to five+ years, during which turtles feed opportunistically and omnivorously - eating what they can find - along ocean current boundaries. Eventually these juveniles are large enough settle into nearshore subtidal algae and seagrass ecosystems to feed and grow (Carr 1987, Reich et al. 2007). If they were to live in nearshore habitats too soon, they would be too small and would become prey to larger fish, birds and other common predators in coastal ecosystems. After growing even more in these nearshore habitats, turtles reach maturity, at around thirty years of age, and return to their nesting sites to breed approximately every three years. In between breeding times, the turtles migrate between nesting grounds and foraging areas in Mexico (both along the Pacific coast and in the Gulf of California) and some even live as far north as California, USA. Main foraging (or eating) grounds include bays and lagoons where green turtles eat several species of eelgrass, marine algae, and invertebrates like crabs and snails (Seminoff et al. 2002a, Lopez-Castro et al. 2010, Lemons et al. 2011).
|Larger region, zoomed in area to the right -->|
|Region of East Pacific Green Turtles: San Diego shown to the north, nesting area in Michoacán in the south.|
This population of green turtles is considered a regional management unit - meaning it has its own management plans and and protection. It was also once the target of a large turtle fishery that reached its peak in the 1970s. The fishery harvested turtle meat and eggs that were sold and consumed in large numbers. Due to this fishery, this once plentiful population of green turtles is currently a fraction of the size of what it once was (National Marine Fisheries Service 2007a). Cooperative efforts, including protective measures put in place in 1990 by the Mexican government, prevented further collapse of the green turtle population, and evidence of slow recovery continues to be observed (Koch et al. 2006). Despite international legal protection, impacts beyond poaching, such as fishing bycatch, boat strikes, development of nesting beaches, and development of coastal foraging areas, continue to threaten these sea turtles.
North Pacific Loggerhead Sea Turtles (Caretta caretta)
In the Pacific, two genetically distinct stocks of loggerhead turtles have been identified (National Marine Fisheries Service 2007b). One in the South Pacific, which nests in and around Australia, and one in the North Pacific, which nests on multiple islands of Japan (Bowen et al. 1995). Both of the Pacific population stocks are currently designated as endangered due to the decline in nesting in recent decades (Kamezaki et al., 2003). In the North Pacific, over 40 nesting beaches are known and monitored, yet approximately 30-40% of this population’s nesting occurs at three main beaches on the small island of Yakushima at the southern end of Japan (Kamezaki et al., 2003). Hatchlings from these beaches, once they swim offshore, are moved north and then east by swimming with the Kuroshio Current and the North Pacific drift - eventually being guided toward the Central North Pacific (CNP), close to Hawaii. Immature loggerhead turtles have been found as fisheries bycatch in the CNP around the extended Hawaiian Islands. Satellite tracks of juvenile loggerheads show extended and concentrated foraging in the areas where currents help create highly productive waters (specifically near the Kuroshio Extension Bifurcation Region and the Transition Zone Chlorophyll Front) (Polovina et al. 2004, Polovina et al. 2006, Kobayashi et al. 2008). Eventually some of these turtles are guided by the California Current, and a large aggregation of loggerheads, estimated to be in the tens of thousands, forage off the coast of the Baja California Peninsula (BCP) (Lewison et al. 2004, Wallace et al. 2008, Peckham et al. 2011).
|Loggerhead sea turtle. Image by author.|
Studies examining body size and stomach contents of fishery-bycaught and dead-stranded turtles show that turtles near Hawaii (CNP turtles) are smaller in size and foraged primarily on pelagic invertebrates and fish commonly associated with fisheries bycatch and bait. In contrast, turtles examined from the Baja region (BCP) were slightly larger in size, although still juveniles, and ate primarily pelagic red crabs (Pleuroncodes planipes) and fish associated with fishing operations (Peckham et al. 2011). Upon reaching maturity, adult loggerheads migrate back across the Pacific to return to the neritic waters of the Western Pacific where they remain to forage and breed for the rest of their lives (Resendiz et al. 1998, Nichols et al. 2000, Hatase et al. 2002, Hatase et al. 2004).
