Here is some data on Lake Zumbro from the DNR website:
Status of the Fishery (as of 07/15/2002)
Lake Zumbro is a long, narrow impoundment about 600 acres in size created by a hydro-electric dam on the Zumbro River. It is characterized by a rocky shoreline that drops off quickly into deep water. Because the lake is technically a river, it provides unique deepwater habitat for native fish such as channel catfish and smallmouth bass. Other, more traditional “lake” species are also present and include largemouth bass, bluegill, black crappie and yellow perch. In 1994, muskie fingerlings were stocked into Lake Zumbro to provide anglers a new fishing opportunity. Since then, over 2000 muskie fingerlings have been stocked. The lake was chosen to receive muskie introductions because of its relatively good size, low abundance of northern pike (a direct competitor) and abundance of forage fish such as sucker and redhorse. Adult muskie are present in Lake Zumbro with some perhaps >40 inches.
A 2002 population assessment generally found gamefish and non-gamefish abundance to be stable. Large and smallmouth bass are present in good numbers across all size ranges. Bluegill and black crappie averaged 6.7 and 8.5 inches in length, and are present in good numbers across several size ranges. Black crappie abundance is above average in Lake Zumbro, providing good fishing during various times throughout the year. Spring trapnetting to assess muskie stocking success was completed in 2001 and 2002. Special, large nets were used to target adult muskie near shallow shoreline areas. A total of 4 muskies have been sampled, ranging in size from 11 to 36 inches.
Probably the two most significant findings from the 2002 assessment is the apparent trends we’re seeing with regard to northern pike and channel catfish. Northern pike abundance has increased from 0/net prior to 1996 to 2.8/net in 2002. Large northern pike have been collected during spring muskie netting, and it is apparent that successful spawning is occuring somewhere within Lake Zumbro or in one or more of its tributaries. Because spawning habitat and other water quality characteristics are less than ideal, it is unlikely that northern pike will ever reach densities high enough to negatively impact the muskie fishery.
Perhaps of more concern is the downward trend in channel catfish abundance. From 1977 through 2002, channel catfish have gone from a high of 34.4/net in 1985 to 6.2/net in 2002. Several reasons for this decline are possible, including changes in spawning habitat. Property owners around Lake Zumbro often like to clear logs and other woody debris from around their shorelines. Since channel catfish are cavity nesters, this practice can be destructive to spawning success. Spawning takes place at water temperatures ranging from 70-75 degrees F. Rapid changes in water levels as a result of the Rochester Flood Control Project can trigger abrupt changes in water temperature, negatively impacting spawning success. Anecdotal information from anglers suggest that fishing pressure is light, and harvest is probably minimal, though we do not have creel survey data to verify this. It is also possible that the population will rebound naturally and that it is simply at the downward end of a cycle. We will continue to monitor the situation and continue to manage Lake Zumbro primarily for channel catfish and muskellunge, with secondary management priority given to bluegill, black crappie, smallmouth bass and largemouth bass.
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Just a comment on the 2 years not included in the data: I believe the Lake Zumbro fishery has improved quite a bit in this time. The crappies and bluegills are very nice size and plentiful. There is some debate about the numbers of toothy critters, but I have seen them consistently this year. The other species are also available in abundance! In my opinion, Lake Zumbro is a quality fishery and well worth getting to know. 
Good Fishing,
Mike