Estimating Age at a Specified Length from the von Bertalanffy Growth Function

Abstract

Estimating the time required (i.e., age) for fish in a population to reach a specific length (e.g., legal harvest length) is useful for understanding population dynamics and simulating the potential effects of length-based harvest regulations. The age at which a population reaches a specific mean length is typically estimated by fitting a von Bertalanffy growth function to length-at-age data and then rearranging the best-fit equation to solve for age at the specified length. This process precludes the use of standard frequentist methods to compute confidence intervals and compare estimates of age at the specified length among populations. We provide a parameterization of the von Bertalanffy growth function that has age at a specified length as a parameter. With this parameterization, age at a specified length is directly estimated, and standard methods can be used to construct confidence intervals and make among-group comparisons for this parameter. We demonstrate use of the new parameterization with two data sets.

Received March 30, 2017; accepted June 9, 2017 Published online September 8, 2017

ACKNOWLEDGMENTS

We thank M. Belnap for use of the Lake Whitefish data, which were obtained during a project funded by the Great Lakes Fishery Commission. We also thank the Minnesota Department of Natural Resources for use of the Walleye data from Lake Winnibigoshish. This paper was improved by discussions with and reviews by T. Brenden and two anonymous reviewers. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.