Abstract
Flood pulses affect floodplain enrichment via the incorporation of nutrients and terrestrial biomass. As a result, they positively affect the body condition of aquatic organisms. This paper evaluates whether the absence of floods (resulting from dam control) affects the feeding activity and body condition of piscivorous fish. In addition, whether piscivores respond similarly to alterations in the flooding regime was assessed. Five piscivorous species were selected (Acestrorhynchus lacustris, Hoplias aff. malabaricus, Plagioscion squamosissimus, Rhaphiodon vulpinus, and Salminus brasiliensis). The fish were captured in four distinct years and in three river subsystems with differentiated water level fluctuations (Ivinheira = not regulated; Baía = regulated by the Paraná River level; Paraná = regulated by dams). Feeding activity and body condition were evaluated using the mean values of the standard residuals generated by regression models between body and stomach weights and standard length and body weight (all log-transformed). Differences among years and subsystems were evaluated via two-way analysis of variance. Pearson and Spearman correlations were performed between flooding attributes (duration, amplitude, timing, and daily variability) and feeding activity and body condition. Feeding activity differed across subsystems, whereas body condition varied across years, depending on the subsystem. Hoplias aff malabaricus (an ambusher adapted to starvation) presented feeding activity independent of the flooding regime and also presented better body condition in times of high water levels. Rhaphidon vulpinus exhibited variations in feeding activity but did not present alterations in body condition. The other species presented poorer body condition in years or subsystems with regular floods. Correlations identified that the duration and timing of floods had negative effects on body condition, whereas amplitude and mean annual water level improved feeding activity. Therefore, regular floods affect the feeding activity and body condition of piscivorous fish, and the response of each species depends on the existence or absence of pre-adaptation to long periods of starvation. Consequently, considering the role of the flood pulse in determining the availability of feeding resources for early life stages of piscivores, control of the flood pulse by dams is also disadvantageous for piscivorous fish.
Highlights
The integrity of processes that operate in floodplains is closely associated with the hydrological regime, and the alternation between flooding and drought periods determines the structure and functioning of these environments (Junk et al, 1989; Neiff, 1990; Junk and Wantzen, 2004; Thomaz et al, 2004; 2007; Raniere et al, 2008)
In the upper Paraná River floodplain, upstream reservoirs have caused a redistribution of historical seasonal water flows throughout the year, especially after the Porto Primavera reservoir - the nearest - was impounded
It is possible that piscivorous fish species are affected and show distinct responses according to the strategy adopted
Summary
The integrity of processes that operate in floodplains is closely associated with the hydrological regime, and the alternation between flooding and drought periods determines the structure and functioning of these environments (Junk et al, 1989; Neiff, 1990; Junk and Wantzen, 2004; Thomaz et al, 2004; 2007; Raniere et al, 2008). In the upper Paraná River floodplain, upstream reservoirs have caused a redistribution of historical seasonal water flows throughout the year, especially after the Porto Primavera reservoir - the nearest - was impounded This event, which directly affected the flooding regimes of the Paraná and Baía Rivers and, to a lesser extent, the low portions of the Ivinheima River, is expected to have caused changes in the feeding and nutritional features of fish that live transiently or permanently in that region. It is expected that species that belong to different trophic categories present differentiated responses to water level variations This possibility can even be extended to species with similar diets but different food intake strategies. It is possible that piscivorous fish species are affected and show distinct responses according to the strategy adopted
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