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e e e abundance of white sucker is lower (Propst 1982a). This phenomenon could occur when the high tolerance of white suckers allows them to survive periods of stress that are lethal to other species, thus affecting the HSI and relative abundance correlation. In order to achieve a better correlation between HSl1s and white sucker relative abundance in riverine habitats, an additional model (Model 5) was developed. More importance was given to cover with the addition of variables for stream cover, percent shade, and pool depth. The importance of pool velocity was considered by adding pool velocity and gradient variables. These variables were intended to better define adult white sucker habitat. This model has not been field tested. Decisions on which, if any, model to use should be made by the potential user and will depend on the userl s needs and resources available. The lacustrine model was tested by entering reasonable combinations of habitat variable values into the model (Table 4) and examining the model output. The HSl1s resulting from the model reflected assumed carrying capacity trends in habitats with the same characteristics as the sample data sets. Model Description Because white suckers are opportunistic feeders, we did not consider food to be important in determining white sucker habitat suitability. In unproduc- t i ve 1 akes and streams food may i nfl uence abundance. However, measures of lake and stream productivity for white suckers have not been quantified. We assumed that habitat quality is determined primarily by cover, water quality, and spawning habitat. These were used as model components. Component ratings were derived from individual variable suitability indices (Figs. 1 and 2). Reasons for placing individual variables in specific components and assumed variable interactions are described below. Model Description - Riverine Water quality component. The water quality component consists of turbid- ity (V1), pH (V2), dissolved oxygen (V3), and temperature (V4). These vari- ables affect growth, survival, and/or distribution of white suckers. Sub- optimum levels of these variables, as defined by the suitability index graphs, result in negative effects on individuals. Toxic substances are not considered in th is mode 1 . Reproduction. Temperature during spawning (V,) is included in the repro- duction component because it is believed to be a primary factor influencing initiation of spawning migration. Discharge may also be important, but would be variable depending on the size of a river or stream. Also, less suitable discharge levels can likely be compensated for by selection of more favorable 7