South of the Villa Mercedes city (San Luis province, Argentina), there are over one hundred shallow lakes of different surface. Many of these lakes are exploited as recreational fishery resources. However, the information available about the ecology and fishes is very limited. In october 2007, the main ichthyofauna characteristics and different ecological aspects of six shallow lakes (34º00'S, 65º24"W) were studied with special focus on the pejerrey Odontesthes bonariensis population. The

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... e area of shallow lakes was 17.8 (±9.9) ha. Turbid (green) and clear shallow lakes were observed with great abundance of Potamogeton berteroanus and Secchi disk values of were up to 2.30 m. All the lakes showed a reed belt of Schoenoplectus americanus with a different degree of development. The abundance of zooplankton was variable with a maximum of 338167 org/m3. The water was classified as oligohaline and very hard. In total, 7 species of fishes were caught distributed in 4 orders and 5 families: O. bonariensis, Astyanax eigenmanniorum, Cheirodon interruptus, Oligosarcus jenynsii, Corydoras paleatus, Cnesterodon decemmaculatus y Jenynsia multidentata. The Shannon-Wiener diversity index was low to medium (0.91-1.55 bits). The largest capture per unit effort of O. bonariensis (CPUEp) was 28.6 kg/20hs/net with a proportional stock density (PSD) of 60.7.