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Development of Suitability Index Graphs: Rationale and Assumptions The preceding suitability index graphs should be regarded as tentative and open to modification. The prospective user should understand that the curves are not the products of extensive laboratory or field investigations. Rather, they reflect the ,author's subjective integration of the literature, personal experience, and reviewer comments. The following discussion documents some of the thought process that went into constructing the curves. Some curves are better documented than others. In many cases, there is information about preferred and limiting or unsuitable conditions, but little information on which to base ratings of intermediate conditions. No particular signif- icance should be attributed to inflection points unless specifically noted in the text. Straight line segments can be substituted for~the sigmoid portions of the curves. The model is offered as a starting point, with the hope that refinements will be made as additional information, including results of testing the model, becomes available. Ratio ofi spawning habitat to summer habitat (V1~. Spawning habitat avail- ability is a logical .starting point in the development of a habitat suitability index model for northern pike. High spring water levels can create spawning habitat if they flood terrestrial and wetland vegetation. The amount of new spawning habitat that results depends on shoreline topography and the amount of adjacent vegetation. The relative availability of spawning habitat can be estimated by the ratio between the amount of surface area in the spring that is shallower than 1 m and vegetated and the entire surface area of the water body in summer. The minimum ratio, below which spawning habitat availability limits population size, depends on the carrying capacity, or productivity, of summer habitat. Other model variables account for factors which limit post-fry stages. Therefore, the minimum ratio is estimated for the theoretical maximum density of northern pike. Snow (1978) presented data suggesting a maximum density of 40 pike greater than 35 cm in length per ha (20 kg/ha). Higher densities could not be maintained even though food .was plentiful. This maximum density value was used to develop the suitability index curve for spawning habitat availability. It is assumed that spawning habitat suitability is positively, but asymptotically, related to the area of spawning habitat per spawning female. This means that an increase in the amount of spawning habitat per female is more beneficial when spawning habitat is relatively scarce than when it is plentiful. Limited evidence in support of this assumption is available. Female northern pike have typically been stocked in managed spawning marshes at densities between 5.4 and 23.2/ha (1 fish per 1,847 m2 and 431 mZ, respec- tively) (McCarraher 1957; Forney 1968; Royer 1971; Fago 1977). Increasing the density of females from 11.4 to 22.4 fish/ha (5.9 to 17.6 kg/ha) only slightly increased the yield of fingerlings from an experimental marsh in Saskatchewan (Royer 1971). Maximum production from managed spawning marshes in Minnesota was obtained at stocking rates of 11.2 kg of femaleslha (Jarvenpa 1962b). 21