Salmon and sea trout
- fishing out the differences

Fishing instructor, fly fishing tuition and fishing trips based in Scotland. Salmon and trout fishing advice, flies and articles.

Distinguishing salmon and sea trout

Distinguishing features of a grilse

Rear of maxillary and eye are in line


Dorsal fin

Adipose fin

Claudal fin


Pectoral fin

Pelvic fin


Anal fin

10 to 13 scales from adipose to lateral line

Salmon and sea trout are migratory species, (Latin names Salmo salar and Salmo trutta respectively) the latter being the same species as brown trout, differentiated because they have migrated to salt water, therefore look different and in the past were thought to be a distinct species, which lead them being classed legally with salmon. Atlantic salmon are truly a migratory fish although some populations have become "land locked" by choice where the fish feed in freshwater lakes that have an abundance of food and spawn in the feeder streams. Fish that spawn in fresh water and feed in the sea are called anadramous fishes. Both salmon and sea trout can grow to large size, certainly in excess of 10 kilos and sometimes much larger, however whilst salmon of that size are not exceptional specimens a sea tout of such dimensions would be regarded as a really great fish.

Distinguishing features of a sea trout

Rear of maxillary bone is behind eye

sea trout

Thick wrist and square claudal fin

sea trout

13 to 16 scales along line from adipose to lateral line

The difficulty for the inexperienced angler is correctly identifying each species when the fish are of comparable size. For illustration here I have used a 3 kg sea trout and a 2.75 kg grilse (1 SW salmon). Large sea trout are often mistaken for salmon, to distinguish between them there is a waterside test that clearly establishes the species. Taking a scale count an oblique line from the rear end of the adipose fin down to the lateral line. For a salmon there will be 10 to 13 scales and for a sea trout 13 to 16 The maxillary bone is the external bone that extends backwards on the top side of the fishes mouth. Note the relative positions of maxillary bone to the eye, a sea trout has a larger and longer mouth than a salmon. The rear edge of a trout's mouth extends a little beyond an imaginary line drawn down from the eye, whereas with a salmon the eye and mouth edge are almost in line. (See the illustrations). The tail of a large sea trout may also be convex when opened out and in any case is nearly square. Salmon, and especially small grilse have distinctly forked tails.

I hope that this page will help fly fishermen and anglers to distinguish between the Atlantic salmon (Salmo salar) and the migratory trout or sea trout (Salmo trutta) at the waterside.

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Copyright 2007 Alastair Gowans AAPGAI and FFF Master and THCI, APGAI. All rights reserved.