Brook, Brown or Rainbow? Trout Identification That Works

One question splits the three, and it is the same question that works on the char family generally.

Are the markings lighter or darker than the body?

• Light markings on a dark body — brook trout. Which is not a trout at all, but a char.
• Dark markings on a light body — brown or rainbow.

Everything after that is a two-way decision.

Brook trout

• Dark olive-green back covered in pale vermiculation — wandering wormlike markings, like a contour map. No other stream fish in the east has this.
• Cream and yellow spots on the flanks, with a scattering of red spots surrounded by blue halos.
• Lower fins — pectoral, pelvic, anal — with a crisp white leading edge backed by black. This is the character to use at a glance, and it is visible even on a fish in the water.
• Tail squared or very slightly forked, which is where "squaretail" comes from.

Brook trout are the native eastern char, and they are the ones most sensitive to warming and siltation. A stream that still holds wild brookies is telling you something about the stream.

Brown trout

• Butter-yellow to bronze body.
• Black spots on the back and upper flanks, and typically red or orange spots with pale halos along the side.
• Tail unspotted or nearly so — this is the character that separates brown from rainbow when colour has faded.
• Adipose fin often orange-edged.
• Large adults develop a hooked lower jaw.

Sea-run and lake-run brown trout go bright silver and lose most of the spotting, at which point the clean tail is what you have left.

Rainbow trout

• Heavy black spotting over the whole tail, in radiating rows. This is the single most reliable rainbow character.
• Spots on the dorsal fin as well.
• A pink to red lateral stripe, strongest in spawning fish and in stream residents; steelhead and lake fish are chrome and may show almost none.
• No red spots with halos.
• White mouth and gums.

The comparison

The two that get missed

Cutthroat — the red or orange slash under each side of the lower jaw. Western, and it hybridises freely with rainbow, producing "cutbows" with a faint slash and rainbow spotting.

Lake trout — another char, so light spots on a dark body like a brook trout, but with a deeply forked tail and no vermiculation and no red spots. Big, deep, cold.

Atlantic salmon parr are frequently taken for brown trout in the northeast. The parr marks, the more deeply forked tail and the longer maxillary separate them, and where Atlantic salmon are being restored the distinction is legally serious.

Why it matters more here than elsewhere

Trout regulations are often species-specific, water-specific and season-specific in the same sentence. Wild brook trout waters frequently carry different limits from stocked rainbow waters on the same river system, and native cutthroat populations are protected in ways that hatchery rainbows in the same drainage are not.

The white-edged fin on a brookie is worth being able to see from the bank before you decide whether to bring the fish out of the water at all.

Handling

Trout are less robust than warmwater fish and the marginal minute matters. Wet hands, no gills, no squeezing, and get it back before the photograph if the water is warm — mortality after release rises sharply above about 20 °C.

For the general habit of using countable characters rather than colour, see counting fin rays and spines.