Rainbow trout are the mainstay of Union Valley Reservoir, with regular stockings during the cooler months. The lake also harbors brown and brook trout. The real behemoths, however, are the Mackinaw, which have been known to reach up to 20 pounds.

Kokanee salmon fingerlings are also stocked in Union Valley Reservoir by the CDFW, and grow quickly to catchable size. They tend to have a cyclical presence, with some years being plentiful and easily caught by trolling anglers, and other years being scarce and difficult to locate.

Smallmouth bass are also available in solid numbers, though the bigger fish are quite line-shy due to water clarity. It's recommended anglers use as small a pound-test line as possible to prevent spooking in these crystal clear waters.