THE LIFE CYCLE OF SALMON & SEA TROUT
Salmon (I) can be distinguished from large sea trout (II) by a more streamlined shape, concave tail, slimmer tail wrist, upper jaw reaching no further than rear of the eye, few if any black spots below lateral line, 10-15 (usually 11-13) scales counted obliquely forward from adipose fin to lateral line – trout have 13-16.
The difference between a Salmon & a large Sea Trout - Courtesy the Atlantic Salmon Trust.
The differences between salmon parr, salmon smolts and young trout.
Salmon parr (I) can normally be distinguished from young brown/sea trout (II) by the more streamlined shape, deeply forked tail, longer pectoral fin, lack of orange on adipose fin, smaller mouth, sharper snout, only 1-4 spots on gill cover (often one large spot), well defined parr marks.
SALMON SMOLTS.
When the salmon parr begin to migrate to the sea, usually in March, April and May, they gradually become more elongated and the fins darken. A layer of guanine crystals is laid down in the skin. rendering the body more silvery in colour and obscuring the spots and finger-marks, except on the gill-covers. They then become Smolts.