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You’ve caught an unfamiliar fish. What is it? You can look at a picture or drawing, ask someone who you think is knowledgeable, or you can consult a fish identification guide or field guide. Taxonomy is what science calls classifying living things according to their similar and dissimilar physical characteristics, and putting them in like-appearing groups.

Using a key, and without any previous knowledge or even a good guess, you can look at a fish, note physical items like the shape of its tail, the pattern of its scales, and whether it has a lateral line. You progress from point to point, clue to clue, as a detective, to “key out” what fish you wish to identify. A fish can be scientifically keyed out by counting fin rays and scales in certain positions, looking at the arrangement of pharyngeal teeth, and so on. For most people, the use of field marks, easily visible external features, is enough. Some fish of the same family may look nearly alike. Their color patterns fade as they grow older or when they are found in certain water conditions. Thus, field marks, like the number of pores under the jaw, can help you tell the difference between a muskellunge and a northern pike.

Beginning with 1a and 1b below, match the information to the characteristics of the fish you wish to identify. When you have placed the fish in its proper family, click on the link to the family for help in determining the specific species. As you progress through the key, refer to the illustrations in Chapter 1 and the glossary.

1a. Head with jaws; no pelvic fins; body long and “snakelike”: Eel Family - Anguillidae.

1b.Pelvic fins present; body not long nor “snakelike”: Go to 2.

2a. Upper portion of caudal (tail) fin base longer than lower portion: Go to 3.

2b. Upper and lower portions of caudal fin base about equal in length: Go to 5.

3a. Vertebrae extend into elongated upper portion of caudal fin; snout with four barbels on underside; body with separate rows of bony plates: Sturgeons family - Acipenseridae.

3b. Vertebrae not noticeably extended into upper portion of caudal fin; tail somewhat rounded: Go to 4.

4a. Jaws long and thin with many teeth; scales thick and bony; dorsal fin short: Gars family - Lepisosteidae.

4b. Jaws short and thick; dorsal fin very long: Bowfins family - Amiidae.

5a. Adipose fin present: Go to 6.

5b. Adipose fin not present: Go to 7.

6a. Body scaleless with chin barbels present: Catfishes family - Ictaluridae.

6b. Body with small scales: no chin barbels; body distinctly spotted; thin pointed flap of skin (axillary process) at base of pelvic fins: Trouts and Salmons family - Salmonidae.

6c. Body with large scales; no axillary process at base of pelvic fins: Smelts family - Osmeridae.

7a. Stout duck-bill snout and jaws with obvious sharp teeth; single dorsal fin with no spines: Pikes family - Esocidae.

7b. Not with duck-bill snout; dorsal fin single or double with both spines and soft rays or soft rays only: Go to 8.

8a. Single soft-rayed dorsal fin completely forward of anal fin; mid-belly scales sharp and “saw-toothed”; silvery body strongly flattened from side to side; no lateral line: Herrings family - Clupeidae.

8b. Single or double dorsal fin; lateral line present; “saw-toothed” mid-belly scales absent; body not strongly flattened from side to side: Go to 9.

9a. Single soft-rayed dorsal fin (include carp and goldfish which have first ray hardened); one or more large pharyngeal (throat) teeth on separated fifth gill arches: Go to 10.

9b. Dorsal fin single or double with both spines and soft rays; pharyngeal teeth present or absent (freshwater drum has teeth on fused pharyngeals): Go to 11.

10a. Mouth directed downward with thick fleshy lips; distance from back edge of gill flap to beginning of anal fin much greater than distance from beginning of anal fin to base of tail fin; pharyngeal teeth more than 15 and in single row: Suckers family - Catostomidae.

10b. Mouth not with thick fleshy lips; distance between back edge of gill flap to beginning of anal fin only slightly greater than distance from beginning of anal fin to base of tail fins; pharyngeal teeth no more than 6 in one to three rows: Minnows family - Cyprinidae.

11a. Body largely scaleless (small prickles present); large pectoral fins; head broad and flattened: Sculpins family - Cottidae.

11b. Body with scales: Go to 12.

12a. Anal fin with 3 or more spines: Go to 13.

12b. Anal fin with 1 or 2 spines: Go to 14.

13a. Gill cover with spine: Temperate Basses family - Moronidae.

13b. Gill cover without spine: Sunfishes family - Centrarchidae.

14a. Lateral line ending at base of caudal fin: Perches family - Percidae.

14b. Lateral line continues to end of caudal fin; soft-rayed portion of dorsal fin longer than spiny portion; large flat teeth present on heavy, fused lower pharyngeals; Drums family - Sciaenidae.