Long‐Term Effects of Electrofishing on Growth and Body Condition of Brown Trout and Rainbow Trout

Brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss (≥18 cm total length) were captured by electrofishing, tagged, and left at large in three Colorado streams. The streams were electrofished again after 1 year, and recaptured tagged fish were tested against unmarked fish (assumed to be unshocked the previous year) for differences in growth and condition. Significant differences (P ≤ 0.05) in last annual length increment between shocked and unshocked fish were detected in two of the four comparisons possible, indicating that shocked age‐5 rainbow trout in the Arkansas River and age‐4 and age‐5 (pooled) brown trout in the Rio Grande grew significantly less over the year of study than did unshocked controls. Significant differences (P ≤ 0.05) in body condition were found in 2 of 10 possible comparisons. Shocked brown trout and rainbow trout recaptured in the Arkansas River in 1994 were in significantly poorer condition than unshocked controls. Brown trout captured for the first time had weighted mean weights exceeding those for recaptured shocked brown trout by 10.5% for 22–36‐cm fish. Rainbow trout captured for the first time on average were 9% heavier than recaptured shocked rainbow trout over the 32–41‐cm size range. No effects on condition were detected in the other study streams. The longterm effects of electrofishing on individual fish in some of these streams suggest that steps should be taken to minimize electrofishing injury. This may include more extensive training of new electrofishing personnel and the use of existing electrofishing equipment in the least injurious manner known.