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.-.- - ',y~ J '4,.- ... rqqo' - - - - -----.--...------------------..------------/ / / _ 01Q73 ..... ~ -~'''' -........ ------ '--:\'\.....:... }:.o...I.., . Effects of Altered Stream Flows on Fishery Resources This policy statement revises the 1981 version by J. c. Peters entitled "Effects of River and Streamflow Alteration on Fishery Resources" (Fisheries 7[2]: 20-22). The following paper was prepared in July 1989 and approved by the Executive Committee on 3 September in Anchorage, Alaskn. A. Issue Definition Alteration of the quantity and timing of river or stream flow can significantly affect fisheries resources. The Ameri- can Fisheries Society (AFS) is alarmed at the loss of natural streams in North America, and greatly concerned with management of fisheries in streams that have been altered'. The AFS be- lieves that public trust rights need more recognition and stronger consideration in the management of stream re- sources. Severe, long-term adverse impacts and irreplaceable loss of stream fishes have occurred throughout North America. Some river basins, particu- larly in the more arid American South- west, have been so radically altered by dams and diversions that an almost total replacement of native fish faunas has occurred. Many of our historic and valuable riverine fishes are declining in distribution and abundance, and some are threatened with extinction. In addition, adverse affects of - water development have resulted in inter- national problems in water supplies, and loss in estuarine productivity. Deg- radation of stream water quality has, in some instances, made the remaining water questionable for further use. There are few river basins in North America where natural flow regimens have not been altered. Dams built by public and private agencies regulate impounded water supplies by storing and releasing water for power, flood control, irrigation, navigation, munic- 18 Harold M. Tyus ipal and industrial uses, and others. Water diversions further reduce the amount of water available to stream resources, and also return the remain- der to a point distant to the intake. Diversions may degrade water quality parameters such as sediment load, temperature, pesticides, herbicides, and others. Regardless of environmen- tal impacts of water diversions or im- poundments, these developments are considered "beneficial" by state and provincial law. Conversely, instream flow use of water for fishes is not universally recognized. Most agencies accept fishery protec- tion as conditional to awarding water rights. However, quantification of stream flow needs of fishes, and op- erational and legal protection are not presently adequate. The importance of protecting stream fishery resources from a water rights perspective is little understood by the public at large, and fishing as a recreational use of larger streams and rivers has been increas- ingly de-emphasized by agencies in deference to reservoirs. The AFS believes that governmen- tal and private sectors do not place sufficient emphasis on protection of instream water for management of stream fisheries resources. In this state- ment, we stress a need for more careful consideration in planning future stream alterations, discuss potential impacts of alterations, and provide guidelines for future management of stream fisheries. B. Impacts of Altered Stream Flows Changes in stream flow can affect fishes directly and indirectly. Direct effects of flow alterations are certainly important if migrations are blocked, fish are trapped in dewatered sections, or reproduction is disrupted. However, insidious effects may be far more det- rimental, and include alteration and loss of stream habitat, introduction of competing non-native fishes, degra- dation of water quality, and other ef- fects. As an example, it is difficult to assess fishery impacts associated with a reduction in stream flooding; how- ever, changed nutrient cycles and dis- ruption of food webs may have serious ecosystem consequences. Decreased stream flow can contrib- ute to direct mortality if fish eggs are exposed, covered with silt, or left with- out _ sufficient, oxygenated water. Re- duction in usable habitat can result in decreased abundance, size, and con- dition of fishes. Water velocities and the amount of appropriate substrate can be so changed that spawning sites become limited, and in some species, an increase in interspecific hybridiza- tion may occur. Anadromous or resi- dent species may not move to appropriate spawning sites if attractant flows are lost or stream passage is inadequate. Reduction in spawning or nursery habitats by stream regulation or diver- sions can concentrate eggs and young, encouraging increased predation by resident or introduced fishes. Produc- tion of essential food organisms, or their availability in occupied habitats, may be reduced for all life stages of fishes. Productivity of riverine systems may be reduced by storage projects that trap nutrients or release water at unfavorable temperatures. Water resource developments and operations may affect stream resources both beneficially and adversely. Return flows from irrigation projects may be wanner, sediment laden, and contam- inated with chemicals, including bio- cides and fertilizers. Conversely, return flows into river channels during droughts can prO'fide Some beneficial effects. Flows from hydroelectric plants may fluctuate greatly and create un- Fisheries, Vol. 15, No.3 ~