Union Lake continues to support a diverse fishery with several popular fish species including walleye, northern pike, black crappie, largemouth bass, and bluegill. Most of these species naturally reproduce in Union Lake, despite water level fluctuations, shoreline development, aquatic vegetation removal, and fishing pressure. The lake's walleye population has generally required stocking to sustain the fishery, although some natural reproduction has occurred recently.

At 16.1 fish per gill net, walleye catch rates in 2012 were the highest ever recorded in this lake and two of the three most abundant year classes were attributed to natural reproduction. It is possible that fry stocking has increased the walleye population to the point where natural reproduction can be more successful than in the past. It is also possible that water levels were favorable for providing good walleye spawning habitat during the two successful years. Walleyes sampled in 2012 averaged 15.2 inches in length and 1.4 pounds in weight.

Catch rates for northern pike, largemouth bass, and black crappie were near their historical averages for Union Lake. Northern pike averaged 22.4 inches (about 2.4 pounds) and pike over 30 inches in length were caught in test nets. Largemouth bass averaged 11.3 inches, but there were good ranges of size and age classes found. Black crappies were relatively small on average (8.3 inches), but fish over 10 inches in length were not uncommon.

Catch rates for bluegill and yellow perch were lower than historical catches in Union Lake but similar to those of the two prior evaluations since high water levels began in the late 1990's. As a primary forage fish for the predator fish in Union Lake, yellow perch are an important part of the fish community. However, in 2012 northern pike and walleye growth rates remained strong compared to statewide and historical lake indices. At an average length of 5.4 inches, bluegills were not large, but several over eight inches were sampled.

Eurasian watermilfoil was found in the lake in 2007. Initial sampling during that year revealed no signs of Eurasian watermilfoil outside of Union Lake after a series of river inspections and spot checks of the watershed that meanders southwest of Union Lake to the Red River. The primary concern is to contain the plants so they don't spread to other lakes, or downstream into the Sand Hill River to the Red River and into Manitoba, where Eurasian watermilfoil has never been found. More monitoring will occur, with effort focused on the drainage pump located on the southwest corner of the lake, where lake water passes into the local watershed. Because of drier conditions recently, the pump has not been operating. Boaters entering and leaving the lake are required to drain all water and check boats, trailers and equipment for vegetation and invasive species.

Lakeshore and watershed property owners can help to slow the aging of the lake (eutrophication) by using the best management practices available. These practices include (1) exceeding setbacks for structures and septic systems, (2) leaving buffer strips of natural vegetation along the shoreline, (3) leaving vegetation stands (especially bulrush and cattail) intact, (4) and minimizing fertilizer and other chemical use in the watershed.