Spinal Injury Rates in Three Wild Trout Populations in Colorado after Eight Years of Backpack Electrofishing

Abstract

We examined long-term effects of annual intensive backpack electrofishing on rates of spinal injury and population abundance of three salmonid and one catostomid species in three small northern Colorado streams and compared rates of externally evident injuries to actual injuries determined by X ray. After 6–8 years of annual three-pass removal electrofishing, injury rates for brook trout Salvelinus fontinalis in treatment sections of two streams averaged 10.4% and 12.3%, but were 0% in previously unshocked controls. In a third stream containing all species in sympatry, longnose suckers Catostomus catostomus sustained the highest average injury rates (9.6%), followed by brown trout Salmo trutta (6.9%), rainbow trout Oncorhynchus mykiss (4.0%), and brook trout (3.5%). These rates were based on external evaluation and probably greatly underestimated actual rates of healed injuries because 44% of X-rayed fish with no externally evident spinal injury showed previous injury. Despite the high incidence of spinal injury, abundance of all salmonid species remained stable or increased in the three streams during the 8-year study, which indicated that there were no detectable adverse population effects of repeated electrofishing. In contrast, abundance of longnose sucker declined significantly. Our results indicate that spinal injuries accumulated over time and that species probably differ in susceptibility to the deleterious effects of electrofishing.