Balancing Fisheries Management and Water Uses for Impounded River Systems

Abstract

<em>Abstract</em>.—Reservoir fishery managers have traditionally viewed reservoirs as stand-alone systems and emphasized in-lake management practices such as controlling selected fish populations, restraining and promoting harvest, and enhancing fish habitat. However, reservoirs do not always respond to in-lake approaches that ignore important factors operating outside the reservoir. I propose an expanded concept where reservoirs are viewed as parts of the landscape and influenced by tributaries, riparian zones, watersheds, and position in the river basin. The influence of tributaries over reservoir fish assemblages ranges from almost none in reservoirs positioned high in a basin where lacustrine fish assemblages prevail to a large effect in downstream reservoirs where riverine fish assemblages prevail. Many species inhabiting reservoirs typically require tributaries to complete their life cycle, or at least their abundance in the reservoir is enhanced by access to flowing water and upriver floodplain lakes. Riparian and buffer zones surrounding tributaries and the reservoir trap sediments and nutrients, reduce wind and associated wave action, provide bank stability and woody debris, and improve esthetics. Direct links between riparian zones and reservoir fish assemblages have received limited research attention, but evidence indicates that riparian plant debris enhances fish species richness, predator–prey interactions, and recruitment of selected species in the littoral zone. Imports from watersheds, including sediments, nutrients, and carbon from dissolved or particulate organic matter, interact to influence turbidity, water quality, primary production, and habitat quality. Fish assemblages are shaped by eutrophication, and organic detritus imported from highly disturbed watersheds may play a major role in promoting key detritivores. At the basin scale, abiotic characteristics, species richness, species and trophic composition, biomass, and population characteristics show longitudinal gradients along reservoir series. Basin-scale variables constrain the expression of processes at smaller scales but are seldom controllable, although an appreciation of basin patterns helps set limits for smaller-scale determinants and thereby management expectations. Extending the scale of reservoir management can enhance the manager’s ability to impact reservoir fish populations and assemblages and increase the effectiveness of traditional in-lake management measures. Nevertheless, reaching outside the reservoir through potentially segregated efforts of isolated managers may not be sustainable, especially if reservoir managers lack jurisdiction and training to reach beyond the reservoir shores. Thus, managers must participate in landscape-level partnerships to advocate landscape changes likely to benefit reservoir environments. Extending the scale of reservoir management does not mean that reservoir managers must become watershed managers, but simply that they should think about reservoirs as part of bigger systems and thereby network with those working upstream and in the watershed to advance reservoir issues.