Use of a modified form factor to compare condition among North American lake sturgeon stocks

Abstract

In fisheries management it is often useful to compare length and weight relationships or condition among populations across a species' range. Currently, the most commonly used metric for this is relative weight (Wr), although some problems have arisen with the use of Wr including the impact of seasonal changes in body condition due to reproduction, and length-related biases in standard weight equations. We propose the use of a modified form factor (mFF) based on the regression of log10α vs β (weight-length model parameters) within a species, to provide a quick and meaningful comparison of mean condition among North American lake sturgeon populations. We used the α and β parameters from 63 lake sturgeon weight-length models from 43 lake sturgeon populations from throughout their range in the equation to calculate the mFF for the 63 samples. Modified form factor values of juvenile, adult male, and female lake sturgeon from the Winnebago System, Wisconsin, in various stages of reproductive development had a 98.0% correlation with their respective relative condition values over a wide range of mFF values. Simple t-tests on sets of mFF values can be used to test the condition differences between populations or sub-samples within populations. Lake sturgeon from the Winnebago System, Wisconsin, USA were found to show W-L relationships best described in two stanzas: all juveniles <71.1cm, and juveniles and adults combined, but separate by sex, ≥71.1cm. Likelihood ratio tests found significant differences between male and female (>71cm) W-L models; juvenile (≤71cm) and male (>71cm) models; and juvenile (≤71cm) and female (>71cm) models. © 2011 Blackwell Verlag, Berlin.