Immobilization Threshold and Fish Conductivity of Two Small Fishes

Abstract

AbstractPower transfer theory (PTT), a useful paradigm in electrofishing, asserts that the threshold power density (Dt, μW/cm3) transferred from water to fish for immobilization is dependent on the conductivity (μS/cm) of the water (σw) and of the fish (σf). Larger differences between the two conductivities require higher applied power densities and may make capture more difficult. Over a range of water conductivities, Dt as a function of σw will form a U‐shaped curve, and the minimum provides an estimate of σf and Dm (the lowest value of Dt that occurs when σw and σf are equal). Studies have demonstrated a narrow range of σf (56–160 μS/cm) relative to the conductivity of natural waters (10–5,000 μS/cm). However, these σf values are for larger species. Electrofishing may also be directed at the capture of smaller species or diverse communities. In an experimental tank, we estimated Dm and σf for a small cyprinid, the Red Shiner Cyprinella lutrensis (TL = 35–64 mm) and a smaller poeciliid, the Western Mosquitofish Gambusia affinis (TL = 23–46 mm). We estimated Dm and σf to be 120 μW/cm3 and 23 μS/cm, respectively, for Red Shiners (n = 72) and 143 μW/cm3 and 23 μS/cm, respectively, for Western Mosquitofish (n = 23). These values for σf are less than half the minimum of previously reported values, and our Dm estimates are less than those that have been reported for game species of similar length (200–300 μW/cm3). Our results serve to explain why smaller species require higher applied power densities and are more difficult to capture from an assemblage of various species and sizes.