How to Identify Rainbow Trout

To identify Rainbow Trout (Oncorhynchus mykiss), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Highly variable. Typically olive-green to bluish-black on the back, fading to silvery sides and a white belly. A distinctive pink to red lateral stripe runs along the sides, most prominent in breeding males. Numerous small, dark spots are present on the back, sides, and fins (dorsal, adipose, and caudal). Anadromous Steelhead are typically more silvery with less prominent spotting and a fainter lateral stripe when in saltwater.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Pink to red lateral stripe (often iridescent), Numerous small, dark spots on body and fins, Adipose fin present, Forked caudal fin, White mouth lining (distinguishes from Cutthroat Trout)

Dorsal fin

Single, relatively tall dorsal fin, typically with 10-12 rays, heavily spotted.

Tail fin

Moderately forked caudal fin, heavily spotted.

Pectoral fin

Paired pectoral fins, typically clear or lightly spotted.

Distinctive fins

Adipose fin present (a small, fleshy fin without rays, characteristic of salmonids), usually spotted.

Identification Tips

1. 1

   Lateral Stripe

   Look for the prominent pink to red iridescent stripe along the lateral line, especially noticeable in stream-dwelling and spawning individuals. This stripe can be faint or absent in Steelhead in marine phase.

2. 2

   Spotting Pattern

   Observe the numerous small, dark spots covering the back, sides, dorsal fin, adipose fin, and caudal fin. The spots are generally more evenly distributed and smaller than those on some other trout species.

3. 3

   Mouth Lining

   Carefully check the inside of the mouth. Rainbow Trout have a white or pale mouth lining, which helps distinguish them from Cutthroat Trout (Oncorhynchus clarkii) that typically have red or orange streaks in the lower jaw fold (the 'cutthroat' mark).

4. 4

   Caudal Fin

   The caudal (tail) fin is moderately forked and typically heavily spotted, unlike the more square or slightly emarginate tail of some other trout species.

5. 5

   Adipose Fin

   Presence of a small, fleshy adipose fin between the dorsal and caudal fins is a key salmonid characteristic. In Rainbow Trout, it is often spotted.

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