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Temperatures below 4° C do not appear to be stressful for juvenile or adult northern pike when cooling is gradual (Casselman 1978). In fact, male northern pike in an Alberta lake achieved about 35% of their total annual growth during the winter at temperatures near 1° C (Diana and MacKay 1979). Sudden temperature drops can, however, be lethal. A sizeable northern pike kill occurred when water temperatures in an Alberta lake receiving heated effluent dropped from 21.8° C to 4.9° C within 30 minutes after a power plant shutdown (Ash et al. 1974). HABITAT SUITABILITY INDEX (HSI} MODELS Model Description - Lacustrine and Riverine The Habitat Suitability Index (HSI) models which follow are attempts to condense the preceding observations into a manageable set of habitat evalua- tion criteria, structured so as to produce an index between 0.0 and 1.0 of overall habitat quality for northern pike. A positive relationship between HSI and carrying capacity of the habitat is assumed (U.S. Fish- and Wildlife Service 1981). Separate HSI models are presented for lacustrine (Fig. 1) and riverine (Fig. 2} habitats, but the two are very similar and are more appropriately described as variations of a single general model. The only differences are that the riverine model contains two additional variables (V8 and V9) to account for the availability and accessibility of low velocity habitat, and TDS (V4) can sometimes be excluded from the riverine model. It is implicit that V8 and V9 are not limiting in lentic environments; that is, both would have suitability indices equal to 1.0. The model includes habitat variables believed to be of general importance in limiting the occurrence of northern pike. The assumed functional relation- ship between each habitat variable and habitat suitability for northern pike is represented in a suitability index (SI) graph. An SI graph associates SI ratings (on a 0.0 to 1.0 scale) with different levels of an environmental variable. It is assumed that SI ratings for different habitat variables can be compared. Overall habitat suitability is assumed to be determined by the variable with the lowest suitability index; that is: HSI (lacustrine) = minimum value for suitability indices V, to V, HSI (riverine) = minimum value for suitability indices U1 to V9 12