I've caught more Walleye and Sauger this year than I did last year. Here is a good article on these fish I found somewhere else.
The Sauger (Sander canadensis) is a fish which closely resembles its more popular cousin the walleye. The sauger, however, are usually smaller and will tolerate murky waters unlike the walleye. Other distinguishing characteristics are spots on its dorsal fin, and its brassy color. Although the average sauger is only a couple of pounds or so, the world record weighed 17 pounds, 12 ounces. Sauger generally inhabit rivers more so than lakes and reservoirs. Being part of the fresh water Percidae family, it is highly prized for its tasty white meat.
Hybridization between sauger and walleye is not uncommon; the hybrids are referred to as saugeye, and exhibit traits of both species. Being intermediate in appearance between the two species, they are sometimes difficult to differentiate from either species, but they are best distinguished by the dark blotches characteristic of the sauger.
Physical description There are three main physical differences between the Walleye and the Sauger. First, the sauger has no white area on the tail, and walleye does. Second, saugers have specks or dots in the first dorsal fin. Third, saugers are generally smaller in size than the walleye.See photos below
Walleye image: Note the white on the lower lobe of the tail. This is clearly a walleye.
Sauger image: Note the lack of white on the tail of the lower lobe, and the specks on the first dorsal fin. This is clearly a sauger.
Saugeye image: The fish in this image is a Saugeye, a walleye-sauger hybrid. It can be somewhat tricky to properly identify a saugeye versus a sauger (like the sauger, they have no white tail lobe). Saugeyes most commonly have fewer dorsal specks that true saugers.
Habitat The sauger is very adapted to rivers and other turbid environments, whereas the walleye prefers cooler, clearer water.
Reproduction Spawn between March and June in pairs or small aggregations.
Interaction with humans Anglers commonly make no differentiation between the walleye and sauger, both intentionally and unintentionally. The flesh and taste of the sauger is identical to walleye, to most individuals. Especially with smaller specimens (i.e. those under 15"), most fisherman and even government Natural Resource agencies treat walleye and sauger identically.