Lake Johanna is a shallow prairie lake located in southeastern Pope County. The lake is comprised of two basins having a combined surface area of 1,584 acres. The main basin has a maximum depth of 13 feet. Water quality and clarity can be poor during summer months in response to wind and wave-induced re-suspension of sediments and subsequent algal blooms. Water transparency exceeded 10.0 feet at time of the June 2016 survey. With exception of a dense curly-leaf pondweed stand during early summer months, aquatic vegetation coverage and densities can be considered "moderate". Fish community structure and fishing quality are influenced by occasional winterkill events.
A standard survey was conducted in 2016 to assess the current fish community. A partial winterkill event in the winter of 2013-2014 may have claimed some of the older, larger fish in the population. However, as is the case in many winterkill situations, fish growth remains fast and the fishery should recover quickly. Compared to other gamefish in Lake Johanna, Northern Pike are more tolerant of winterkill conditions and thus proved to be very abundant. Mean length was almost 24 inches. Several pike over 30 inches were also recorded in the catch. Walleye catch rates declined during 2016, but a modest Walleye fishery still remains in response to supplemental fry stockings and some natural recruitment. Largemouth Bass and Bluegill were abundant, but most captures were smaller, younger individuals. Average length of Largemouth Bass was only 7 inches. A few older fish up to 19 inches in total length were observed. Most Bluegills were less than 6 inches long, however, a modest number of memorable-size fish measuring up to 11 inches were caught during the survey. Black Crappie catch rates remained low, but crappie are not always sampled efficiently during summer surveys. Bullhead abundance has remained well below historic levels during the last few surveys. As long as no severe winterkills occur during the next few years, Lake Johanna should provide excellent angling opportunities.
Water clarity in the winter months is typically very high. With an elevated density of Northern Pike in the lake, winter spearing opportunities should be good.
The Lake Johanna Scientific and Natural Area (SNA) is located in the southern basin of Lake Johanna. This SNA is a unique island that is home to one of the largest and most diverse nesting colonies for colonial waterbirds (great blue herons, double-crested cormorants, great egrets, black-crowned night herons, and American white pelicans) in Minnesota. The SNA is designated as a public use unit and is open to the public for nature observation and general educational and research activities. However, it is closed to the public from April 1 through July 15 to avoid disturbance during the potential nesting season.