Evaluation of length distributions and growth variance to improve assessment of the loggerhead sea turtle, (Caretta caretta) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gf06g709j

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  • Nest counts from the largest subpopulation of the loggerhead sea turtle (Caretta caretta) in the Atlantic, peninsular Florida, have been declining since 1998. Analyzing trends of nests and adult females is insufficient to determine the overall status of a sea turtle population, due to variation in nesting frequency and a 25 – 35 year time lag between hatching and maturation. While nest numbers of this threatened species have declined by 50% since 1998, catches of juvenile turtles in many areas have increased. Better assessment of population status and diagnosis of the causes of population trends are possible if we examine existing data on changes in abundance and size distribution from in–water surveys of juvenile and subadult turtles as well as adults. I have plotted the length frequency data and catch rates of neritic turtles (juvenile, subadult and adult turtles that inhabit nearshore waters) collected from 1980 – 2007 at 4 in–water sites ranging from North Carolina to Florida (N=10,486). I also examined the length distributions recorded from strandings of dead turtles found on beaches in the Gulf of Mexico (N=4,308) and along the U.S. east coast (N=10,918) from 1990 – 2005. I found a similar pattern in length distribution shifts over time, which provides compelling evidence that the data are revealing true population change. The data suggest an increase in the overall abundance of juvenile turtles in the southeast U.S. since the late 1990s and an increase in the median size of neritic juveniles (40 – 90 cm) since 1990. There has also been a reduction in the number and proportion of small (<55 cm) juveniles since 2000 in some areas; these are considered “recruits” to the neritic feeding areas on the continental shelf. The shift in median size of juveniles coupled with the increased abundance in North Carolina and Florida indicate that there may be a large cohort or group of similar sized turtles that should reach maturity in the next decade. Changes in length frequency through time could indicate changes in recruitment, survival rates, or behavior, and translating length to age requires a good estimate of variance in growth rates. I developed an individual–based growth model using variance estimated through a new technique that measures growth using increments measured in cross sections of the humerus bone of stranded turtles. The technique permits a reconstruction of the growth history of individual turtles. While the growth patterns are clearly not a smooth, asymptotic function over length and time, there is some consistency among individuals. My re–sampling analysis of growth from 92 turtles provided an estimate of 30.74 years (95% CI = 23.82 – 37.66) required for a U.S. loggerhead turtle to grow from 20 cm (age 1 or 2) to 90 cm. This analysis will improve our estimates of the mean and variance in the length of time loggerheads are spending in various habitats and life stages, and improve future demographic models to determine population status. My analyses incorporate multiple data sets and techniques to improve our understanding of how the whole loggerhead population is changing through time, leading to more accurate diagnosis and predictions about population trends.
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