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4. Adult. From sexual maturity until death. Spawning/embryo. Optimal spawning substrate for northern pike is a dense mat of short vegetation (Fabricius and Gustafson 1958) in a shallow, wind- sheltered area. The type of vegetation does- not appear to be critical (Fabricius 1950; Forney 1968), although grasses or sedges seem to be preferred (Jarvenpa 1962b; Franklin and Smith 1963; Robertson 1969; June 1971; McCarraher and Thomas 1972). In Nebraska, the greatest densities of eggs were found in flooded prairie grasses at depths of 0.2 to 0.45 m (McCarraher and Thomas 1972). Mowed hay and flooded hay bales were used when flooded natural grasses were not available. Similarly, Forney (1968) found that pike would spawn over prepared plots of winter wheat (approximately 15 cm tall), as well as over natural grasses (Spartina spp.), sedges (Cyperaceae), and water plantain (Alisma sp.). Spiicerushes (Eleacharis spp.) and canary grasses (Phalaris spp.) are also commonly used (Franklin and Smith 1963; Forney 1968; Mississippi River Work Unit 1978). Scattered vegetative debris may be used when more preferred substrates are not available (Priegel and Krohn 1975). Grossman (pers. comm.) reported that northern pike in Lake Simcoe (Ontario) often spawn over "finely shredded remains of dead cattails loosely scattered on black muck", and over deciduous leaves from the previous autumn. Most spawning occurs in water shallower than 0.5 m (Williamson 1942; Clark 1950; Fabricius 1950). Northern pike have been observed spawning with their backs out of water (Clark 1950). The vegetation mat should provide abundant surface area for eggs to adhere to and yet allow the circulation of water. Eggs which fall to the bottom are unlikely to hatch due to anoxic conditions in the organic-rich sediments typical of pike spawning grounds. Northern pike embryos are sensi- tive to high siltation rates (? 1 mm/day), such as occur in waters with exten- sive wave action and bank slumping (Hassler 1970). The rate of embryo devel- opment, the percent normal hatch, and larval vigor are dependent on oxygen availability during incubation (Gulidov 1969; Siefert et al. 1973). A dissolved oxygen concentration of 4.5 mg/1 is adequate for embryo development and survival at temperatures up to 19° C (50% saturation) and a low flow rate (30 ml/min); concentrations of 3.2 mg/1 (33% saturation) or less appear to be unsuitable for embryos and larvae (Siefert et al. 1973). Gulidov (1969) reported a much higher percent normal hatch for northern pike embryos incubated at dissolved oxygen concentrations greater than 35 to 40% saturation (approx- imately 4 mg/1 at a temperature of 12 to 12.5° C) than for those incubated in lower oxygen tensions. Hydrogen sulfide may accumulate when the dissolved oxygen concentration is low. Exposure to hydrogen sulfide levels greater than 0.014 to 0.018 ppm for 96 hours decreases the viable hatch of northern pike eggs (Adelman and Smith 1970a). Incubation time is inversely related to temperature (Swift 1965; Lillelund 1966; Walker 1968). Average length of incubation is approximately 26 days at 6° C, 17 days at 8° C, 12 days at 10° C, 9 days at 12° C, 6 days at 14° C, and 5 days at 16 to 20° C (Swift 1965; Walker 1968.). 9