Well, "large numbers" might be a little generous, but they certainly are becoming more numerous. A few years ago, expecting the coming trend, I had played with the notion of trying to establish small reproducing populations of muskie through stocking in some grated marshes off the western basin; grant money never panned out. Travis Hartman, a friend of mine with the Division of Wildlife (ODW) and very avid muskie angler, has been informally following recent muskie catches. Travis heard of 19 fish coming from Ohio waters in 2004: 1 was taken in an ODW fall gill net, 4 were taken in commercial trap nets and released, and 14 were taken through recreational angling. Only a few of the 14 recreationally caught fish were documented in formal creel survey interviews; word of many came from word of mouth, photos posted at bait shops, etc. (mighty informal indeed). It doesn't sound like much, but that number is way up from recent years. Fish ranged in size from 24" to 41.25". There was a 45.5" fish taken in 2003! Most of these fish came from W. Sister and the Erie Islands, Sandusky Bay, the Portage River mouth, and some from the Huron and Vermilion Rivers. John Hageman even took an 8"-er off Gibraltar Island in, I believe, 2002.
Before I was with Ohio Sea Grant, I spent a good deal of time following the evolution of the fish assemblage associated with the restoration of Metzger Marsh, a site between Port Clinton and Toledo. In 1999, the first year the Marsh's hydrology had been reopened to Lake Erie, Susan Wells, USFWS collected a single small northern pike in a herp trap. By 2003, I expected to see a pike or two ranging from young of the year to 30" adult fish every time I ran monthly trap nets from spring into fall.
The success of esocid spawn (that of the pike and muskie family) is closely linked to quality, shallow-water vegetation. When Erie's vegetation was devastated by eutrophication and poor water clarity, esocid reproduction in viable numbers was functionally eliminated from all but a tiny few sites on Erie proper. With recent increases of healthy marshlands and associated vegetation, esocids are making a comeback. This return will be gradual because esocids have relatively strong spawning site fidelity; i.e., muskie like to spawn in the place they themselves were spawned. The spread of a reproducing population requires fish that are willing to stray or that have been far enough removed from their original spawning site to make return impractical (e.g., washing into Erie out of Lake St. Clair).
There aren't enough data to draw good conclusions yet, but it doesn't appear to me that harvest could be having much impact on the growing population. Of course, serious muskie fisherman are famous for their complete lack of harvest. Of the 19 fish that Travis knows were caught in 2004, only four were removed from the population and only three through recreational angling. The more likely reason for the length of time it's taking for muskie numbers to increase in Erie is their site fidelity (currently to other places) and the lack of a preexisting reproducing population in Erie. Barring unforeseen circumstances, I'd guess esocid populations in Erie would continue to increase slowly, gradually spreading in from the far ends of the Lake.