Reproduction of muskellunge Esox masquinongy has failed in many waters that formerly supported self-sustaining populations. Laboratory experiments were conducted to isolate causes of such failures. Differential mortality occurred among lots of muskellunge eggs incubated in jars of unaceated lake water over substrates of sand, gravel, silt, aquatic macrophytes, wood, tree leaves, polyethylene screen, and bare glass. High and rapid early mortality (days 1#x2013;2), attributable to low dissolved oxygen (DO) concentrations (0#x2013;0.1 mg/liter), occurred among eggs incubated on leaves and macrophytes. After day 3, Saprolegnia sp. fungus was implicated in high egg mortalities in jars with inorganic substrates and moderate DO concentrations (3.8#x2013;4.1 mg/liter). Lowest mortality rates occurred on organic substrates (silt and wood) amidst intermediate DO concentrations (0.4#x2013;1.7 mg/liter) and limited fungal infestation. Among eight midwestern lakes and reservoirs, measured DO at the substrate-water interface in four of them was high (means, 6.0#x2013;8.4 mg/liter) and showed little microstratification; these lakes contain self-sustaining muskellunge populations. The other four lakes showed extreme DO microstratification and virtual anoxia (means, 0.4#x2013;2.4 mg/liter) at the substrate-water interface; muskellunge populations in these lakes are supported almost wholly by stocking. Suitable spawning substrates in these lakes are aerated by annual reservoir drawdown, have inherently low biological oxygen demand, or support dense beds of stonewort Chara sp. Reproductive failure is associated with spawning areas having deep accumulations of organic matter and dense macrophyte growth. Improvements of spawning habitat to prevent or alleviate hypoxia are among the options available to manage this species.
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