Small fish diet in connected and isolated lakes in a Neotropical floodplain

Hydrological connectivity interferes directly in dispersal rates of organisms and in similarity of environmental conditions among floodplain environments. Consequently, connectivity promotes changes in food resources availability to fish. Here we tested the predictions that (a) isolated floodplain lakes have greater environmental heterogeneity than connected lakes, (b) fish diet differs more among isolated than among connected lakes and (c) trophic niche breadth of the species is smaller in isolated than in connected lakes. We used one invertivorous and one algivorous species of fish to test these two last predictions. The environmental heterogeneity (evaluated in relation to abiotic variables and the macrophyte composition) tended to be greater in isolated lakes. Diet of both species differed significantly in all isolated lakes populations, and in the majority of the populations in connected lakes; however, the main items consumed for invertivorous species were common in all connected lakes. Trophic niche breadth of the invertivorous fish was significantly greater in the connected lakes; however, the algivorous species showed no significant difference between connected and isolated lakes. Our findings suggest that connectivity can facilitate the dispersal of organisms, allowing the increase in food item richness and consequently, trophic niche breadth, as observed for invertivorous species. However, connectivity likely plays a minor role on the algivorous species, whose diet may be more affected by local conditions, such as habitat structure. Our results support the view that regional (dispersion) and local (habitat structure) factors interact with feeding habit to determine fish food composition in the floodplain lakes we studied.     

An invasive and a native macrophyte species provide similar feeding habitat for fish

Invasive macrophytes usually increase the homogenisation of habitats and can affect local communities with negative effects on the food chain. We evaluated experimentally the composition, density, richness and diversity of the invertebrates in the invasive macrophyte Hydrilla verticillata and assessed the potential effects of this macrophyte on the diet and foraging efficiency of small native fishes. The native macrophyte Egeria najas was used as control, since it has very similar physical structure and physical complexity to H. verticillata. Macrophytes of both species were maintained separately in aquaria with the fishes, and the invertebrates richness and abundance associated with plants and within fish stomach were evaluated. No significant difference was found in the richness and diversity of invertebrates attached to macrophytes; however, the two macrophytes differed in regard to invertebrate composition, and the density was significantly higher in H. verticillata. The fish foraging efficiency was high in both macrophytes, and the difference between them was not significant. The fish diet did not differ significantly between the native and the invasive macrophyte in respect to invertebrate composition, abundance and richness, and trophic niche breadth. We found no evidence that H. verticillata negatively affects the density of associated invertebrates, and as a consequence, fish diet and foraging efficiency. Our results indicate that the replacement of E. najas by a similar biomass of H. verticillata does not affect the feeding behaviour of associated fish species.     

Fish feeding in changing habitats: effects of invasive macrophyte control and habitat complexity

Abstract –  Many freshwater lakes have been invaded by non-native aquatic plants. Although managing the invasions is necessary to restore native macrophytes, little is known about the effects of invasive macrophyte eradication on fish foraging. This study was designed to determine whether fish feeding was affected by large-scale changes in plant composition after selective control of invasive Eurasian watermilfoil, Myriophyllum spicatum , and whether feeding was correlated with habitat complexity or abundance of invasive plants on a smaller scale. Popnets were used to collect plants, invertebrates and fishes in the littoral zone of four Minnesota lakes twice a year for 4 years. The degree of stomach fullness in fishes was not correlated with the proportion of invasive plants or habitat complexity. Bluegills, the most abundant fish species, exhibited high selectivity for Diptera and their preferences varied little in different plant habitats. Changes in niche width of bluegills were not consistent with invasive plant control. Overall, invasive macrophyte control with timely restoration of the native plant community did not affect the characteristics of fish feeding we investigated. This study also contributes to our understanding of fish–macroinvertebrate interactions as a function of habitat complexity.     

Invasive molluscs in the diet of Pterodoras granulosus (Valenciennes, 1821) (Pisces, Doradidae) in the Upper Paraná River floodplain, Brazil

Abstract –  Diet composition of Pterodoras granulosus was investigated from June 2004 to June 2005 in the Upper Paraná River floodplain, following invasion by two mollusc species Corbicula fluminea and Limnoperna fortunei . Stomach and intestinal contents analysis showed spatial differences in the diet composition. In the Baía River, P. granulosus diet consisted mainly of C. fluminea , whereas plant items dominated in the Ivinheima River, and L. fortunei was the most important food in the Paraná River. There was little selectivity in shell length consumed. In spite of C. fluminea being a larger mollusc, L. fortunei appears to be energetically more profitable because its shell is more fragile and is rapidly opened and digested. A considerable amount of closed shells of C. fluminea at the end of the digestive tract points to a possible role of P. granulosus as a mollusc disperser.