Kevin L. Pope and Carter G. Kruse
The analysis of fish condition has become a standard practice in the management of fish populations as a measure of both individual and cohort (e.g., age- or sizegroup) wellness. Condition has been generically described as the well-being or robustness of an individual fish (Le Cren 1951; Bulow et al. 1981; Blackwell et al. 2000). It has typically been estimated by comparing an individual fish weight to a standard weight for a given length and assuming that larger ratios (condition index) reflect a healthier physiological state (Bolger and Connolly 1989; Murphy et al. 1991) or by directly measuring physiological parameters related to the energy stores, such as tissue lipid content (Craig 1977; Fechhelm et al. 1995). All methods of calculating condition share the common goal of controlling for or removing the confounding effects of absolute body size when comparing body mass or other measures of nutritional state (Jakob et al. 1996). This is particularly important for organisms with indeterminate growth, such as fishes (Reist 1985).
Measures of condition are generally intended to be an indicator of tissue energy reserves, with the expectation that a fish in good condition should demonstrate faster growth rates, greater reproductive potential, and higher survival than will a lesser-conditioned counterpart, given comparable environmental conditions. Subsequently, fish condition is of keen interest to fisheries scientists, and numerous studies have investigated the relationship between measures of fish condition and parameters such as growth, fecundity, population structure, life history adaptations, environmental conditions, or management actions such as stocking (Cone 1989; Brown and Murphy 1991; Gabelhouse 1991; Blackwell et al. 2000). Although measures of condition in fish can be sensitive or related to factors that might logically affect energy storage or fitness in an individual, there is commonly substantial interspecies, seasonal, environmental, and spatial variation that influences our ability to interpret changes in fish condition.