AUTHORS: Theo Huber, University of Nebraska at Kearney; Alex Keiler-Klein, University of Nebraska at Kearney; Melissa Wuellner, University of Nebraska at Kearney; Keith Koupal, Nebraska Game & Parks Commision.
ABSTRACT: Chemicals such as rotenone are used to remove undesirable fishes from lakes and promote the establishment of more desirable, recreationally important species. However, the application of piscicides can directly or indirectly impact zooplankton in both positive and negative ways. Lake Ogallala, a 263-ha reservoir directly below Lake McConaughy, has had several rotenone treatments dating back decades, with the most recent application in October 2023. Here, we compare taxa richness and densities from the zooplankton community in Lake Ogallala before and after the 2023 renovation. Zooplankton were collected monthly from April to November 2023 and from April to October 2024 at nine locations throughout the reservoir using an 80-μm Wisconsin net towed vertically, beginning 1 m from the bottom at each site. For each sample, four 1-mL subsamples were drawn, and zooplankton were identified to the lowest possible taxon and enumerated. Mean total density (number/L) and mean density by taxon were calculated across all sites for each month. In 2023, 14 taxa were found, and densities were highest among cyclopoids, Daphnia, and rotifers. In 2024, 13 taxa were identified, with densities highest for cyclopoids, Bosmina, and rotifers. Two taxa (harpacticoids and Diaphanosoma) were only present in 2023, and one taxon (Leptodora) was only present in 2024. This research adds to what has been found on the impacts that piscicides may have beyond those for fish. Understanding these changes is important in deciding whether to apply piscicides, given the possible benefits, costs, and drawbacks may be.
