Biotic Impacts in Shallow Water Bodies of South Dakota Resulting From Common Carp Invasion

Common carp Cyprinus carpio (hereafter referred to as carp) are one of the most common freshwater fish invaders worldwide that are known to have adverse ecological effects. They are generalists that possess a suite of life history traits that make them very efficient at invading new water bodies and rapidly expanding their populations. Their impacts on lentic systems as a result of their feeding behavior are thoroughly documented, and include increases in turbidity, nutrient and phytoplankton concentrations, and cyanobacteria blooms, decreases in benthic macroinvertebrate densities, submergent macrophyte abundances, and dissolved oxygen concentrations, and varied effects on zooplankton assemblages. How these alterations in ecosystem functioning affect fishes at higher trophic levels has been insufficiently studied in natural systems, with the exception of effects on relative abundances of sport fishes. Thus, the objectives of this study were to determine the effects of carp on water quality, invertebrates, and submergent macrophytes in shallow water bodies of South Dakota, and how these effects of carp may impact population characteristics and trophic interactions of adult and age-0 sport fishes, namely bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides. This was done by comparing three shallow reservoirs that contained carp to three that did not, all of which were located in eastern South Dakota. Water quality and invertebrate densities were measured during spring, summer, and autumn in each reservoir, from summer 2010 through spring 2012. Turbidity and chlorophyll a concentrations were significantly greater in the carp systems. Dissolved oxygen, total phosphorus, and total nitrogen concentrations did not differ between carp and non-carp systems, and nitrogen appeared to be the limiting nutrient in both types. Total benthic macroinvertebrate densities also did not differ between carp and non-carp systems, nor did dipterans when analyzed separately. Gastropods were significantly less dense in systems that contained carp. Total zooplankton densities were not different between carp and non-carp systems, nor were any individual genera, with the possible exception of Bosmina spp. Body lengths of Daphnia spp. also did not differ. Submergent macrophyte coverages were compared during August 2012, and systems without carp had significantly greater coverages. The disparity in submergent macrophytes may explain differences found in gastropod and Bosmina spp. densities. Although some of the typical effects of carp were observed in these systems, others were not. The complexity within and variability among natural ecosystems may override some of the effects of carp. Also, the fact that all the study systems were reservoirs located in agriculturally dominated watersheds may mean that allochthonous inputs are very influential in the dynamics of these water bodies and nutrient turnover rates may differ substantially among systems. Lastly, carp relative abundances may not have been high enough to induce some of the ecosystem differences reported in the literature. Adult fishes were sampled with nighttime electrofishing and fyke nets in 2011 and 2012; age-0 fishes were sampled with daytime electrofishing in 2010 and 2011. Scales were used to age adult fishes and otoliths were used to estimate daily ages of age-0 fishes. Relative abundance, growth, mortality, and condition were calculated for both adult and age-0 bluegill and largemouth bass and compared between carp and non-carp systems. Additionally, age structure, size structure, and recruitment were compared for adult fishes and hatch dates were compared for age-0 fishes. Adult bluegill did not differ between carp and non-carp systems in any of the population characteristics. Adult largemouth bass differed in growth and size structure; fish in non-carp systems grew significantly faster and had size structures similar to or larger than fish in carp systems. Conversely, both age-0 fishes tended to grow faster in the presence of carp, although the difference was not as great in both years. Significant differences in relative abundances of age-0 fishes were also observed, but the effects were opposite between species. Age-0 bluegills were more abundant in the carp systems and age-0 largemouth bass were more abundant in the non-carp systems. Differences that were found in mortality rates, condition, and hatch dates lacked substantial replication. Higher turbidity in the carp systems may have negatively impacted adult largemouth bass while simultaneously benefitting age-0 fishes in terms of feeding, hence the patterns detected in growth rates. Competition among age-0 fishes may also be relaxed in habitats devoid of vegetation. The lack of differences among the other population characteristics again might be due to moderately low carp relative abundances. Also, macroinvertebrate prey should not have been limited for juvenile fishes in the carp systems as was hypothesized. Lastly, the complex natural processes occurring in these lentic systems may reduce any effects of low to moderately dense carp populations