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Summer habitat for northern pike is limited in some lakes by a combination of high surface temperatures and low oxygen concentrations in cooler, deeper strata. Krohn (1969) reported a mid-August die-off in a Wisconsin lake follow- ing several very warm days. Temperatures ranged from 32° C at the surface to 24° C at a depth of 3.7 m, the lower limit of oxygenated water. High surface temperatures (32° C) coincided with the beginning of a northern pike die-off in a Missouri reservoir (Goddard and Redmond 1978). Dead fish were found to have bacterial, fungal, and protozoan infections. Nigh surface temperatures can contribute to high incidences of infection by lowering resistance to disease and by concentrating the pike in a narrow depth stratum. Substantial mortality of northern pike occurred in an Iowa marsh when temperatures reached 35.6° C at the surface and 31.7° C at the bottom (Ridenhour 1957). In this case, oxygen concentrations were adequate, but temperatures were too high at all depths. It seems likely that high summer and/or winter temperatures limit the southern distribution of northern pike. Although present in the Mississippi River drainage, pike are absent from waters south of the 32.2° C isotherm for normal July daily maximal air temperature, except in thermally stratified reservoirs where they have been introduced (Visher 1954; Grossman 1978). Northern pike are absent from all but the upper reaches of the Ohio River and its tributaries, although muskellunge (Esox masquinongy), a closely related species with similar spawning habitat requirements, are widely distributed in the drainage (Forbes and Richardson 1920; Gerking 1945; Trautman 1957; Grossman 1978). Much of the Ohio River is south of the 31° C isotherm for mean maximal July air temperature.(National Oceanographic and Atmospheric Administration 1974). Surface water temperatures of 29 to 31° C are not unusual for Ohio lakes (Tressler et al. 1940). In the. laboratory, young northern pike do not grow at temperatures above 28° C, even when fed ad libitum (Casselman 1978). Temperatures greater than 32° C can cause death within Z days (Scott 1964). High temperatures at other times of the year may limit reproductive success. In a laboratory study, the percentage of yellow perch that spawned was highest when they were exposed to temperatures <_ 6° C for a minimum of 185 days, beginning October 30 (Jones et al., unpublished data cited in Hokanson 1977). No successful spawning occurred among perch held at a minimum tempera- ture of 12° C or higher. Northern pike, which occur farther north than do yellow perch (Scott and Grossman 1973), are presumably also adapted to a seasonal temperature cycle involving cold winters. Dissolved oxygen concentration is usually the most important habitat variable affecting overwinter survival of northern pike. Shallow, heavily vegetated lakes and rivers with low discharges - habitats that might be quite favorable for northern pike at other times of the year (Johnson and Moyle 1969) -frequently develop l.ow dissolved oxygen concentrations during winter. Laboratory and field observations indicate that pike are more tolerant of low dissolved oxygen conditions during winter than are many other temperate species, including black crappie (Pomoxis nigromaculatus), largemouth bass (Micropterus salmoides), bluegili (Lepomis macrochirus), walleye (Stizostedion vitreum vitreum), and yellow perch (Cooper and Washburn 1949; Moyle and Clothier 1959; Patriarche and Merna 1970; Petrosky and Magnuson 1973). If the 6