Identifying trout species: brook, brown, rainbow, sea, marble and cutthroat

While there are numerous trout species populating the waters around the globe, this article will focus on five of the most popular: the brook trout, brown trout, rainbow trout, sea trout, marble trout, and cutthroat trout. These iconic species are not only a delight for anglers, but they also hold a significant place in the ecosystem and have fascinating traits that set them apart.

How they're different: Taxonomy

Trout, as part of the Salmonidae family, are divided into various genera and species. This table will compare the full taxonomy of our five trout species. As mentioned they're all part of the same Family, meaning that up until Family the taxonomy is the same, and only in Genus and Species you will see differences.

Note that we haven't included sea trout in this table given that it is not a distinct species, but a name used for anadromous (sea-run) forms of brown trout (Salmo trutta).

The brook trout, classified under the Salvelinus genus, is distinct from the others, sharing its lineage with chars rather than true trout. The brown and marble trout belong to the Salmo genus, signifying a closer relation between them. Meanwhile, the cutthroat and rainbow trout are both classified under Oncorhynchus, indicating another closely related pair.

How to identify each of these trout species

Some of these trout species look very familiar. How they look also depends on their life stage, size and the type of water and environment that they live in. Still, with some background information you'll be able to tell these species apart.

Understanding fin types

What greatly helps in identifying different trout types is the different fin types. Just looking at colors and spots can be confusing for two reasons: species can look very similar just based on these traits, plus these traits differ greatly depending on the type of water and age of the fish. In our descriptions for identifying trout below we talk about fin types as one of things that will help identify what trout you've landed.

Identifying Brook Trout

Brook trout, also known as "brookies," are a native species of the eastern United States but are introduced around the world so you can find them in many different countries.

Their unique characteristics make them fairly easy to identify. The most distinctive feature is their coloration. Brook trout have dark green to brown backs with lighter, often mottled sides, transitioning into a pale, yellowish belly. A unique trait of the brook trout is their red spots encircled by a bluish halo, scattered across their sides.

Their dorsal fin, adipose fin, and tail are also adorned with a maze-like, wavy pattern, which often looks like a marbling effect. These patterns are not present in most other species, making it a reliable identification feature. Furthermore, their lower fins, including the pectoral, pelvic, and anal fins, have a distinct white leading edge followed by a black stripe and then a vibrant reddish-orange color.

Identifying Brown Trout

Brown trout, on the other hand, are native to Europe but have been widely introduced elsewhere, including North America. They possess a somewhat more drab coloration compared to brook trout, which aids in their camouflage against predators.

Brown trout have a dark brown back, transitioning to a lighter brown or yellowish color on their sides, and then to a cream-colored belly. Unlike brook trout, brown trout have large, black spots scattered across their back and sides, with some red or orange spots, usually encased in a lighter halo, particularly on their sides.

One major differentiator for brown trout is their spots that extend into the adipose fin, a trait not present in brook trout. Moreover, their lower fins are usually plain and dull, often ranging from pale yellow to a more vivid orange without the distinctive white and black margins found in brook trout.

Key differences: Brook vs. Brown Trout

As fly fishers, there are a few key points you need to remember when identifying whether your catch is a brook trout or a brown trout:

1. Coloration: Brook trout are usually more vibrant, with a dark back, mottled sides, and a lighter belly, whereas brown trout have a more consistent brownish coloration from back to belly.
2. Spots: Brook trout have small red spots encased in bluish halos, while brown trout have larger black spots and some red or orange spots encased in lighter halos.
3. Fins: Brook trout have lower fins with a white leading edge followed by a black stripe and a vibrant reddish-orange color. In contrast, brown trout's lower fins lack these striking color contrasts.
4. Marbling: Brook trout have marbled patterns on their dorsal, adipose, and tail fins, which is absent in brown trout.

Identifying Rainbow Trout

Rainbow trout are one of the most recognizable species among fly fishers due to their distinct and vibrant coloration. They have a broad, reddish-pink stripe running horizontally down each side, from their gills to their tail, against a silvery to dark green or bluish background. This "rainbow" coloration gives them their name.

Unlike brook and brown trout, rainbow trout have small black spots scattered across their entire body, including the tail, and these spots can extend onto their dorsal and adipose fins. Their belly is usually whitish, and the lower fins can be slightly orange with a narrow white margin.

Identifying Sea Trout

Sea trout is a common name, typically referring to anadromous (or sea-run) brown trout. They are born in freshwater but migrate to the ocean as they grow, returning to freshwater only to spawn.

The coloration of sea trout can vary greatly, mostly due to their unique life cycle involving both freshwater and saltwater habitats. Generally, they have a silvery, almost metallic appearance that helps them camouflage in the ocean environment. This is a drastic shift from the brown and yellowish hues seen in their freshwater, or brown trout, counterparts.

Their backs are typically dark grey, and they have a vibrant silver flank that fades into a white or cream-colored belly. The spotting pattern seen in brown trout can be faint or even absent in sea trout, due to the color shift during their transition to saltwater. However, it's common for the body to be covered in spots. The spots are not present on the fins.

Their fins do not have any remarkable coloration—typically, they match with the body color, being dark on top and lighter underneath. The sea trout's overall streamlined, torpedo-like body shape is another distinguishing feature that aids them in their long, oceanic journeys.

Identifying Marble Trout

Marble trout, as the name suggests, are recognized by their distinctive, marble-like color pattern. They are native to rivers of the Adriatic Sea basin in Italy and Slovenia. As they mature, they develop a unique marbled appearance, with large, dark round or oblong spots on a lighter background that can range from silver-gray to brownish-yellow. This pattern, which extends across their entire body, is uneven and can differ drastically between individuals, but it’s generally more evident on their back and sides.

Marble trout lack the red or orange spots found in some other trout species. Additionally, their adipose fin is typically without any spots, and the lower fins have no distinct coloration—usually matching the belly color.

Identifying Cutthroat Trout

Cutthroat trout are native to the western United States and are named for the vivid red or orange slash-like marks on the underside of their lower jaw. Their coloration varies widely depending on their habitat and subspecies but is generally a greenish to brassy yellow, fading to a yellowish or silvery belly.

Cutthroat trout bear round to oval-shaped spots that are more concentrated towards the tail and along their back. Notably, the spots are generally absent on the front half of their body. Their lower fins can be somewhat yellowish to orange without any particular markings. Remember, the definitive identification feature is the distinctive "cutthroat" mark under the jaw.