Despite the tendency to winterkill periodically, Lake Fifteen's fishery is relatively diverse and productive. The most recent partial winterkill in 1980 resulted in rapid expansions in bullhead numbers (bullheads are tolerant of low dissolved oxygen levels). Conversely, gamefish populations were decimated and required reintroduction into the lake. Since then, Northern Pike, Walleye, Bluegill, Black Crappie, Yellow Perch, and Largemouth Bass populations have recovered while bullhead populations have decreased to more normal levels.
Populations of most fish species are self-sustaining under normal conditions in this lake. Spawning habitat is available for all present species except Walleye, which are stocked annually as fry. In 2014, test net catches of Northern Pike were above the normal range for this type of lake. These pike averaged 22.9 inches and 2.7 pounds. Walleyes were not abundant but were relatively large with a mean length of 21.4 inches. Recent natural reproduction of Bluegills continues to be successful as indicated by trap net catches over 40 fish per net. The majority of these Bluegills were rather small and slow growing with a mean length of 5.7 inches. Very few Bluegills over eight inches were found. Modest populations of Black Crappie and Largemouth Bass were also found in 2014. Crappies were mostly small and young with two fairly strong year classes on the verge of becoming large enough for anglers to keep.
The Cormorant Lakes Sportsmen's Club has seasonally operated a small aeration system off the north shore since 1988. Low dissolved oxygen events such as what was experienced in 1980 have not occurred, so the effectiveness of the aeration system has not been rigorously tested. A very light winterkill occurred during 2004 with the aerator operating. Also, dissolved oxygen dropped to critical levels in 2006 and 2008, but no serious winterkill is believed to have occurred. It is not known whether the aerator was the primary reason further mortality of gamefish was not observed during these years.
Good land stewardship is important throughout the watershed to maintain water quality. It is particularly critical near Lake Fifteen which is already a borderline winterkill lake. Lakeshore and watershed dwellers can help protect water quality, fish populations, and property values by (1) leaving or establishing buffer strips of natural vegetation along shorelines, (2) leaving aquatic vegetation stands intact, especially bulrush and cattail, (3) not fertilizing or using phosphorus free fertilizers, (4) ensuring compliance of septic systems, (5) participating in the lake association and in the Minnesota Pollution Control Agency's Citizen Lake-Monitoring Program, and (6) utilizing practices illustrated in the DNR document "Lakescaping for Wildlife and Water Quality" when landscaping shoreline property.