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(~ Average length of the frost-free season (V6~. The length of the frost- free season correlates with the length of the growing season for northern pike; however, if the cold weather season is too short, normal gonadal devel- opment may be impaired. The suitability index curve is based on growth data for northern pike, laboratory observations on yellow perch spawning (see text), the relative position of isopleths for length of frost free season (Visher 1954), and the southern limit to the native range of northern pike (Grossman 1978). Maximal weekly average temperature of surface water (1 to 2 m deep) (V,~. Summer water temperature affects growth and survival of northern pike. The optimal temperature for growth of fingerling and older northern pike is 19 to 20° C (Casselman .1978). Temperatures will be in the optimal range during warming and cooling phases if the maximal summer temperature is slightly higher than the growth optimum. Maximal suitability of V, is assumed for peak weekly average temperatures of 20 to 25° C. Laboratory studies show that temperatures over 32° C can kill northern pike within several days (Scott 1964). An oxygenated metalimnion, mouths of tributaries or inlets, or springs may be used when temperatures elsewhere are too high. Their effectiveness as thermal refugia depend on their size and the duration of hot spells. Percent pools and backwaters during midsummer (U$) and stream gradient ,(.V9~. Riverine habitat may be suitable to the extent that it resembles a lake ,~ environment. Northern pike require standing, or very slowly moving, water. Two variables are included as indicators of the presence of low velocity habitat: percent pools and backwaters during midsummer and stream gradient. A direct, proportional relationship between the availability of standing water habitat (V$} and habitat suitability is assumed. However, a high gradient stream is likely to be of low suitability even if pools are present, because movement between pools would be restricted. It is assumed that northern pike are less well adapted to high gradient streams than are muskellunge, based on patterns of distribution for the two species (Grossman 1978; Harrison and Hadley 1978). Muskellunge occur in stream reaches with gradients as high as 6.9 mlkm (Parsons 1959), but gradients less than 2 m/km seem to be more typical of riverine muskellunge habitat (Brewer 1970; Miles 1978). It is assumed that stream reaches with gradients greater than 5 m/km (0.5%) have little or no habitat value for northern pike. Field Use of the Model Year to year variation in weather complicates the application of this habitat model. Such variability is most pronounced for variables related to temperature (V6 and V,) and water level (U1 and V2). Between-year variation does not pose a problem if the objective of an HSI model application is to obtain an HSI rating for a single year. More often, however, the goal is to obtain an average or representative HSI for a particular body of water that can be compared with HSI`s for other bodies of water or' for the same body of 25