Lake Henry, along with Lakes Agnes and Winona, comprise a chain of lakes within the city limits of Alexandria. Lakes Agnes and Henry are broadly connected basins with a common "narrows" on the north side of Lake Agnes and south side of Lake Henry. A 200-foot long sand and rock bar separates the two basins. This bar is one to two feet under the water surface during normal water levels. These lakes received partially treated sewage effluent from the City of Alexandria dating back to the 1920's. A new tertiary sewage treatment plant (ALASD) began operating on the south shore of Lake Winona in 1977. Total phosphorus discharges were reduced by approximately 90 percent. Water quality in Lakes Henry and Agnes improved greatly with less nutrient loading, but the basins remain nutrient rich.
A partial fish kill event was reported on March, 20, 2011. Numerous dead fish were observed at the inlet to Lake Agnes and outlet of Lake Henry. An ice-out electrofishing survey in May, 2011 yielded very few fish. Adult broodstock largemouth bass, bluegill and black crappie were stocked in the spring of 2011 to accelerate restoration of the fishery.
A follow up fish community assessment was completed in June, 2011. No largemouth bass were sampled during the electrofishing survey. However, mid-summer shoreline seining efforts yielded numerous juvenile largemouth bass. The 12.0-inch maximum with one over 20.0 inches harvest regulation established in 2006 remains in effect and will provide additional protection to this recovering population.
Black crappie, bluegill, and yellow perch abundance declined since the 2007 survey. Black crappie remained fairly abundant, but trapnet catches did decline substantially since the 2007 survey in probable response to the fish kill. Mean length of sampled fish was 7.8 inches. Bluegill trapnet catches averaged 1.8 fish/net in 2011 as compared to 35.8 fish/trapnet in 2007. Average size of bluegill was 5.8 inches in length. Yellow perch gillnet catches averaged 20.3 fish/net in 2011, compared to 141.0 fish/trapnet in 2007. Most perch captures were too small to interest anglers.
Northern pike numbers remain stable. Gillnet catches averaged 4.7 fish/net in 2011. Average weight of sampled northern pike was 3.8 pounds. Fishing quality, expressed in terms of average size captured, was relatively good. Twenty eight percent of the sample exceeded 28.0 inches in total length. The largest northern pike captured in the 2011 survey was 34.4 inches. Some portion of the adult pike population was likely comprised of adult pike that migrated upstream from Lake Le Homme Dieu.
Common carp were captured from in Lake Agnes in 2007 after many years of absence. Abundance has increased with reproduction and additional migration from downstream lakes. Average length of sampled fish was 17.2 inches.