Comparison and Validation of Nonlethal Techniques for Estimating Condition in Juvenile Salmonids

Fish proximate composition influences growth, reproduction, and survival and is commonly used to determine physiological condition; however, proximate composition is time-consuming to measure and typically requires a lethal sample that cannot be analyzed in situ. As a result, researchers are increasingly interested in using nonlethal techniques that are quicker and less expensive to estimate proximate body components in fish. Bioelectrical impedance analysis (BIA) and microwave energy meters have been used with success on large-bodied fishes, although no evaluation on small-bodied Pacific salmon Oncorhynchus spp. has been conducted. We evaluated three nonlethal techniques (BIA, energy meter, and condition factor) as predictors of individual body composition (determined through lethal whole-body proximate analysis) in juvenile hatchery steelhead O. mykiss (minimum fork length = 138 mm; minimum wet mass = 24.5 g). The BIA values were positively correlated with measured total body water (R(2) = 0.77), protein (R(2) = 0.74), and ash (R(2) = 0.46) in steelhead; energy meter values were positively correlated with total body water (R(2) = 0.83). Only condition factor was predictive of total body lipid, and both BIA and energy meter measures reflected somatic energy in muscle tissue rather than the total body energy reserves. Although modification of nonlethal methodologies is still required to generate accurate measures of total body energy reserves, BIA and energy meter technologies provide relatively quick and detailed data on a number of proximate body components without the need to kill the individual fish.