Tuesday, December 4, 2012

The Importance of Understanding Life Stage Duration

Did you know that it is currently not possible to accurately age a living turtle? Yet successful conservation of marine turtles relies on having the best possible understanding of habitat use and complete life history. Therefore, age-at-size estimates are used to construct population structure and abundance models for management (Crouse et al. 1987, Heppell et al. 2003). Hamann et al. (2010) identified the need to “develop methods to accurately age individual turtles, determine a population’s (or species’) mean age-at-maturity, and define age-based demography” as one of the top 20 global research priorities for marine turtle conservation and management. This report also concluded that researchers must identify the “parameters [that] influence the biogeography of sea turtles in the oceanic realm” and resolve the “demography of turtles in the oceanic stage, including the duration of the oceanic juvenile stages”.
Source: http://seaturtlefoundation.com/

Determining age of turtles and life stage-duration through my research will address this priority to improve the population ecology and conservation of sea turtles. In addition, each distinct life stage of marine turtles experiences different types and intensities of threats. For example, a small juvenile turtle living out in the open ocean is exposed to threats (i.e. large industrial fishing fleets, natural oceanic predators, ghost-fishing nets & etc.) that are very different from the types of threats that a much large turtle will be faced with while living in a coastal area or lagoon (i.e. boat traffic, coastal pollution, smaller scale fishing efforts & etc.).

Assessing the variability of these impacts at each stage and habitat is difficult when stage duration is uncertain, making it a challenge to prioritize and implement protection measures to mitigate these impacts. By more accurately determining the duration of each life stage (i.e. how many years a turtle is in each stage) the full impact of particular sources of mortality on the overall population can be prioritized and more effectively managed.

My research will uses skeletochronology- a method to age dead turtles, together with other emerging techniques to generate age-based demographic parameters, including stage duration, for two populations of marine turtles in the Pacific. This research will contribute important information to marine turtle managers by directly addressing two of the top research priorities identified for marine turtle conservation.

Next blog entry: Techniques used to address these questions - learn more about how skeletochronology and stable isotope analysis can determine the age and past locations of sea turtles. 

And if you have not already - please see the previous entry, "A call to help: Support Community Based Sea Turtle Conservation & UCSD Research" and watch the video to learn more about my research and how YOU can help out

Some images in video obtained under NMFS Permit #1591


Avens, L., and L. R. Goshe. 2007. Comparative skeletochronological analysis of Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) humeri and scleral ossicles. Marine Biology (Berlin) 152:1309-1317.
Bjorndal, K. A., A. B. Bolten, R. A. Bennett, E. R. Jacobson, T. J. Wronski, J. J. Valeski, and P. J. Eliazar. 1998. Age and growth in sea turtles: Limitations of skeletochronology for demographic studies. Copeia:23-30.
Casale, P., N. Conte, D. Freggi, C. Cioni, and R. Argano. 2011. Age and growth determination by skeletochronology in loggerhead sea turtles (Caretta caretta) from the Mediterranean Sea. Scientia Marina 75:197-203.
Coles, W. C., J. A. Musick, and L. A. Williamson. 2001. Skeletochronology validation from an adult loggerhead (Caretta caretta). Copeia:240-242.
Crouse, D. T., L. B. Crowder, and H. Caswell. 1987. A STAGE-BASED POPULATION MODEL FOR LOGGERHEAD SEA TURTLES AND IMPLICATIONS FOR CONSERVATION. Ecology (Washington D C) 68:1412-1423.
Goshe, L. R., L. Avens, F. S. Scharf, and A. L. Southwood. 2010. Estimation of age at maturation and growth of Atlantic green turtles (Chelonia mydas) using skeletochronology. Marine Biology 157:1725-1740.
Hamann, M., M. H. Godfrey, J. A. Seminoff, K. Arthur, P. C. R. Barata, K. A. Bjorndal, A. B. Bolten, A. C. Broderick, L. M. Campbell, C. Carreras, P. Casale, M. Chaloupka, S. K. F. Chan, M. S. Coyne, L. B. Crowder, C. E. Diez, P. H. Dutton, S. P. Epperly, N. N. FitzSimmons, A. Formia, M. Girondot, G. C. Hays, I. J. Cheng, Y. Kaska, R. Lewison, J. A. Mortimer, W. J. Nichols, R. D. Reina, K. Shanker, J. R. Spotila, J. Tomas, B. P. Wallace, T. M. Work, J. Zbinden, and B. J. Godley. 2010. Global research priorities for sea turtles: informing management and conservation in the 21st century. Endangered Species Research 11:245-269.
Heppell, S. S., M. L. Snover, and L. B. Crowder. 2003. Sea turtle population ecology.
Parham, J. F., and G. R. Zug. 1997. Age and growth of loggerhead sea turtles (Caretta caretta) of coastal Georgia: An assessment of skeletochronological age-estimates. Bulletin of Marine Science 61:287-304.
Reich, K. J., K. A. Bjorndal, and A. B. Bolten. 2007. The 'lost years' of green turtles: using stable isotopes to study cryptic lifestages. Biology Letters 3:712-714.
Snover, M. L., L. Avens, and A. A. Hohn. 2007a. Back-calculating length from skeletal growth marks in loggerhead sea turtles Caretta caretta. Endangered Species Research 3:95-104.
Snover, M. L., and A. A. Hohn. 2004. Validation and interpretation of annual skeletal marks in loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii) sea turtles. Fishery Bulletin (Seattle) 102:682-692.
Snover, M. L., A. A. Hohn, L. B. Crowder, and S. S. Heppell. 2007b. Age and growth in Kemp's ridley sea turtles: evidence from mark-recapture and skeletochronology.
Snover, M. L., and A. G. J. Rhodin. 2008. Comparative ontogenetic and phylogenetic aspects of chelonian chondro-osseous growth and skeletochronology. Biology of Turtles:17-43.
Zug, G. R. 1985. Skeletochronological age estimates for Hawaiian green turtles. Marine Turtle Newsletter:9-10.
Zug, G. R., G. H. Balazs, and J. A. Wetherall. 1995. Growth in juvenile loggerhead sea turtles (Caretta caretta) in the North Pacific pelagic habitat. Copeia 1995:484-487.
Zug, G. R., G. H. Balazs, J. A. Wetherall, D. M. Parker, and S. K. K. Murakawa. 2002. Age and growth of Hawaiian green seaturtles (Chelonia mydas): an analysis based on skeletochronology. Fishery Bulletin 100:117-127.
Zug, G. R., M. Chaloupka, and G. H. Balazs. 2006. Age and growth in olive ridley seaturtles (Lepidochelys olivacea) from the north-central Pacific: a skeletochronological analysis. Marine Ecology-an Evolutionary Perspective 27:263-270.
Zug, G. R., and R. E. Glor. 1998. Estimates of age and growth in a population of green sea turtles (Chelonia mydas) from the Indian River lagoon system, Florida: a skeletochronological analysis. Canadian Journal of Zoology-Revue Canadienne De Zoologie 76:1497-1506.
Zug, G. R., H. J. Kalb, and S. J. Luzar. 1997. Age and growth in wild Kemp's ridley seaturtles Lepidochelys kempii from skeletochronological data. Biological Conservation 80:261-268.
Zug, G. R., A. H. Wynn, and C. Ruckdeschel. 1986. Age determination of loggerhead sea turtles, Caretta caretta, by incremental growth marks in the skeleton. Smithsonian Contributions to Zoology:i.