A primary threat to the North Pacific loggerheads in the East Pacific is the intense mortality associated with small-scale artisanal fisheries. Conservative estimates indicate that 1000+ individual loggerheads are killed per year (Peckham et al. 2007, Peckham et al. 2008) from bycatch related to BCP small-scale fisheries. Upwelling in the East Pacific creates productive waters that result in large amounts of biomass (plankton, fish & other animals) in a concentrated area. While the loggerheads in this area are foraging primarily on swarms of red crab, many types of fish also forage in these productive waters. The concentration of fish attracts local fishers to the same location, thereby contributing to high turtle-fishery interaction making this area a bycatch “hotspot” (Peckham et al. 2007, Wingfield et al. 2011).
The figure below is from Peckham et al., 2007 (click for access to full article and figures).
Notice the concentration of loggerhead turtles (red areas) in the vicinity of small-scale fishing efforts (white boat logos and outline).
In the past fifteen years, significant efforts have been made to engage local fishers and address the problem of this turtle hotspot. Gradually, fishing practices are being adjusted in order to reduce interactions with turtles and a protected area has been established (Lewison et al. 2011). Cooperative work by Grupo Tortuguero and their partners in communities throughout Baja and Pacific Mexico states have been addressing this issue of bycatch, as well as poaching and other threats to marine turtles and sea life through diverse community engagement programs. And while much has been learned about this population of loggerheads and their use of this productive yet dangerous East Pacific habitat, it remains unclear how long the juvenile loggerheads forage in this BCP hotspot region prior to migrating to the West Pacific. Understanding the duration of exposure to sources of high mortality is important information for population management (Hamann et al. 2010, Wallace et al. 2011).
My research will address these some of these unknowns related to habitat use and duration of different life stages of turtles of each of these two populations.
|Me helping H. Peckham tag and measure a loggerhead in the "loggerhead hotspot" off of||San Juanico, BCP, MEX|
Arthur, K. E., Boyle, M. C. and Limpus, C. J. (2008) 'Ontogenetic changes in diet and habitat use in green sea turtle (Chelonia mydas) life history', Marine Ecology Progress Series, 362, 303-311.
Bowen, B. W., Abreugrobois, F. A., Balazs, G. H., Kamezaki, N., Limpus, C. J. and Ferl, R. J. (1995) 'TRANS-PACIFIC MIGRATIONS OF THE LOGGERHEAD TURTLE (CARETTA-CARETTA) DEMONSTRATED WITH MITOCHONDRIAL-DNA MARKERS', Proceedings of the National Academy of Sciences of the United States of America, 92(9), 3731-3734.
Cardona, L., Campos, P., Levy, Y., Demetropoulos, A. and Margaritoulis, D. (2010) 'Asynchrony between dietary and nutritional shifts during the ontogeny of green turtles (Chelonia mydas) in the Mediterranean', Journal of Experimental Marine Biology and Ecology, 393(1-2), 83-89.
Carr, A. (1987) 'New perspectives on the pelagic stage of sea turtle development', Conservation Biology, 1(2), 103-121.
Hamann, M., Godfrey, M. H., Seminoff, J. A., Arthur, K., Barata, P. C. R., Bjorndal, K. A., Bolten, A. B., Broderick, A. C., Campbell, L. M., Carreras, C., Casale, P., Chaloupka, M., Chan, S. K. F., Coyne, M. S., Crowder, L. B., Diez, C. E., Dutton, P. H., Epperly, S. P., FitzSimmons, N. N., Formia, A., Girondot, M., Hays, G. C., Cheng, I. J., Kaska, Y., Lewison, R., Mortimer, J. A., Nichols, W. J., Reina, R. D., Shanker, K., Spotila, J. R., Tomas, J., Wallace, B. P., Work, T. M., Zbinden, J. and Godley, B. J. (2010) 'Global research priorities for sea turtles: informing management and conservation in the 21st century', Endangered Species Research, 11(3), 245-269.
