SFCC Introduction to salmon scale reading; Terminology and Common Scale Patterns
Introduction to Salmon Scale Reading
• Common scale patterns
• “Problem” scales
• Spawning marks
Salmon Scale Reading
• Terminology and the parts of a scale
This is the worn,
posterior, protruding part
of the scale
Visible on the outside of
the fish under the
Epidermis, which covers
This is anterior,
embedded part, the
“root” of a scale.
This is the part that
These “rings” are the
growth circuli, that
are laid down as the
scale gets bigger as
the fish grows.
At the centre of the scale is the “Focus”, the original, small,
scale formed towards the end of the fish’s first Summer
Scales can come off fish, and in the resulting gap,
“Replacement” scales are grown. These do not show the
growth pattern of the fish before they themselves formed
and so cannot give the complete history of a fish.
On adult salmon scales there are two phases of growth shown – the River
stage when the fish was a Fry and Parr in fresh water and the Marine stage
when it was living in the sea. The transition between the two should be very
obvious – the sea has much better feeding
Readings “inside” the transition give data on the freshwater phase
and show how old the fish was when it smolted.
River Age Sea Age
Winter Bands form when the growth circuli are closer together
because the fish & its scales are growing more slowly in colder
Note how narrow the first Summer band is – just a couple of
circuli between the Focus and the first Winter band.
This fish therefore smolted after two Winters in fresh water.
The Marine phase is read in the same way – counting the
tight bands of circuli that show Winter (closed) growth.
First summer at sea.
Widely spaced circuli.
Winter band of
Second summer at sea.
Marine Stage / SEA AGE
Transition from river to sea.
This fish therefore had two Summers and one Winter in the
sea – so it is a Grilse :
( 1 Sea-winter + Summer growth afterwards )
A “Winter” indicates a full year’s growth as it marks the end
of full year, “+” means part of a year’s growth after the last
And as it had two Winters in the river, it is a :-
The .between the two numbers is the
transition between the river stage and the
The most common range of patterns
likely to be read from Tweed fish.
( The .shows that the number is
before the transition to the sea)
2 River Winters,
with a straight and
from river to sea
One River Winter
and a gradual
transition to marine
growth, “Run Out”
2 + .
A “Summer Grilse” – with 1Sea Winter, and Open, plus,
growth right to the the edge
growth at the edge
Growth Checks: the main problem in scale reading
This is not a Winter band, it
is a growth “Check”. It is
weaker than a Winter band –
it is narrower and is not
An “Autumn Grilse” – with 1 Sea Winter, open, plus
growth and then Closing growth at the edge
The same pattern, but as this fish was caught in
Spring, the two Winter bands of closed growth
represent two full years = a Spring Salmon, .2
This is why the date a fish is caught is important
Three closed bands, but without a date could be an
Autumn Salmon ( . 2+cl ) or a Spring Salmon ( .3)