Effects of Nutritional Regime On the Bioelectrical Impedance (BIA) and Lipid Content of Fathead Minnows (Pimephales promelus)

Bioelectrical impedance analysis (BIA) is a useful and non-invasive condition indicator for fish. Previous studies have shown that tissue reactance increased with lipid content of field caught brook trout, and that both decreased through the winter period. Phase angle (reactance/resistance) is considered to be a sensitive indicator of the integrity of membrane systems, and has been shown to be sensitive to short-term changes in nutritional state.   While tissue impedance measures have been shown to reflect tissue composition and body size metrics of a wide range of fish species and sizes (length 100–1000 mm), no studies had yet examined very small fishes. Reactance, resistance and phase angle  of body tissues were measured weekly in fathead minnows (40 – 57 mm, 0.7 – 1.9 g) raised under different experimentally imposed nutritional regimes (control vs triple ration).  Differences in growth rate, Fulton’s condition index, lipid content (2:1 chloroform/methanol solvent extraction) and water content (wet vs dry mass) were observed. Both reactance and phase angle increased with lipid content and responded on a weekly time scale to changes in the nutritional regime. We chose the fathead minnow for this study, since a great deal of laboratory work is carried out on such small fishes and the BIA techniques offers numerous potential applications for such experimental work.