The lookalike pairs
These are the confusions that matter, because in most states each pair carries different size limits, seasons or daily bags. Every one is settled by a single check that takes seconds.
Count dorsal spines. Seven or eight means black crappie, five or six means white. White crappie also has vertical bars rather than scattered mottling.
Pumpkinseed has a bright red or orange spot on the trailing edge of the ear flap and wavy blue lines on the cheek.
Brook trout has light markings on a dark background and white-edged lower fins.
Blue cat has a straight-edged anal fin with 30 to 36 rays and no spots. Channel cat's anal fin is rounded with fewer rays.
Flathead has a squared-off tail, a flattened head and a protruding lower jaw.
Jaw length. Largemouth upper jaw extends past the eye; smallmouth stops at or before it. Smallmouth also has vertical bars, not a horizontal stripe.
Spotted bass has rows of small dark spots below the lateral line and a rough tooth patch on the tongue.
Pattern direction. Light spots on dark is pike; dark bars or spots on a light body is muskie. Count jaw pores too: pike five or fewer, muskie six or more.
Brown trout has red or orange spots with pale halos and few or no spots on the tail.
Brook trout has light worm-like markings on a dark back and white leading edges on the lower fins.
Spotted bass has spot rows below the lateral line and a tongue tooth patch; smallmouth has neither.
Sauger has distinct rows of dark spots on the spiny dorsal fin membrane and no white tail tip. Walleye is the reverse.
Perch has bold dark vertical bars and no canine teeth, and rarely exceeds a pound.