Fits the re-parameterized von Bertalanffy growth equation of Francis (1988) by using nonlinear least-squares
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age |
Vector of ages of individual fish. |
L |
Vector of lengths of individual fish. |
agephi |
Arbitrary reference age phi |
agepsi |
Arbitrary reference age psi. agepsi>agephi. |
graph |
Logical specifiying whether observed versus predicted, and residual plots should be drawn. Default=TRUE. |
gestimate |
Logical specifying whether automatic generation of starting values of lphi, lchi and lpsi should be used. Default=TRUE. If gestimate=FALSE, user-specified starting, lower and upper limits of parameters must be entered. |
Lphiparms |
If gestimate=FALSE, starting value, lower limit and upper limit of lphi used in nls. |
Lchiparms |
If gestimate=FALSE, starting value, lower limit and upper limit of lchi used in nls. |
Lpsiparms |
If gestimate=FALSE, starting value, lower limit and upper limit of lpsi used in nls. |
control |
see |
Francis (1988) re-parameterized the von Bertalanffy growth equation for age-length in order to make equivalent comparison of parameters to parameters of a common model used to estimate growth from tagging data. Three parameters, lphi, lchi and lpsi, are estimated. The re-parameterization also has better statistical properties than the original equation.
The formulae to get the conventional von Bertalanffy parameters are:
Linf = lphi + (lpsi-lphi)/(1-r^2) where r = (lpsi-lchi)/(lchi-lphi)
K = -(2*log(r))/(agepsi-agephi)
t0 = agephi + (1/K)*log((Linf-lphi)/Linf)
If gestimate=TRUE, unconstrained nonlinear least-squares (function nls) is used to fit the model. If gestimate=FALSE, constrained nonlinear least-squares is used (algorithm "port" in nls).
nls object of model results. Use summary to extract results.
Gary A. Nelson, Massachusetts Division of Marine Fisheries gary.nelson@state.ma.us
Francis, R. I. C. C. 1988. Are growth parameters estimated from tagging and age-length data comparable? Can. J. Fish. Aquat. Sci. 45: 936-942.
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