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The impact of fish predation on epibenthic organisms at different trophic levels was studied in a series of replicated experimental outdoor channels fed by a boreal forest stream (Québec, Canada). Brook trout (Salvelinus fontinalis) were introduced into five of 10 channels according to a randomized block experimental design. Periphyton biomass (expressed as total chlorophyll a and total organic matter) and macroinvertebrate biomass were measured four times during the summer. The biomass of small insects such as Chironomidae was always higher in the presence than in the absence of fish, while the biomass of larger invertebrates such as Baetis (Ephemeroptera) and Psychoglypha subborealis (Trichoptera) was reduced in the presence of fish. Periphyton biomass was significantly greater in channels with fish on the first sampling date after fish introduction but not at later dates. Complementary cage experiments, involving both the inclusion and exclusion of selected invertebrates from periphyton-covered surfaces, demonstrated that the presence of Baetis can reduce the periphytic biomass, while the presence of P. subborealis can reduce the biomass of Chironomidae. Overall, these results suggest that size-selective predation by brook trout can cause profound changes in the structure of epibenthic communities at primary as well as secondary trophic levels.

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The effect of supplemental feeds with different levels of crude protein on pond water quality and food utilization efficiency by pacú (Piaractus mesopotamicus) was evaluated in a semi‐intensive culture system. Fish were stocked at a density of one individual per m2, raised for 299 days, and fed isocaloric diets containing soybean meal, blood and bone meal, and yellow maize. Dietary treatments consisted of diets containing 25%, 35%, and 45% crude protein. Ponds receiving the two highest protein levels showed significantly higher values of alkalinity, conductivity and nitrites. High dietary protein diets resulted in no significant improvements in final weight (336.4±77.2, 308.2±92.6 and 368.4±82.2 g, mean±standard deviation, for 25%, 35% and 45% protein levels respectively). Moreover, fish carcass composition was not significantly different among treatments. Instead, significant augmentations in feed conversion ratio (1.36, 1.54 and 1.73 respectively) and reductions in protein retention (35.7%, 33.5% and 29.0% respectively) occurred as protein levels increased. Results indicate that at the lowest protein level, pacú growth rates and carcass composition were similar to the other experimental treatments, with a significant improvement in pond water quality and feed utilization efficiency.

The riverine complex Paraguay-middle Parana-Rio de la Plata extends more than 3700 km southwards from its sources in the western hills of the Brazilian Shield to its discharge into the Rio de la Plata River. The high variety of habitats is reflected in the large diversity of its fish communities, which are dominated by characiform and siluriform fish species. The potamic axis is evolutionarily and ecologically open to fish movements and there are more than 400 fish species listed for the whole system, but only 100 species are common to both upper and lower basins. However, data limitations in some portions of the system need to be addressed before creation of an ichthyogeographic classification. The river basins that make up the potamic axis are low to medium developed and environmental pressures are unevenly distributed. Chemical pollution is a concern throughout. In the Pantanal, small hydro-projects and sedimentation from agricultural activities have had adverse effects on fish habitat inducing a loss of fish diversity. Un-dammed but more regulated and developed lowland rivers, may be impacted by upstream dams that may create unsuitable habitats for fish adapted to normal main channel conditions because they increase river flows during periods that were formerly low waters or change flows at random. The fisheries are lightly to moderately exploited compared to other subtropical and tropical riverine fisheries, and retain several of their original characteristics in less developed river reaches, although changes are evident. Large potamodromous fish are usually present in the catch, but the abundance of large piscivores is lower and fish size at catch is smaller. The development of the riverine system is expected to continue throughout the basin. If it is implemented as it was executed in the past, a continual loss of fish habitats and a general decrease in ecosystem health can be predicted.