Changes in roach (Rutilus rutilus L.) population structure induced on draining a large reservoir

The influence of draining a large reservoir (Pareloup, South of France) on the population structure and the growth of roach is studied in this work. This reservoir, with an area of 13 km², drained in June 1993, was refilled 2 months later. Fish were sampled monthly before draining (1981) and after refilling (1997) by nylon monofilament gill nets used at the same catching effort for both periods. This permitted a comparison to be made between the density of roach and length distribution before and after refilling. Changes in growth were determined by scalimetry. The observed difference in population structure and growth underlined that the differences are mainly due to a greater growth rate after refilling during the first 2 years of roach life. These growth changes are explained by the decrease in roach density and the increase in 1993 of the biomass of Zooplankton, which constitute the main feeding resource for 0⁺ and 1⁺ roach. The study led us to propose hypotheses on the ecological significance of growth changes induced by refilling, which can be considered as a rejuvenation of the aquatic ecosystem.