Armed with a half-dozen backpack electrofishers, a crew of “netters” and “shockers” waded through the Lower Kings River last week in search of rainbow trout. FISHBIO assisted with multi-pass depletion electrofishing efforts as part of the Kings River Conservation District’s (KRCD) annual rainbow trout population study in the Kings River below Pine Flat Dam in California’s Central Valley. The annual monitoring is preformed to assess the response of the trout population to habitat enhancements conducted over the past decade, as required by the Federal Energy Regulatory Commission (FERC). The Kings River Fisheries Management Program (KRFMP) has implemented extensive gravel augmentation, boulder placement, side channel enhancement, and riparian planting. The KRFMP is a partnership between the Kings River Water Association (KRWA), California Department of Fish and Game (CDFG) and the KRCD working to enhance and maintain a sustainable trout fishery on the lower Kings River.

Backpack electrofishing has been a common tool for sampling salmonids for decades. Similar to the snorkel surveys we use in deeper rivers, backpack electrofishing is often used to estimate the abundance of fish in a shallow river or stream. During this process, there are teams composed of a “shocker,” the person operating the electrofishing unit, and at least one “netter” to quickly scoop up the momentarily stunned fish. The number of teams depends on the width of the stream, since the teams form a line and move together upstream as they shock. The goal of staying in line is to ensure uniform sampling effort across the width of the stream and to prevent fish from slipping back downstream behind the shockers. There are several assumptions associated with this abundance estimation method (Temple and Pearsons 2007), including the important assumption that the fish are in a “closed population.” Essentially, we assume that as the teams pass upstream and sample, the fish are not eluding capture by moving further upstream. To avoid this, we place a block net at the top and bottom of the stream segment, temporarily trapping the fish. Of course, there are always some fish that elude capture the during the first pass upstream; thus, a common method is to use three or four passes in each stream segment to account for this, a technique called “depletion” or “multiple-pass” electrofishing.