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Habitat variables Suitability indices '% <br />Ratio of spawning habitat <br />to summer habitat [area that <br />is less than 1 m deep and <br />vegetated (spring) divided <br />by total midsummer area] (V1) SI, <br />Drop in water level during embryo <br />and fry stages (U2) SIZ <br />Percent of midsummer area with <br />emergent and/or submerged <br />aquatic vegetation or remains <br />of terrestrial plants (bottom <br />debris excluded) (V,) SI, <br />Loglo TDS during midsummer (V4) SI4 <br />Least suitable pH in spawning <br />habitat during embryo and <br />fry stages (VS) SI5 <br />Average length of frost-free <br />season (V6) SI6 <br />Maximal weekly average <br />temperature (1 to 2 m <br />deep) (V,) SI, <br />Area of backwaters, pools, or <br />other standing/sluggish <br />(< S cm/sec) water during <br />summer, as a percent of <br />total area (Vs) SI, <br />Stream gradient (V9) 519 <br />HSI <br />Figure 2. Diagram showing habitat variables included in the riverine <br />model for northern pike and the aggregation of the corresponding <br />suitability indices (SI's) into an HSI. HSI = the lowest of the nine <br />suitability index ratings. V4 should be eliminated from the model under <br />certain conditions (see p. 24). <br />C <br /> <br />14 <br />