JAMES B. LAYZER AND LESA M. MADISON, Tennessee Cooperative Fishery Research Unit, U.S. Geological Survey, Tennessee Technological University, Cookeville, Tennessee 38505, USA

Defining the instream flow needs of freshwater mussels presents a unique challenge. Mussels, unlike other macroinvertebrates and fish, have limited mobility and cannot respond to rapid changes in stream discharge caused by storm events or operation of hydroelectric projects. This sessile behavior poses several problems for determining flow requirements for mussels. Firstly, methods such as the Instream Flow Incremental Methodology (IFIM) require data on the differential use of microhabitats (suitability curves) by the target organism. The sessile nature of mussels means that data obtained on microhabitat use are flow-conditional. That is, measured values of variables such as water depth are dependent upon the flow conditions when the measurements are made. Secondly, typical applications of IFIM assume that target organisms can track hydraulic conditions and move to suitable microhabitats as discharge changes. Thus, the total weighted usable area (WUA) for a stream reach may change little with a change in discharge, but the specific location of the suitable habitat may change and may not be readily available to the existing population of mussels. Options are available in the Physical Habitat Simulation Model (PHABSIM) that don’t require making this assumption; however, the output is less easily summarized and interpreted. Although the boundaries of established mussel beds often are well defined, attempts to define them in terms of simple hydraulic variables such as water velocity are futile. Recently completed research in small and medium-sized streams suggest that complex hydraulic characteristics such as shear stress at the time of juvenile excystment, or shortly thereafter, may be a key factor in determining the location of mussel beds. Finally, the life history requirement of a fish host for mussel larvae necessitates identifying the host(s) and providing for suitable habitat conditions for the host in the immediate area of the established mussel bed.