An influx of cattle waste from nearby rangelands adds to the lake's natural eutrophic state.

[2] The Bureau of Land Management operates the Mann Lake Recreation Site, about 100 miles (160 km) southeast of Burns by highway and unpaved road.

Recreation at or near the site includes fishing, camping, hiking, wildlife viewing, and amateur geology.

[5] A large population of goldfish, introduced illegally, have at times multiplied in Mann Lake and competed for food with the smaller trout.

[5] In 2010, the Oregon Department of Fish and Wildlife treated the lake with rotenone, a piscicide, killing around 200,000 goldfish and fathead minnows while sparing nearly all the trout.