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MUSSELS:THE FORGOTTEN FAUNA OF REGULATED RIVERS. James B. Layzer and <br />Mark E. Gordon, Tennessee Cooperative Fishery Research Unit, Tennessee <br />Technological University, Box 5114, Cookeville, TN 38505. <br />During the past century, freshwater mussel populations have declined precipitously <br />throughout North America. Much of this Loss resulted from [he construction of dams. <br />In the Cumberland River system, 23% (22 species) of the historic mussel fauna is <br />extinct or listed as endangered. Several additional species have either been <br />extirpated from the Cumberland River or exist only in small, nonreproducing <br />populations. Mussels of headwater streams have been severely impacted by coal mining <br />and poor land-use practices. For instance, nearly all mussels have been eliminated <br />from the Lower 17 km of the Little South Fork Cumberland River due to coal mining. <br />The eleven major dams constructed on the 1,105-km-long Cumberland River and its major <br />tributaries have had devastating effects on species typical of larger rivers. For <br />example, construction and operation of Center Hill Dam inundated about 150 km of <br />riverine habitat, altered the temperature regime of the tailwater, blocked movements <br />of host fishes, and alternately scours and exposes much of the streambed during <br />hydroelectric peaking operations. These perturbations have eliminated more than 40 <br />species of mussels (including several endangered or extinct species) from the first <br />15 km of tailwater; less than 50% of the historic fauna remains in the lower reach of <br />tailwater. Recognition of mussel life history requirements during preconstruction <br />could have reduced many of these impacts. <br />STAB]LTTY W HYDROPSYCHE GUILDS ALONG A REGULATED STREAM: THE ROLE <br />OF COMPETITIVE INTERACTIONS VERSUS ENVIRONMENTAL PERTURBATIONS. <br />Julio A. Camazgo, C1T-INIA, Valdeolmos, Madrid, Spain and (present address) Department of <br />Biology, Colorado State University, Fort Collins, CO 80523, USA <br />Temporal alterations in the structure of Hydropsyche guilds were studied at three sampling stations along a <br />regulated stream to examine the role of competitive interactions and environmental perturbations in determining <br />species persistence. S-1 was placed upstream from adeep-release hydropower impoundment located in the middle <br />Rio Duraton (northern Spain). S-2 and S-3 were respectively placed 2.5 and 7.6 km below the dam. Dischazge <br />fluctuations of hypolimnial waters with a significant oxygen deficit were produced daily by the dam. Under <br />unregulated conditions, I assume that the coexistence of competing species is a function of the species dominance <br />(d') and the resource limitation (R), as major causes of interaction strength between competitors, and the niche <br />overlap (O) as a major cause of connectance between competitors. The species dominance tends to increase with <br />the number of subordinate species (d' = S2/S). S is the number of competing species and S2 represents the absolute <br />difference between competing species with regazd to their relative abundances. Thus, and according to May's <br />model of stability, a Hydropsyche guild would tend to be unstable if d'2 RZ O S >1 or 822 R2 O >S. In this work, <br />I consider that the values of R and O are each 1. Therefore, a Hydropsyche guild would tend to be unstable when <br />822 >S. Riffle macrobenthic surveys were undertaken in June and December of 1987, and in June of 1988. At <br />S-1, values of S were 6, 6 and 5 and values of 822 were 3.8, 12.7 and 4.1. At S-2, values of S were 4, 3 and 3 <br />and values of 822 were 1.$, 0.2 and 1.6. At S-3, values of S were 4, 4 and 3 and values of f22 were 2.5, 2.3 and <br />1.3. Values of total density and total biomass for Hydropsyche guilds were significantly (P<0.05) higher <br />downstream than upstream from the dam. It is concluded that competitive interactions upstream and environmental <br />perturbations downstream determined species persistence in Hydropsyche guilds. The perturbation tolerance <br />appeazs to be a primary biotic factor affecting competitive abilities of Hydropsyche species. <br />Ecological Responses to River Regulation: Macroinvertebrates and <br />Fish <br />THE EFFECTS OF RIVER-BED RESTRUCTURING ON FISH AND BENTHIC <br />COMMUNITIES OF A 4th-ORDER STREAM (NECK, AUSTRIA). <br />Mathias Jungwirth, Otto Moog & Susanne Muhar, Dept. <br />Hydrobiol., State Univ. Agric., A-1180 Vienna, AUSTRIA <br />Fifteen sections of seven different epipotamal streams have been studied between 1981 and 1984. These <br />studies proved the impact of river bed structures on fish communities. Reduced spatial heterogeneity due to <br />river straightening results in decreasing numbers of fish species, stock density and biomass. The variance of <br />maxima] depths (VMD) is a practical and sensitive measure of habitat structure which significantly correlates <br />with the fish species diversity (FSD). <br />1987 these model was used for the evaluation of a planed restructuring project on a channelized section of <br />the River Melk. After its realization in 1988, the reaction of the fish and benthic fauna was studied over <br />three years. The results verified the VMD/FSD model. Number of fish species changed from 10 to 19, fish <br />density and biomass as well as annual production of 0+ fish increased threefold. <br />Tl~e number of benthic invertebrate taxa changed from 202 in the channelized section to 273 in the <br />restructured area (in total 347 taxa have been recorded). Increased habitat variability - on the bank as well as <br />on the channel bottom - is the explanation. A more diversified benthic community developed, favouring <br />epipotamalic elements. In the restored stretch the drift of aquatic specimens and terrestric organisms <br />decreased significantly in comparison with the ehannelized section. Channel structures and their impact on <br />drift are discussed. _ <br />