The Need to Reconsider the Use of Condition Indices in Fishery Science

Condition indices are used in fishery science as indicators of the weight‐to‐length relationship of a population or subgroup (e.g., strain, sex, or age category). This conversion of the two‐dimensional weight‐length relationship into a single statistic results in a loss of information and, in many cases, an inaccurate representation of that relationship. Fultonˈs condition factor ( K ) is based on the assumption that the slope of the weight‐length relationship is 3.0. The relative condition factor ( Kn ) is based on the assumption that the slopes of all samples to be compared are equal to some specific value. Relative weight ( W r ) is based on the assumption that the slope and the intercept of the weight‐length relationship are the same as those in the “ideal” equation. Instead of providing simple, interpretable descriptions of the weight‐length relationship in fish populations, all three of these commonly used condition indices have critical flaws that can result in incorrect conclusions. Relative weight is arguably the most misleading of these indices, and its use should be reconsidered before it becomes as misused as Fultonˈs K . The calculation of estimates of ordinary least‐squares regression parameters is a more accurate method of examining the weight‐length relationships for fish populations, and I suggest that it is the preferred method for evaluation of fish condition.