Does boat electrofishing harm aquatic insects?

In an effort to reduce the population of non-native Rainbow Trout Oncorhynchus mykiss and benefit native Yellowstone Cutthroat Trout O. virginalis bouvieri in the South Fork Snake River, the Idaho Department of Fish and Game (IDFG) has used boat electrofishing, a fish sampling technique that temporally stuns fish by applying electrical current to the waterway. This technique is used to capture and relocate Rainbow Trout from the South Fork to other waters where Rainbow Trout make fishing better without impacting native species.  Electrofishing efforts have been met with some controversy from the angling public. A public meeting was held by IDFG in the spring of 2023 to address the concerns. During the meeting, a member of the public questioned whether these electrofishing efforts have had a negative impact on aquatic insect populations. While a few studies have researched impacts of backpack electrofishing on aquatic insects, we are not aware of published articles investigating impacts of boat-based electrofishing. Stable aquatic insect populations are vital to maintain a healthy fishery, so IDFG, in collaboration with BYU-Idaho, studied whether jet-boat and raft electrofishing increased the rate in which aquatic insect are disturbed from the riverbed (drift rate) and insect mortality.  

To determine if boat electrofishing causes an increase in aquatic insect drift rate, we used a series of drift nets to sample three electrofishing sites. Sites were selected to overlap with IDFG’s 2024 boat electrofishing population estimates of both the Henrys Fork Snake River and the South Fork Snake River. At each site, we calculated the average number of insects drifting at three distinct times: four days before electrofishing occurred, during electrofishing efforts, and three days after electrofishing. We then compared the drift rates of each sample time at each site. Comparing insect drift rates before, during, and after electrofishing allowed us to establish a baseline drift rate and determine if electrofishing increased drift rates the day of electrofishing and three days after electrofishing. 

To determine if boat electrofishing causes mortality of aquatic insects, we used kick nets to collect a sample of forty Hydropsychid Caddisflies and forty Perlid Stoneflies (Image 1). We separated each group of forty insects into two groups of twenty with half as a control groups and the other half as the test groups. The control group insects were not exposed to electrofishing and served as a baseline to compare survival rates to the test group that was exposed to electrofishing. While the control groups were placed into an aerated cooler, the test groups were placed into a temporary trap in the South Fork Snake River. The test groups were then exposed to a single jet-boat electrofishing pass. All insects were transported to BYU-Idaho and randomly assigned to tanks in a recirculating rearing chamber (Image 3). For three days, we monitored all insects and checked for mortalities every ten to twelve hours.  

At each of the three electrofishing sites, we compared the average rate of drifting insects between the three sample times: before, during, and after electrofishing. We found no significant difference in insect drift rates between sample times at either of the three electrofishing sites (Figure 1). Our findings show no impact on aquatic insect drift rates attributable to raft and jet boat electrofishing.