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Aerial or ground surveys can be used in conjunction with past or projected water level data to estimate spawning habitat availability (V1) over a series of years. Many plants have luxuriant above-surface growth during summer and early autumn only to die back during late autumn and winter. Spawning habitat quality depends on vegetative cover during spring. Also, if vegetation is examined at a time when it is not under water, its characteristics when sub- mersed should be estimated. The time and duration of the embryo and fry stages must be determined in order to properly estimate V2 and V5. Spawning typically begins when the water temperature reaches 8° C and is usually complete when water temperatures have remained at or above 13° C for several days. Allowing 2 weeks for incuba- tion, 2.5 weeks for fry to grow to the size at which emigration begins, and 3 weeks for most of the fry to depart from the nursery grounds, there appears to be a critical period of 7.5 weeks during which water level drawdowns can affect reproductive success. Absorption of the yolk sac requires about 10 days in Minnesota (Franklin and Smith 1963). The first 3.5 weeks after spawn- ing (incubation and sac-fry stages) would, therefore, seem to be especially critical. These values can serve as general guidelines, but should be revised if there is reason to believe that the critical intervals are different for a particular study site. The percent of midsummer area covered with vegetation (V3) and the percent of area as pools, backwaters, or other standing/sluggish water (V$) can be estimated by following procedures outlined in Terrell et al. (in press). Several of the variables included in these models do not require field measurements. As noted above, average length of frost free season (V6) can usually be obtained from a local weather station or from a published climatol- ogical summary (National Oceanographic and Atmospheric Administration 1978). If it is necessary to interpolate a value,- the relative elevations of the study site and of reference stations should be considered. Stream gradient (V,) can be calculated from U.S. Geological Survey topographic maps. It is not necessary to measure TDS (V4) or pH (VS) if it is known that the measure- ments would fall within the broad ranges in which suitability indices equal 1.0 or 0.0. Otherwise, the measurement techniques described by Lind (1974) can be used. Rivers, streams, and surface waters of lakes are usually well-mixed. Horizontal variation in water quality variables, such as temperature, pH, and TDS, are rarely so large as to affect suitability index ratings. Sheltered bays, mouths of tributaries and inlets, and springs are possible exceptions to this generalization. If measurements made at different sites result in dif- ferent suitability indices, the suitability indices, rather than the raw habitat measurements, should be averaged. Each suitability index should be weighted according to the fraction of the total area over which it is assumed to apply. 0 27