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Common carp Cyprinus carpio is a nonnative invasive nuisance species to North America. Many authors have documented the detrimental affects of common carp invasions on waterfowl habitats (Chamberlain 1948; Robel 1961), game fish habitat (Cahn 1929), and the overall decline in native fishes (Bernstein and Olson 2001; Koehn 2004). Common carp reduce water quality by mobilizing nutrients and increasing turbidity; therefore, increasing phytoplankton biomass and reducing zooplankton biomass and rooted aquatic vegetation (Lougheed et al. 1998). Common carp are capable of rapidly colonizing shallow lakes and altering a body of water from a clear stable state, dominated by submergent vegetation to a more turbid state, dominated by phytoplankton (Northcote 1988; Parkos et al. 2003).
Management and control of common carp has been well documented through much of North America (Meronek et al. 1996; Wydoski and Wiley 1999) with millions of dollars invested on research and control (Pimentel et al. 2000). Removal projects included mechanical harvest by netting (Ritz 1987; Pinto et al. 2005), water level manipulation to disrupt spawning (Summerfelt 1999), exclusion from spawning habitat (Lougheed and Chow-Fraser 2001), and piscicide application (Meronek et al. 1996). Northern pike Esox lucius have additionally been used as a biological tool to control common carp recruitment in the Sandhill lakes in Nebraska (Paukert et al. 2003). All methods of carp control have had varying degrees of success (Meronek et al. 1996).
Common carp gained access to the U.S. Fish and Wildlife Service (USFWS), Valentine National Wildlife Refuge (NWR) lake system through Gordon Ditch, which was dug during the 1930's (Wanner 2009). The ditch was plugged shortly after completion to eliminate fish movement onto the Refuge. Refuge lakes have a long history of chemical renovation to remove common carp (Wanner 2009). For approximately five years after renovation and the re-stocking of game fish, angling is excellent, waterfowl use is high; however, both decline soon after carp recolonization and subsequent habitat degradation (M. Lindvall, Valentine NWR, personal communication). Fisheries biologists from the USFWS and Nebraska Game and Parks Commission (NGPC) have also experimented with the use of northern pike and largemouth bass Micropterus salmoides to control common carp recruitment. Early attempts were unsuccessful because northern pike were introduced after carp populations were well established and subsequently the population and individual fish were too large to be controlled by predation (Wanner 2009). Common carp recruitment in the Refuge lakes is low due to predation or other abiotic factors (Phelps et al. 2008).
Common carp have also been physically removed on Valentine NWR lakes by releasing water through control structures between lakes, luring fish into ditches during spawning migrations where they are subsequently trapped. In the ditches between Whitewater and Dewey lakes and Dewey and Clear lakes (Figure 1), thousands of common carp, with an estimated biomass of several tons, were trapped in 1993 and 2008 (Wanner 2009). Trapping was also attempted in 2003 with little success due to scour holes around the trap that allowed carp to escape (M. Nenneman, unpublished data). These methods of controlling common carp have never been thoroughly evaluated; therefore, the objectives of this study were to 1) estimate abundance, biomass, and size structure of common carp in Dewey Lake, 2) estimate the proportion of the abundance, biomass, and size structure of the common carp removed from the lake during the trapping operation, and 3) monitor water quality and carp relative abundance before and after carp removal.