Effects of decadal changes in the hydrological regime of the middle reach of the Parana River (Argentina) on fish densities

Changes in fish densities recorded over 14 years (1996-2009) were studied for effects of long-term variation of the hydrologic regime. We collected field data with an echo sounder in scour holes of minor channels draining an area of floodplain in the middle reach of the Parana River. Fish densities in 2000-2009 were significantly lower than in the previous decade. The decrease was associated with a marked reduction of water levels, flood magnitudes and connectivity of channels with the nearby floodplain lakes. This distortion of the flood pulse likely had an effect on the life history strategies of the fishes. The effects of damming in the upstream basin and other man-made perturbations are minor in the middle reach. However, the decadal alterations of regime are intimately linked to climate fluctuations in the Parana River basin during the past century. Tendencies of observed fish densities are similar to results reported in literature on the influence on fishes for similar long-term alterations of the flood regime in river flood plain systems.