Working Together to Ensure the Future of Wild Trout_____________________________________________________________1 Impacts of Electrofishing Injury on Idaho Stream Salmonids at the Population Scale

ABSTRACT—This study assesses the mortality impacts of electrofishing at the population scale based on levels of sampling by Idaho Department Fish and Game (IDFG) and non-IDFG projects during the 1995 and 1996 field seasons. We estimated electrofishing induced population mortality by considering the proportion of stream reach shocked during sampling, the probability of fish exposure to an electric field based on sampling method used, and a hypothesized worst-case (25%) mortality rate for all electroshocked fish. For IDFG mark-recapture estimates the mean mortality from shocking was 1.05 % with a range of 0.13-4.02%. For Idaho Department of Fish and Game (IDFG) removal sampling, we estimate a mean population mortality of 0.38 % with a range of 0.02-2.91%. For non-IDFG sampling mean population mortality averaged 1.11 % with a range of 0.05-7.71%. Fifty-one percent of all mortality estimates were less than 0.50%. These low estimates are likely worst-case electrofishing effects because the high assumed mortality value used is not supported by any literature values. We conclude the impacts due to sampling using electrofishing methods does not constitute a meaningful impact to Idaho stream trout at the population level, especially when compared to annual natural mortality levels for most stream salmonids which typically equal 30-60%