Exploring Fabens' growth interval model with data on a long-lived vertebrate, Trachemys scripta

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We used data from a natural population of slider turtles, Trachemys scripta, (n = 70) to compare a von Bertalanffy growth equation constructed from known-age data with one constructed using Fabens' method (under the assumption that ages were not known). The 95% confidence intervals revealed no significant differences in estimates of variables a, b, or k for the two equations, both of the form: Lt=a(1-be-kt). We also examined the effects of truncated samples on Fabens' method by omitting large and small individuals. Fabens' method underestimated the asymptotic value, a, and overestimated the intrinsic growth factor, k, when larger individuals were omitted. The omission of small individuals resulted in little change in estimated values. Our findings confirm the utility of Fabens' method in estimating growth curves for animals of unknown age if data are available across all size classes. Although data on large individuals may be more important in arriving at accurate estimates of variable values, data on small individuals may be necessary to determine if another model (i.e., logistic) might provide a better descriptor of growth trajectories.

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