We examined the effects of adult common carp (Cyprinus carpio) on shallow aquatic ecosystems and compared the effects with those of a native benthic fish, channel catfish (Ictalurus punctatus). Experimental ponds contained enclosures (0.06 ha) with a low carp biomass (174 kg·ha–1), high carp biomass (476 kg·ha–1), high catfish biomass (416 kg·ha–1), and no fish. We measured abiotic factors (turbidity, suspended solids, total phosphorus), as well as effects on adjacent trophic levels (aquatic macrophytes, zooplankton, and aquatic macroinvertebrates) from July to September. Common carp was positively related to total phosphorus, turbidity, suspended solids, and zooplankton biomass, and negatively related to macrophyte and macroinvertebrate abundance. Suspended solids in the carp treatments consisted primarily of inorganic particles. Carp were either positively or negatively related to phytoplankton, depending on zooplankton abundance. A high biomass of carp had greater effect on nutrients, turbidity, and suspended solids than a low biomass. Channel catfish was positively related to total phosphorus concentrations and altered zooplankton composition, but did not affect turbidity, suspended solids, macroinvertebrates, and macrophytes. These results suggest that common carp have a stronger influence on water quality and aquatic community structure than benthic fish native to North America.

We examined the effects of low-head dams on aquatic biota, habitat, and water quality in a 171-km reach of a midwestern warmwater river that was fragmented by 15 dams into a series of free-flowing and impounded habitats. Dams impounded 55% of the river's surface area within the study reach and influenced distributions of 30 species of fish by restricting upstream movements. Values for the Illinois index of biotic integrity (IBI) were higher in free-flowing areas (mean IBI ϭ 46 out of a possible 60 at below-dam and midsegment free-flowing locations) than impounded areas (mean IBI Ͻ 31 for above-dam and midsegment impounded locations). Likewise, scores from a macroinvertebrate condition index (MCI) were higher at stations in free-flowing reaches (mean MCI Ͼ 415 out of a possible 700) than in nearshore areas of impounded reaches (mean MCI Ͻ 210). Ponar dredge samples taken only from open-water impounded areas showed an offshore invertebrate community that consisted almost entirely of tolerant oligochaetes and chironomid larvae. Qualitative habitat evaluation index (QHEI) scores indicated good-quality habitat in free-flowing areas (mean QHEI Ͼ 70 out of a possible 100) and severely degraded habitat at impounded sites (mean QHEI Ͻ 45). In impounded reaches, dissolved oxygen and pH showed wide daily fluctuations (2.5-18.0 mg/L and 7.0-9.4 units) and often failed to meet Illinois water quality standards. In free-flowing portions of river, fluctuations in these parameters were less extreme and water quality standards typically were met. We found little evidence of cumulative effects of dams; however, our data suggest that low-head dams adversely affect warmwater stream fish and macroinvertebrate communities by degrading habitat and water quality and fragmenting the river landscape. These results should aid river managers and stakeholders in determining appropriate restoration practices (i.e., dam removal versus fish passage structures) for warmwater rivers and streams that contain low-head dams.

We compared survival and growth of fry and small (mean total length = 48–61 mm), medium (132–145 mm), and large (186–216 mm) fingerling walleye (Stizostedion vitreum) stocked for 4 yr in a centrarchid-dominated impoundment. Mean survival based on fall population estimates 1 and 2 yr after stocking indicated highest survival for large fingerlings (mean survival = 31 and 10%, respectively), followed by medium ones (7 and 4%). Few individuals from the fry and small fingerling size groups were recovered in extensive field sampling. Creel census data reinforced these findings. Thermal stress at stocking and predation by largemouth bass (Micropterus salmoides) were more important than either hooking mortality or spillway escapement in influencing survival. Walleye diets were dominated by bluegill (Lepomis macrochirus) in volume (87%) and frequency of occurrence (84%). Growth rates were slower with bluegill as predominant prey compared with walleye growth in waters containing clupeids and cyprinids and may have been influenced by the abundance and size distribution of bluegill. Based on benefit/cost analysis (survival or catch/cost of rearing), stocking walleye > 200 mm provided the highest return on investment.

Previous work with centrarchid fishes suggests that recruitment success is higher for fish hatched early in the spawning season. Similar recruitment patterns have been shown for bluegills Lepomis macrochirus, but only in piscivore-free waters and at the northern extent of their range. We investigated the role of predation and first-winter mortality in governing bluegill recruitment. Spawning date distributions for larval bluegills taken in ichthyoplankton tows were compared with distributions for juveniles in shoreline rotenone samples from the fall and the following spring. Daily otolith rings were used to determine ages and spawning dates for bluegills in Ridge Lake, Illinois, during 3 years. Spawning took place from mid-May to mid-August of each year and produced several peaks in larval abundance. Differences between the spawning date distributions of larval bluegills and juveniles surviving to fall suggested that fish spawned early in the season experienced higher mortality than those spawned later. Extensive field sampling and bioenergetic model estimates of age-0 bluegill consumption by largemouth bass Micropterus salmoides indicated that predation was an important source of mortality for early-spawned fish. Estimates of bluegill abundance and size structure in fall and spring showed that losses over the first winter were high (75% to 88%); however, unlike in studies at more northern latitudes, there was little evidence of size-specific mortality. Our results suggest that predation is an important mechanism regulating the recruitment success of young bluegills and that early cohorts produced during protracted spawning must endure high mortality to become spring yearlings.

Body size governs predator-prey interactions, which in turn structure populations, communities, and food webs. Understanding predator-prey size relationships is valuable from a theoretical perspective, in basic research, and for management applications. However, predator-prey size data are limited and costly to acquire. We quantified predator-prey total length and mass relationships for several freshwater piscivorous taxa: crappie (Pomoxis spp.), largemouth bass (Micropterus salmoides), muskellunge (Esox masquinongy), northern pike (Esox lucius), rock bass (Ambloplites rupestris), smallmouth bass (Micropterus dolomieu), and walleye (Sander vitreus). The range of prey total lengths increased with predator total length. The median and maximum ingested prey total length varied with predator taxon and length, but generally ranged from 10–20% and 32–46% of predator total length, respectively. Predators tended to consume larger fusiform prey than laterally compressed prey. With the exception of large muskellunge, predators most commonly consumed prey between 16 and 73 mm. A sensitivity analysis indicated estimates can be very accurate at sample sizes greater than 1,000 diet items and fairly accurate at sample sizes greater than 100. However, sample sizes less than 50 should be evaluated with caution. Furthermore, median log10 predator-prey body mass ratios ranged from 1.9–2.5, nearly 50% lower than values previously reported for freshwater fishes. Managers, researchers, and modelers could use our findings as a tool for numerous predator-prey evaluations from stocking size optimization to individual-based bioenergetics analyses identifying prey size structure. To this end, we have developed a web-based user interface to maximize the utility of our models that can be found at www.LakeEcologyLab.org/pred_prey.