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COLLECTION OF AQUATIC HABITAT INFORMATION FOR IFIM: SENSITIVITY ANALYSIS OF FRENCH STREAMS. Jean Rene Malavoi, Yves Souchon, Laboratory Quantitative Hydroecology, CEMAGREF, 3 quaff Chauveau 69336 Lyon Cedex 09, France. The quantitative information on habitat needed to effectively use methodologies like the instream flow incremental method (IFIM) of the U.S. Fish and Wildlife Service is dependent on a stratified sampling of physical data, this requires considerable expertise in choosing the measurement points. We analyzed the aquatic habitat sensitivity in some French streams to the information measured. We considered the geomorphological features, the number of cross sections in each typical feature and the number of samples by transact. The result will help the IFIM users to better design their approach to field sampling, especially when the data are to be extrapolated to a whole reach of river or compared to biological data. FLUSHING FLOW ESTIMATION USING POINT-SPECIFIC DETERMINATION OF CRITICAL SHEAR STRESS: AN APPLICATION OF THE HYDRAULIC MODEL IFG4. Richard A. Domingue, Instream Flow Specialist, Stone & Webster Engineering Corp. Flushing flows maybe required to remedy sedimentation problems in stream reaches that have been significantly dewatered. Shear stress determinations are widely used as a method of determining appropriate flushing flows. These determinations generally rely on whole-channel measurements of depth and velocity. Because the distribution of sediment deposits and the point-specific depth and velocity vary dramatically across most channel cross-sections, a method was devised to determine the critical shear stress on apoint-specific basis. The mainstay of the Physical HABitat SlMulation (PHABSIM) library of hydraulic models, IFG4, was employed to determine point-specific shear stress. The ability of fFG4 to simulate cell velocity and depth over a wide range of discharges made it ideal for the purpose. Sediment removal was assumed to occur when the simulated shear stress exerted by a given discharge on a sediment impacted cell was equal to or greater than the critical shear stress required for incipient motion. The discharge at which all sediment-impacted cells within a stream transact were subject to critical shear stresses was termed the Probable Effective Flushing Flow (PEEP). A test was made to measure the effectiveness of the PEFF on removing sediment deposits in a Michigan stream. AN ANALYTICAL APPROACH TO ASSESSMENT OF FISH STOCKS OF THE GANGA RIVER SYSTEM. Arun G. Jhingran and R.A. Gupta, Central Inland Capture Fisheries Research Institute, Barrackpore-'743101, West Bengal, India. Reviewing the recent trends in fish production, an attempt is made to assess the stocks of four Indian major carps viz., Labeo rohita, Cirrhinus mrigala, Catla cat/a and Labeo ca/basu and two major catfishes viz., M. aor and M. seenghala of the selected segment of the Ganga River System using lengtl;-based cohort analysis on the data collected during 1981-85. An assessment of this mixed fishery is also carried out to evaluate stock size, exploitation rate, maximum sustainable yield (MSY) and the corresponding effort (Fmsy) at this level. These parameters have been studied for individual species and the approach extended to mixed fisheries of major carps and catfishes separately in order to provide appropriate management advice. The study has shown that the fishery is operating at a level higher than Fmsy and no further increase in fishing effort is advisable. Such a viewpoint is discussed and feasible approach for management suggested. 17