Temporal, spatial, and body size effects on growth rates of loggerhead sea turtles (Caretta caretta) in the Northwest Atlantic
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In response to a call from the US National Research Council for research programs to combine their data to improve sea turtle population assessments, we analyzed somatic growth data for Northwest Atlantic (NWA) loggerhead sea turtles (Caretta caretta) from 10 research programs. We assessed growth dynamics over wide ranges of geography (9–33°N latitude), time (1978–2012), and body size (35.4–103.3 cm carapace length). Generalized additive models revealed significant spatial and temporal variation in growth rates and a significant decline in growth rates with increasing body size. Growth was more rapid in waters south of the USA (<24°N) than in USA waters. Growth dynamics in southern waters in the NWA need more study because sample size was small. Within USA waters, the significant spatial effect in growth rates of immature loggerheads did not exhibit a consistent latitudinal trend. Growth rates declined significantly from 1997 through 2007 and then leveled off or increased. During this same interval, annual nest counts in Florida declined by 43 % (Witherington et al. in Ecol Appl 19:30–54, 2009) before rebounding. Whether these simultaneous declines reflect responses in productivity to a common environmental change should be explored to determine whether somatic growth rates can help interpret population trends based on annual counts of nests or nesting females. Because of the significant spatial and temporal variation in growth rates, population models of NWA loggerheads should avoid employing growth data from restricted spatial or temporal coverage to calculate demographic metrics such as age at sexual maturity.
KeywordsGrowth Increment Green Turtle Nest Female Somatic Growth Rate Straight Carapace Length
We are very grateful to the many people who assisted with turtle captures and measurements: B. Bolt, A. Brame, B. Brost, R. Burrows, J. Byrd, R. Cancro, E. Chadwick, M. Cherkiss, S. Connett, C. Crady, B. Crouchley, S. Gann, C. Hackett, T. Hirama, R. Lowers, K. Holloway-Adkins, K. Ludwig, K. Minch, H. Nixon, J. Nixon, S. Nixon, L. Ogren, M. Provancha, T. Redlow, E. Reyier, B. Reynolds, A. Sartain, D. Scheidt, J. Schwenter, A. Segars, B. Stephens, and W. Witzell. For logistical support, we thank C. Douglass, J. Douglass, T. Gottshall, K. Nimmo, J. Spade, and T. Ziegler. For data management, we thank P.E. Eliazar. We used the Maptool program (see www.seaturtle.org) to create the map in this paper. Our studies were supported by several funding sources: Disney Worldwide Conservation Fund, Florida Fish and Wildlife Conservation Commission, NASA-John F. Kennedy Space Center, National Marine Fisheries Service, National Park Service, US Geological Survey, and the Wildlife Conservation Society (New York). All turtle captures and handling were conducted with appropriate national and state research permits and approved IACUC protocols, where applicable. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
Conflict of interest
The authors declare that they have no conflict of interest.
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