Ecology of large piscivorous fishes in Guri Reservoir, Venezuela, with notes on fish community structure

Abstract

Venezuela's growing human population is accompanied by a growing need for electricity which has largely been met with hydroelectric power, and yet the full effects of river impoundment on river ecosystems are not known. Venezuela currently has the second largest hydroelectric facility in the world, the Raul Leoni Dam (Guri Reservoir). Formed by the blackwater Caroni River, Guri is characterized by low pH, low nutrients, and high dissolved organic matter. Water level fluctuations associated with hydroelectric facility operations may have large effects on tropical fish spawning, feeding, and survival. The primary sportfishes in the reservoir are the peacock basses (Cichla spp.), that exhibited heightened fish production immediately after inundation. However, during the 1990's, sportfishermen at Guri began complaining about decreased catches. To investigate claims of declining Cichla populations and to compare current fish community structure with a previous survey, the four large piscivorous fishes of Guri Reservoir were sampled. Samples from the northern area of the reservoir had 50 species representing 18 different families. The dominant species in seine samples was the characid Hemigrammus micropterus. In these samples, Cichla temensis, Cichla cf orinocensis, Plagioscion squamosissimus and Hydrolycus scomberoides had greater body condition compared with values for conspecifics from a previous study. Conversely, catch per unit effort for Cichla in gillnets decreased in the current study. Prochilodus rubrotaeniatus, a detritivorous characiform, was the dominant species captured in gillnets. Cichla spp. appear to breed throughout the year with a peak before the rainy season. Hydrolycus scomberoides and Plagioscion squamosissimus partitioned resources, with the former consuming the largest prey and the smallest prey consumed by the latter. Cichla temensis and Plagioscion squamosissimus had high diet overlap among prey types but consumed prey of different sizes. Niche breadths for all species were low. Claims of declining Cichla populations in Guri appear to have some foundation. Blackwater physicochemistry, the reservoir "boombust" cycle, and fishing pressure all influence fish ecology in Guri Reservoir.