Hatase, H., Takai, N., Matsuzawa, Y., Sakamoto, W., Omuta, K., Goto, K., Arai, N. and Fujiwara, T. (2002) 'Size-related differences in feeding habitat use of adult female loggerhead turtles Caretta caretta around Japan determined by stable isotope analyses and satellite telemetry', Marine Ecology-Progress Series, 233, 273-281.
Hatase, H., Matsuzawa, Y., Sato, K., Bando, T. and Goto, K. (2004) 'Remigration and growth of loggerhead turtles (Caretta caretta) nesting on Senri Beach in Minabe, Japan: life-history polymorphism in a sea turtle population', Marine Biology, 144(4), 807-811.
Kamezaki, N., et al. (2003) ‘Loggerhead Turtles Nesting in Japan’. Pages 210-217 in
Bolten, A.B. and B.E. Witherington (editors). Loggerhead Sea Turtles. Smithsonian Books, Washington D.C.
Kobayashi, D. R., Polovina, J. J., Parker, D. M., Kamezaki, N., Cheng, I. J., Uchida, I., Dutton, P. H. and Balazs, G. H. (2008) 'Pelagic habitat characterization of loggerhead sea turtles, Caretta caretta, in the North Pacific Ocean (1997-2006): Insights from satellite tag tracking and remotely sensed data', Journal of Experimental Marine Biology and Ecology, 356(1-2), 96-114.
Koch, V., Nichols, W. J., Peckham, H. and de la Toba, V. (2006) 'Estimates of sea turtle mortality from poaching and bycatch in Bahia Magdalena, Baja California Sur, Mexico', Biological Conservation, 128(3), 327-334.
Lemons, G., Lewison, R., Komoroske, L., Gaos, A., Lai, C.-T., Dutton, P., Eguchi, T., LeRoux, R. and Seminoff, J. A. (2011) 'Trophic ecology of green sea turtles in a highly urbanized bay: Insights from stable isotopes and mixing models', Journal of Experimental Marine Biology and Ecology, 405(1-2), 25-32.
Lewison, R. L., Freeman, S. A. and Crowder, L. B. (2004) 'Quantifying the effects of fisheries on threatened species: the impact of pelagic longlines on loggerhead and leatherback sea turtles', Ecology Letters, 7(3), 221-231.
Lewison, R. L., Soykan, C. U., Cox, T., Peckham, H., Pilcher, N., LeBoeuf, N., McDonald, S., Moore, J., Safina, C. and Crowder, L. B. (2011) 'INGREDIENTS FOR ADDRESSING THE CHALLENGES OF FISHERIES BYCATCH', Bulletin of Marine Science, 87(2), 235-250.
Lopez-Castro, M. C., Koch, V., Mariscal-Loza, A. and Nichols, W. J. (2010) 'Long-term monitoring of black turtles Chelonia mydas at coastal foraging areas off the Baja California Peninsula', Endangered Species Research, 11(1), 35-45.
National Marine Fisheries Service 2007a, National Marine Fisheries Service 2007b,
Available online at: http://www.nmfs.noaa.gov/pr/species/turtles/
Nichols, W. J., Resendiz, A., Seminoff, J. A. and Resendiz, B. (2000) 'Transpacific migration of a loggerhead turtle monitored by satellite telemetry', Bulletin of Marine Science, 67(3), 937-947.
Peckham, S. H., Diaz, D. M., Walli, A., Ruiz, G., Crowder, L. B. and Nichols, W. J. (2007) 'Small-Scale Fisheries Bycatch Jeopardizes Endangered Pacific Loggerhead Turtles', Plos One, 2(10).
