Selective Removal of Gizzard Shad From Southern Iowa Impoundments

Illegal introductions of gizzard shad Dorosoma copedianum have occurred with increasing frequency in Iowa waters.  These introductions typically precede large-scale declines in angling quality for important centrarchid species including bluegill Lepomis macrochirus, largemouth bass Micropterus salmoides, and crappie Pomixis spp..  Traditional management of such introductions was preceded by years of poor angling, reduced angler usage, followed by complete renovation of the fishery.  We attempted to develop methods for selective gizzard shad removal from these systems.  Five-percent rotenone was applied at a concentration of 5.0-9.5µg/l active ingredient at seven lakes in fall 2010 and 2011.  Dilute rotenone was applied to the surface and deep pelagic portions of these systems along transect to minimize localized differences in rotenone concentration (e.g., hot spots).  Post-application water samples demonstrated that target concentrations were reached with reasonable precision.   Subsequent sampling demonstrated non-target loss was low with few population-level impacts on other sport-fish species.  Walleye Sander vitreus appear to be one recreationally important species to have been completely eradicated by low-dose rotenone treatment.  Bluegill , largemouth bass, and crappie were present in post-application samples and relative abundances were similar to pre-application values.  Gizzard shad were completely eradicated from three of the seven lakes treated thus far.