What is the characteristic dark marking reliably used to identify a walleye over a sauger?

A distinct, dark blotch located at the posterior edge of the spiny dorsal fin.

One of the most reliable methods for distinguishing a walleye (*Sander vitreus*) from its close relative, the sauger (*Sander canadensis*), centers on the pigmentation of the first dorsal fin. The walleye typically exhibits one or more dark spots, but the defining feature is a singular, prominent dark blotch concentrated specifically at the very rear tip or posterior edge of this spiny dorsal fin. In contrast, the sauger generally displays rows of distinct dark spots distributed across the entire span of its spiny dorsal fin, rather than consolidating that key marking solely at the rear extremity. This anatomical detail is crucial for accurate identification in the field.