Peckham, S. H., Maldonado-Diaz, D., Koch, V., Mancini, A., Gaos, A., Tinker, M. T. and Nichols, W. J. (2008) 'High mortality of loggerhead turtles due to bycatch, human consumption and strandings at Baja California Sur, Mexico, 2003 to 2007', Endangered Species Research, 5(2-3), 171-183.
Peckham, S. H., Maldonado-Diaz, D., Tremblay, Y., Ochoa, R., Polovina, J., Balazs, G., Dutton, P. H. and Nichols, W. J. (2011) 'Demographic implications of alternative foraging strategies in juvenile loggerhead turtles Caretta caretta of the North Pacific Ocean', Marine Ecology-Progress Series, 425, 269-280.
Polovina, J. J., Balazs, G. H., Howell, E. A., Parker, D. M., Seki, M. P. and Dutton, P. H. (2004) 'Forage and migration habitat of loggerhead (Caretta caretta) and olive ridley (Lepidochelys olivacea) sea turtles in the central North Pacific Ocean', Fisheries Oceanography, 13(1), 36-51.
Polovina, J., Uchida, I., Balazs, G., Howell, E. A., Parker, D. and Dutton, P. (2006) 'The Kuroshio Extension Bifurcation Region: A pelagic hotspot for juvenile loggerhead sea turtles', Deep-Sea Research Part Ii-Topical Studies in Oceanography, 53(3-4), 326-339.
Reich, K. J., Bjorndal, K. A. and Bolten, A. B. (2007) 'The 'lost years' of green turtles: using stable isotopes to study cryptic lifestages', Biology Letters, 3(6), 712-714.
Resendiz, A., Resendiz, B., Nichols, W. J., Seminoff, J. A. and Kamezaki, N. (1998) 'First confirmed east-west Transpacific movement of a loggerhead sea turtle, Caretta caretta, released in Baja California, Mexico', Pacific Science, 52(2), 151-153.
Seminoff, J. A., Resendiz, A. and Nichols, W. J. (2002a) 'Diet of East Pacific green turtles (Chelonia mydas) in the central Gulf of California, Mexico', Journal of Herpetology, 36(3), 447-453.
Snover, M. L., Hohn, A. A., Crowder, L. B. and Macko, S. A. (2010) 'Combining stable isotopes and skeletal growth marks to detect habitat shifts in juvenile loggerhead sea turtles Caretta caretta', Endangered Species Research, 13(1), 25-31.
Wallace, B. P., Heppell, S. S., Lewison, R. L., Kelez, S. and Crowder, L. B. (2008) 'Impacts of fisheries bycatch on loggerhead turtles worldwide inferred from reproductive value analyses', Journal of Applied Ecology, 45(4), 1076-1085.
Wallace, B. P., DiMatteo, A. D., Bolten, A. B., Chaloupka, M. Y., Hutchinson, B. J., Abreu-Grobois, F. A., Mortimer, J. A., Seminoff, J. A., Amorocho, D., Bjorndal, K. A., Bourjea, J., Bowen, B. W., Duenas, R. B., Casale, P., Choudhury, B. C., Costa, A., Dutton, P. H., Fallabrino, A., Finkbeiner, E. M., Girard, A., Girondot, M., Hamann, M., Hurley, B. J., Lopez-Mendilaharsu, M., Marcovaldi, M. A., Musick, J. A., Nel, R., Pilcher, N. J., Troeng, S., Witherington, B. and Mast, R. B. (2011) 'Global Conservation Priorities for Marine Turtles', Plos One, 6(9).
Wingfield, D. K., Hoyt Peckham, S., Foley, D. G., Palacios, D. M., Lavaniegos, B. E., Durazo, R., Nichols, W. J., Croll, D. A. and Bograd, S. J. (2011) 'The Making of a Productivity Hotspot in the Coastal Ocean', Plos One, 6(11).