PiscesLogoSmallerStill Graphical von Bertalanffy

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Each of the growth equations offered  is solved using the Levenberg-Marquardt Method for non-linear regression. The one exception is the Graphical von Bertalanffy. This fits the von Bertalanffy growth curve using a graphical method which was much used before the development of computers.


To allow comparison with older methodologies that are still described in books on fisheries methods, a graphical method for fitting the von Bertalanffy is included. This is based on the Ford-Walford plot of length (or weight) in which the length (or weight) at time t+1 is plotted against the length (or weight) at time t. If a straight line is fitted to the graph then the growth parameter, k, can be estimated from the slope and L inf. is given by the intercept of this line with a 45 degree line from the origin. This method is not as accurate as  nonlinear regression  and should not be the method of choice.



graphical solution von bert




While the graphical method is included mainly for historical interest, but may be useful for diagnostic purposes should the Levenberg-Marquardt Method fail to reach a solution. Growth II estimates the initial guesses of the parameters for fitting the von Bertalanffy curve required by the Levenberg-Marquardt Method using this graphical approximation.