LONG-TERM BROWN TROUT POPULATIONS RESPONSES TO FLOW MANIPULATION CATHERINE SABATON, a YVES SOUCHON, b * HERVE CAPRA, b VERONIQUE GOURAUD, a JEAN-MARC LASCAUX c and LAURENCE TISSOT c a Electricite ´ de France, Research and Development, LNHE, 6 Quai Watier, 78401 Chatou, France b Cemagref, Quantitative Hydroecology Laboratory, Biology of Aquatic Ecosystems, 3 bis quai Chauveau, CP 220, F 69 336 Lyon Cedex 09, France c ECOGEA, 10, Avenue de Toulouse, 31860 Pins Justaret, France ABSTRACT Despite the many habitat simulations that have been undertaken around the world, not enough biological monitoring has been performed following flow manipulations. It is difficult, however, to refine flow management decisions without a better understanding of the links between amounts, durations and seasonality of flow deliveries and population dynamics. Trout populations were monitored before and after flow alterations in five trout streams, involving 17 study sites over a 4- to 12-year period, depending on the sites. A comparison of the trout populations observed to theoretical habitat/population models pointed up several helpful lessons. Various factors slow increases in population size, including the availability and quality of spawning grounds, the general connectivity of the bypass section (BPS) and severe spate events. In addition to these site-dependent factors, hydrological dynamics may explain why it is so difficult to clearly identify relationships between habitat availability and real fish stocks. Moreover, opportunities to observe population changes are improved when the pre-enhancement instream flow value is very low, and when there is a considerable difference between pre- and post-enhancement values. A population dynamics model that incorporates different habitat limitations and demographic background can be a very precious tool to improve understanding of the different situations and to build scenarios of population recovery. Copyright # 2008 John Wiley & Sons, Ltd. key words: minimum flow alteration; habitat modelling; Brown trout population simulation; hydropower; relicensing Received 30 August 2006; Revised 20 June 2007; Accepted 23 November 2007 INTRODUCTION The potential ecological effects of dams and hydropower operations have frequently been described in literature (Petts, 1984; WCD, 2000). In future, one of the major problems in dam management will most certainly be to establish better balance among the uses for which the dams were originally built, and to improve consideration of the functioning of the rivers on which they lie. This will in turn require greater expertise of the various types of corrective measures which can be implemented, where possible, in view of the foreseeable impacts on aquatic populations. The most radical of these measures involve actual dam removal, described in a number of recent papers (Heinz Center, 2002; Poff and Hart, 2002; Graf, 2003). In Europe this will, in all likelihood, affect only lowhead dams or levees or certain dams with strategic importance for migration of diadromous fish. In cases where the dams will be kept, particularly those important for the production of hydroelectric energy, various options need to be examined. Among these are the construction or fine tuning of fish passes for upstream or downstream migration (Marmulla, 2001), improved management of sediment flows and transfers which impact the geomorphological balance in zones downstream of dams (Petts and Gurnell, 2005), greater consideration of riparian areas or establishing a good trade-off between hydropeaking operations and preservation of sufficient spawning grounds and nurseries downstream. Some of these measures have been integrated in recent discussions about ‘green’ energy sources in RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 24: 476–505 (2008) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1130 *Correspondence to: Yves Souchon, Cemagref, Laboratoire d’Hydroe ´cologie Quantitative, Unite ´ Biologie des Ecosyste `mes aquatiques, 3 bis quai Chauveau, CP 220, 69336 Lyon Cedex 09, France. E-mail: yves.souchon@cemagref.fr Copyright # 2008 John Wiley & Sons, Ltd.