Population structure of Uncancylus concentricus da Reserva Natural de Usos Multiplos Ilha Martin Garcia no Rio de la Plata Superior, Argentina

The present work analyzes the population dynamics of Uncancylus concentricus in natural conditions in the northeastern coastal area of the Multiple Use Natural Reserve Isla Martin Garcia (Beach of Basural), Buenos Aires, Argentina. Martin Garcia Island is located in the Upper Rio de La Plata, to the south of the mouth of the Uruguay River (34°11'25"S and 58°15'38"W). Monthly collections were made from August 2005 to December 2006. The size frequency of the U. concentricus population throughout the sampling period ranges from 1.2 to 8.3 m. The changes in the size frequencies throughout the months surveyed indicate a variable distribution pattern because of the increase and decrease in water flow, but we observed a peak in the frequency of individuals within the size range 3.5 to 4.5 mm in the population. It can be concluded that this species lives for at least a year in the wild since the juveniles are highly abundant in spring and summer.