Influence of Electrode Type and Location upon Bioelectrical Impedance Analysis Measurements of Brook Trout

In recent years, bioelectrical impedance analysis (BIA) has started to develop into a low-cost tool that can provide accurate estimates of fish condition. Past researchers have had success predicting mass-based proximate condition components, but attempts to predict percent-based components have not been as successful, suggesting that methodological improvements are needed. The percent dry weight (%DW) of a fish is a desirable value because energy density and body composition estimates can be obtained from it using previously developed or easily developable equations. The primary objective of this study was to determine the locations at which electrodes should be placed to provide the best estimates of %DW for brook trout Salvelinus fontinalis ranging from 140 to 330 mm (total length). A second objective was to determine the effect that electrode type has on the ability to predict %DW. Models developed using two electrode locations performed better than those with only one location. One set of measurements should be made by placing the electrodes along the dorsal midline (DML) of the fish. A second set should be made by placing one electrode on the dorsal midline directly in front of the dorsal fin and another on the ventral midline directly below the first electrode (DTVpre). On average, models developed using these locations explained 13.2% more of the variation in %DW than models developed using the same locations as previous researchers. Validation of the BIA models demonstrated that both subdermal needle (root mean square error [RMSE] = 1.34, R-2 = 0.82) and less-invasive external rod electrodes (RMSE = 1.37, R-2 = 0.79) provided accurate estimates of %DW using the DML and DTVpre locations. More research is needed to determine whether these patterns hold true for smaller fish and species with distinctly different morphologies, bone structures, or scale types.