Chapter 9: Environmental Effects of Bigheaded Carps
Even though most species that are introduced outside their native range cause no appreciable change in the invaded ecosystem (Williamson 1996), the introduction of some nonnative species results in costly economic damages and negative ecological changes (Kolar and Lodge 2002). Documenting and quantifying ecological changes, however, can be challenging. Many authors have commented on the difficulty of documenting the specific role of introduced fishes, even in obvious cases of depletion of native species (Crossman 1991). This may be especially true in large river ecosystems, the typical habitat for bigheaded carps, where relatively little is known about ecology of fishes (Dettmers et al. 2001) or plankton communities (Berner 1951). Covarying factors such as changing hydrology, water temperatures and flow rates, the abundances of other biota, and the introduction of other nonnative species and other human activities confound efforts to document the effects of introduced bigheaded carps. This is also true of the introduction of bighead Hypophthalmichthys nobilis and silver H. molitrix carps into the Mississippi River basin. Here we present documented negative effects of introduced bigheaded carps around the world (and in the United States, although less is known about this new introduction) and speculate about the potential effects of the genus on freshwater ecosystems in the United States.
Kohler and Courtenay (1986), mostly following Taylor et al. (1984), characterized the negative effects of nonnative species in invaded ecosystems into five broad categories: habitat alteration, trophic alteration, spatial alteration, gene pool deterioration, and disease transmission. Below we delineate documented and potential effects of bigheaded carps on each of the categories of negative effects identified in Kohler and Courtenay (1986).
Changes in water quality are probably the most direct effects on the aquatic habitat from the introduction of bigheaded carps. The effects from these fishes on water quality, however, appear to vary. Water nutrient concentrations have been documented to decrease (Opuszynski 1980), increase (Mátyás et al. 2003), and remain unchanged (Starling 1993) in the presence of silver carp. Laws and Weisburd (1990) found that sediment resuspension by silver carp introduced nutrients into the water column, stimulating plankton growth. Vybornov (1989) found decreased dissolved oxygen content of water in the presence of silver carp.