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• change to low dissolved oxygen is gradual,. northern pike appear able to toler- ate concentrations as low as 0.1 to 0.4 mg/1 for at least several days (Cooper and Washburn 1949; Magnuson and Karlen 1970; Petrosky and Magnuson 1973). A prolonged period with dissolved oxygen concentratians less than 1.0 mg/1, however, appears to-cause partial or complete winterkill (Johnson and Moyle 1969; Stewart 1978). The critical oxygen concentration also depends on tem- perature. Requirements for dissolved oxygen increase exponentially as tem- perature increases, but the lower incipient lethal concentration [concentration below which pike cannot survive for an indefinite time (Fry 1947)] is still relatively low (< 1.5 mg/1) at 28° C (Casselman 1978). Tolerance of ]ow dissolved oxygen conditions appears to be inversely related to size for juvenile and adult northern pike. Overwinter mortality in an Ontario lake with low oxygen levels was total for pike older than 2 years and/or longer than 39 cm (fork length), but many smaller, younger individuals survived (Casselman and Harvey 1975). Similarly, very few northern pike older than 2 years were present in a Montana stream in which winter dissolved oxygen concentration dropped to low levels, although young fish were common (Stewart 1981). Fish kills due to low dissolved oxygen concentrations can also occur during summer in shallow, nutrient-rich waters. The cause is often the collapse of a heavy bloom of blue-green algae (Barica 1975). Northern pike were among the many dead fishes observed in a small Wisconsin river when the decomposition of a large algal mass temporarily depleted the dissolved oxygen (Mackenthun et al. 1945).- An extensive review of limnological characteristics of Ontario lakes containing northern pike, walleye, smallmouth bass (Micropterus dolomieui), and lake trout (Salvelinus namaycush), singly and in various combinations, provides a useful overview of northern pike habitat in this region (Johnson et al. 1977). Nearly 70% of the lakes that contain northern pike contain either only northern pike or northern pike and walleye. It appears that lakes with northern pike typically have mean depths of 2 to 6 m, littoral (<_ 6.1 m deep) areas which are 60 to 80% of the total surface area, Secchi disk transpar- encies of 2 to 4 m, total dissolved solids (TDS) levels between 50 and 125 mq/1, and a near neutral or slightly acidic pH, based on average values of these habitat variables for groups of lakes with different species combina- tions. These ranges could reflect availability of habitat conditions as well as habitat requirements of northern pike, particularly .for water quality parameters. In those lakes where both northern pike and lake trout occur, pike are likely to be largely confined to the perimeter or to island/shoal areas. Although northern pike occur in oligotrophic waters, they are more typical of mesotrophic or borderline eutrophic conditions (Casselman 1978; Ryder and Kerr 1978). The mean total phosphorus (TP) concentration in Minnesota lakes containing northern pike in association with walleye and yellow perch was 0.034 ppm, compared to 0.02 ppm for lake trout-cisco (Coregonus artedi) lakes, 0.058 for bass-panfish lakes, and 0.126 for "rough fish" lakes; lakes in which northern pike accounted for a significant fraction of the fish population 7 ~.