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HABITAT SUITABILITY INDEX (HSI) MODELS , Model Applicability Geographic area. This model is applicable throughout North America where white suckers occur. The standard of comparison for each individual variable Suitability Index (SI) is the optimum value that occurs anywhere within this geographic range. Season. The lacustrine model provides a rating for lake or reservoir habi tat based on its abi 1 i ty to support all 1 i fe stages of whi te suckers throughout the year. The riverine model can be used two ways: (1) to provide a rating for streams and ri vers used by resident whi te sucker popul at ions throughout the year; or (2) it may be incorporated into the lacustrine model to provide a rating for tributary streams or rivers during the spawning and fry migration period when these streams serve as recruitment areas for the lake population. Cover types. Riverine and lacustrine. Minimum habitat area. Minimum habitat area is defined as the mlnlmum area of contiguous suitable habitat that is required for a population to maintain itself indefinitely. The minimum habitat area necessary for a white sucker population has not been established. Verification level. The acceptance level of the lacustrine and riverine mode 1 is that it produces an index between 0 and 1 whi ch the authors bel i eve has a positive relationship to carrying capacity for white suckers. Data from LaGarde Creek, Colorado, was used to evaluate the riverine HSI model (Table 2). The low HSI's generated for LaGarde Creek indicated poor habitat, and white suckers did not occur in the study reach. t The riverine HSI model also was tested with data from eight sites in the St. Vrain Creek and Big Thompson River, Colorado, that had viable white sucker populations. Sites with low HSI I S were assumed to have a correspondingly lower relative abundance of white suckers than sites with higher HSI's. A poor correlation or lack of correlation between relative abundance and HSI's might indicate that the model is inadequate or that species interactions may have a significant influence on relative abundance. The correlation co- efficient between the relative abundance of white suckers and the HSI's was moderate (r = 0.477) (Propst 1982a). The data sets determined by sampling are given in Table 3. This moderate correlation between the HSI's and the relative abundance of white suckers may be the result of two factors. The first factor is that there may have been inadequate di fferent i at i on between 1 i fe stage requisites. White sucker fry and juveniles occur in a greater variety of habitats (shallow riffles, slow runs, along edges, and backwater pools), while whi te sucker adults (> 150 TL mm) are usually more common to pool habi tats (Propst 1982b). The second factor is that white suckers of all life stages have the ability to survive in a variety of conditions. In stressed streams (or sections) which would have low HSI's, white suckers may account for 50 to 80% of the relatively few species (3 or 4) of the fish population found. While in less stressed environments more species are present and the relative 6 t