The document discusses the structure and growth patterns of fish scales, particularly in trout and salmon, detailing how scales can be used to determine age and growth history. It explains the significance of ridges, or circuli, on the scales that reflect seasonal changes in growth, and the importance of using 'dirty' scales for DNA analysis. Additionally, the document outlines practical methods for collecting scales from both dead and living fish, emphasizing historical advancements in scale reading and its impact on understanding fish biology and management.

1. SFCC
Introduction to Scale Reading

2. The Trout, Frost & Brown
The scale lies in a pocket in the Dermis (inner layer of the skin).
Embedded scales
Dermis

3. The Trout, Frost & Brown
The part facing towards the tail projects above the surface of the
fish but does not pierce the Epidermis (outer layer of the skin)

4. The Trout, Frost & Brown
The anterior, and much larger part is deeply embedded in the
fish’s skin: like the root of a tooth in the gum
This “root” part of the scale does not get worn and abraded like the
extruding part and is therefore the bit of the scale that is used for
reading

5. This is the posterior,
protruding part of the
scale
Visible on the outside
of the fish under the
Epidermis
This is anterior part of
the scale, its root,
embedded in the
Dermis. This is the
part that is read.
A Scale

6. The Trout, Frost & Brown
A scale is an oval plate with more or less concentric ridges
which are more numerous and continuous on the anterior
(and, in all but the youngest and smallest fish, greater) part,
than on the posterior, exposed, part of the scale
Posterior, exposed part of
the scale. Some of the skin
is still adhering on this
scale
Anterior, embedded part of
the scale
These ridges , some of which are widely and some narrowly spaced, are called circuli.
SCALES AS TISSUE
SAMPLES
With the arrival of new
genetic techniques, scale
samples now have to be
regarded as tissue samples
that can produce DNA as
well as being sources of data
on growth patterns and ages.
It is actually the bits of skin
and fish glaur (mucus)
sticking to scales that can
produce DNA, so clean
scales are of less use for this
than “dirty”

7. Hard “ridges” are left at the edge of each plate as softer material is re-absorbed. The upper
side of a scale is therefore ridged while the lower layer is smooth.
A scale consists of a fibrous, non-calcareous material laid down in thin plates below one
other: Each new layer grows over the whole of the base plus some extra
The Trout, Frost & Brown
A scale is thus made up of plates with ridges at their edges

8. The Salmon, J.W. Jones
These periods have been termed “Summer” and “Winter” growth respectively, though the
correspondence with these seasons is only rough.
It has long been accepted that the bands of wide rings (circuli) represent periods in the life
of the fish when growth was rapid and that the bands of narrow rings represent periods of
less rapid growth.
Winter bands
Summer bands

9. The ridges on
one side of
the scale
mean that
impressions
can be taken

10. This means that
scales do not
have to be
individually
cleaned before
reading as the
impression in the
Acetate shows up
the pattern of the
scale through any
overlying dirt

11. Scale.
Impression
The pattern of widely and narrowly spaced ridges / circuli can
be read to show the seasons that a fish has lived through.
Reading

12. The Trout, Frost & Brown
Scale “papillae” appear when Fry are around 3cms in length and the scales are complete
when the fish are around 50mm in length.
The scales show first along the lateral line, then spread dorsally and ventrally above and
below it, the region posterior to the Dorsal Fin being the first to be colonised.
Trout and Salmon are born “naked”, without scales.

13. The “oldest” scales with the most complete record of seasonal growth are therefore
found in this area, just behind the Dorsal Fin, on the diagonal from the Dorsal to the
Pelvic fins
.
Practical Note: Near spawning time, the skin of male Salmon & Trout becomes very
thick and it can be impossible to scrape scales off without doing damage. Not getting
scales from one sex can bias results. In such cases, it is sometimes possible to get scales
off the wrist of the tail, which also have the full life history on them.

14. 1 Write the details on the front of the packet in pencil or waterproof ink. It is best to do this
before putting the scales in as it is much easier to write on the packet when it is dry. *
* If the fish is being opened up, make a note that the sex has been identified in this way.
Outside the breeding season, sex can be unidentifiable from external features. The
convention is that Mature Males are recorded as MM but Mature Females as FF. This
avoids any confusion that could arise from Mature Females being recorded as MF
TAKING SCALES
.
A: FROM DEAD FISH
2 The scales are taken on a diagonal from the back of the Dorsal fin to the front of the Anal
fin, half way down towards the Lateral Line
3 Run the knife backwards across the area (with the scales) to remove some of the glaur
4 Then run the knife forwards (against the scale) to displace them. They will stick onto the
blade of the knife. Place the blade into the scale packet, press down on it and withdraw
the blade, wiping the scales off into the inside of the packet.
5 Put the packet somewhere dry. Scales will rot if kept damp, but if dried, can last for a very
long time.

15. 1 This is done in the same way as for dead fish, but gently, and limiting the number of scales
taken to about half a dozen
TAKING SCALES
.
B: FROM LIVING FISH
2 Mature males at spawning time, when scales are under thickened skin, are difficult to get
samples from.
They can sometimes be taken more easily from the Wrist of the tail
Tweezers can be used to take them individually (but this only really applies to anaesthetised
fish).
On large fish they can sometimes be drawn out of their skin pockets with the point of a knife
– put the point on the extruding part of the scale, press down and then draw the scale
towards the tail
3 Large and thick scales such as those of Grayling need to be removed individually from
their skin pockets with the point of a knife as above.

16. A bit of HISTORY
Revolution 1: early 19th century – Parr are the young of Salmon
Revolution 2: late 19th century – Scales can tell the age & growth of fish
Revolution 4: 1960s – Electric fishing allows sampling of juveniles
Revolution 3: 1939 – Fertilisation in the wild is highly efficient
Revolution 6: The present – Genetic data
There have been six major revolutions in Salmon management & biology
Revolution 5: 1960s – The marine feeding grounds located

17. The first scientific application of scale reading to salmonids was by Dahl, in Norway,
around 1910 who summarised the matter as :
“It is therefore quite clear that the summer-zones and the winter-bands on the
scales of trout are respectively formed in the corresponding seasons, which affect
their growth, and that these zones indicate annual epochs in the life of the fish. If,
therefore, we count the number of these zones, we can ascertain the age of the fish.

18. During the last three years or so we have been gradually and with increasing certainty
realising that a study of the scales of the salmon yields a most valuable addition to our
knowledge of the fish’s life
The first popular account of scale reading in English was in W. L. Calderwood’s The
Life of the Salmon, published in 1910. He introduced the new technique by writing:-
And outlined its basis:-
… while summer feeding and growth is in progress the lines or ridges on the scales are
added in greater numbers than at other times, and that between the ridges the spaces are
greater, and that while the more moderate feeding of winter is in progress the ridges are few
and close together. The result is that each summer and each winter leaves its indelible trace
on the scale

19. The study of the scales has prominently called attention to the infrequency of spawning
amongst heavy fish. It comes as a surprise, for instance, to learn that very many of the large
spring fish of the Tay – fish almost invariably about 20lb in weight – have never spawned
….. From the study of the scales alone do we gain this information as to the infrequency of
spawning, and the consequent benefit to our stock of salmon in preserving most religiously
the breeding fish which enter our rivers.
The first great revelation from scale reading was that salmon were not frequent and
repetitive spawners, as Calderwood put it:
The legal protection given to Kelts in 1857 was based on the idea that Salmon were
repeat, annual, spawners & therefore if they were not killed they would come back
bigger each year. Scale reading showed that this was wrong !