Selective Eradication of Gizzard Shad from Southern Iowa Impoundments

There has been a recent increase in the frequency of illegal gizzard shad (Dorosoma copedianum) introductions into southern 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 usually was preceded by years of poor angling and resulted in the renovation of the entire fishery.  To avoid such cycles, the Iowa Department of Natural Resources Fisheries Bureau has taken a proactive approach to such introductions and is attempting complete but selective gizzard shad eradication with minimal non-target loss.  Five-percent rotenone was applied at a concentration of 5.0 µg/l active ingredient at two different lakes in November, 2010.  During the application, emphasis was applied to even chemical distribution.  Each zone’s surface area was covered several times to minimize localized “hot spots”.  Post-application water samples demonstrate that the target concentration of 5µg/l was reached with reasonable precision.  Overall, non-target loss was rare with few population-level impacts.  Walleye appear to be the only game species to have been negatively affected by these low dose treatments.  One of these applications resulted in the complete eradication of gizzard shad while the other was unsuccessful.