Introduction to Salmon Scale Reading
• Common scale / growth patterns
• “Problem” scales
• Spawning marks
• One scale to the tune of another
Salmon Scale Reading
• Terminology and knowing your way
around a scale.
This is the worn, posterior,
protruding part of the scale
Visible on the outside of the
fish under the Epidermis,
which covers it.
This is anterior, embedded part, the
“root” of a scale.
This is the part that is read.
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 phase, when the fish was a Fry and Parr
in fresh water and the Marine phase when it was living in
the sea. The transition between the two should be very
obvious – the sea has much better feeding
fresh water to the
sea as a Smolt
Readings “inside” the transition give data on the freshwater
phase and show how old the fish was when it smolted.
How many freshwater Winters ?
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
Winter band of
Second summer at sea.
Reading a marine phase:
Where’s a Sea Winter ?
Transition from river to sea.
This fish therefore had two Summers and one Winter in the
sea – so it is a Grilse, x.1+
( 1 Sea-winter + summer growth afterwards )
A “Winter” is the equivalent of a full year’s growth as it
marks the end of full year, “+” means part of a year’s
growth after the last Winter
And as it had two Winters in the river, it is a :-
The . between the two numbers shows that the first is the
freshwater age and second the marine age
The seasonal runs of salmon are defined by the pattern at
the edge of their scales:
If there is closed (“Winter”) growth at the edge of the scale and it is a fresh fish
after the 1st January, it is a Spring Salmon
If there is open (“Summer”) growth at the edge of its scale, it is a Summer Grilse
If there is closed growth at the edge of its scale and it is fresh before the 1st
January then it is an Autumn Grilse or Salmon
.. But there are some complications !!
2 River Winters, with a
straight and abrupt
transition from river to
One River Winter and a
gradual transition to
marine growth = “Run
This can be written as
“+” growth for the River
1+. 2+. etc
Two River Winters
A “Summer Grilse” – with Summer growth at the edge
x.1+o (o = open)
Sea growth patterns
1SW + and
An “Autumn Grilse” – with Winter 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, x.2
This is why the date a fish is caught is important
3 closed bands, but without a date could be an
Autumn Salmon ( x.2+cl) or a Spring Salmon (x.3)
3 or cl ?
Not all scales conform to the patterns we have just
• What follows are a couple of examples of
Where is / are the winter (s)?
18/08/2006 3lb 50cm.
26/09/2007 5lb 59cm.
Split winter - (Winter checks )
- Which is the actual Winter band?
• When salmon return to fresh water they
• To provide energy, material is reabsorbed
from body tissues, including the roots of
• The degree of erosion can show how long
a fish has been in fresh water – but some
can occur at sea after feeding stops or
even during a Winter out at sea
This means this fish has been has stopped feeding for some
time – usually because it has been in fresh water
50% erosion means half the margin of the scale is
eroded: varies from scale to scale on same fish,
need to take an average over several scales
29/09/2007 10lb 74 cm
This 70% eroded scale
shows that this fish probably
entered the river as a Spring
Salmon, though caught in
100% erosion makes
this scale impossible
to read. 2.NR- the scale
is no longer complete to
10/11/2007 14lb 86cm.
Menzies 1920: Notes on the salmon of Thurso Bay
“A further proof that absorption is progressive is given by a number of fish which were
marked in the sea and subsequently recaptured there prior to a visit to fresh water” e.g.
“Both colouration of the skin and absorption of the scales may take place in the sea. An
exceptional fish may show absorption of the scales in June. A quarter of those taken in
July showed it and the proportion rapidly increased until almost the whole catch had
eroded scales in September. The amount of the absorption in individual fish also
steadily increases from July to September, male fish showed it to a greater extent than
females and small summer fish to a greater extent than grilse at the same dates.”
Tagged in the sea,
recaptured in the
sea 6th September
When scale reading, Erosion is the first thing to look for and assess as it tells
you whether you are dealing with a fish newly in the river and therefore directly
reflecting the part of the season during which it was caught (Spring, Summer or
Autumn) or whether it is from earlier in the year – e.g. a Spring or Summer fish
caught in Autumn and is not therefore reflecting the time at which it was caught.
• If a fish spawns and then returns to the
sea to recondition, evidence of spawning
remains on its scales.
• The old, eroded, scale edge remains
visible as a scar – the “Spawning Mark” or
04/09/2007 9.25lb 72cm.
The spawning mark
(SM) is the old eroded
edge, with new marine
growth beyond it
The SM has replaced the old edge of the scale: it could have been Open or Closed or
a full winter band at the edge. All that is definite is that there was summer growth
before it, which is denoted as “+”
Erosion & Spawning Marks
Loch Ness 35lbs, female, 26-03-1931 2.3.SM.1
• Scales can be collected from smolts and
be read in the same way as other life cycle
16 April 2007 131mm
With a lot of + growth (= “run-out” on the adult scale)
16 April 2007 127mm
With no + growth (or very little) but an average sized smolt
16 April 2007 101mm
With no + growth (or very little) but a small smolt (101mm)
11 April 2007 85mm
:with a lot of+ growth but a very small smolt
We should remember that information should
be read from the image not that we should
force information onto the image to match
what we would expect for a given size of
fish or time of year.
Always be aware of which way information is
actually flowing – it should be from the
screen, not to it.
It is better to have an “unreadable” category
rather than have forced interpretations in
One scale to the tune of another!
However, you can use clearer scales from the
same population of fish to interpret more difficult
- just like some scales from the same fish are
more readable than others.
Scales of different fish from the same time
period and place are most likely to allow this.
The example below is from two fish caught on
the same day at the nets at Paxton on the
The problem scales / fish was 710mm and
though most likely to be a 2SW fish it had
four bands that could be read as sea winters.
First Sea-winter. There are two bands in the right place to be a sea-winter, both are
continuous and strong, so which is the winter? Or, are there two close-together
Winters being shown?
The basic pattern is the same, two bands that could be winters, but on the lower side it can
be seen than both bands emerge from an area that is a single band and on the side at the
top, the outer “band” can be seen to be merging back in to the inner band, to make more of
Based on this, it can be said that in the first fish, both bands are 1st SW bands, but separated
all the way round. The inner band can then be said to show the start of winter and the outer,
the end. This pattern can be called a “Split Winter”
Another fish, much the
same size, caught on
the same day
The problem fish: two possible bands for the second Sea Winter
The other fish, much the same size, caught on the same day
This shows that the inner of the two bands is the Sea Winter – with this other fish, the outer band
One scale to the tune of another!
However, you can use clearer scales from the
same population of fish to interpret more