AC Electrofishing Injury of Large Brown Trout in Low-Conductivity Streams

Abstract

Fifty-seven large brown trout Salmo trutta (mean total length, 407 mm; range, 264–647 mm) were collected by backpack AC electrofishing from seven low-conductivity (12–60 μS/cm) Tennessee streams and examined by X ray and necropsy for injuries. No fish died during sampling or before processing in the field. Overall, 34 fish (60%) had hemorrhagic injuries and 20 of these (35%) had class−1 or −2 spinal compressions or subluxations. Eighteen of the spinal injuries (90%) were accompanied by hemorrhage of adjacent musculature. Injury rates were lower for fish shorter than 305 mm (N = 5, 1 injured) and longer than 500 mm (N = 6, 1 injured) when compared with fish 305–500 mm (N = 46, 32 injured). Temporary skin discolorations (sometimes referred to as “brands”) were observed on 24 fish (42%), 19 of which were injured. These discolorations were between the dorsal and caudal fins (where spinal injuries typically occurred) and were significantly related to the incidence and severity of hemorrhage (P = 0.004) and spinal injury (P = 0.006). The overall spinal injury rate (35%) was within the range reported for large brown trout collected with DC electrofishing gear in streams of higher conductivity (8–84%). Twenty-three percent of the fish we examined had injuries of at least moderate severity (class-2 spinal injuries and class−2 or −3 hemorrhages), but because only a small portion of any stream is actually sampled, we concluded there is limited potential for AC electrofishing injuries incurred by large brown trout in low-conductivity streams to cause population level impacts.