sfcc introduction to scale reading - 2017

SFCC

Introduction to Scale Reading

The Trout, Frost & Brown

The scale lies in a pocket in the Dermis (inner layer of the skin).

Embedded scales

Dermis


The part facing towards the tail projects above the surface of the

fish but does not pierce the Epidermis (outer layer of the skin)


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


A scale is an oval plate with more or less concentric ridges

which are more numerous and continuous on the anterior,

embedded (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. This gets worn down

and is unreadable

Anterior, embedded part of

the scale. This is the part that

is read

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”

Softer material

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


A scale is thus made up of plates with ridges at their edges

The Salmon, J.W. Jones

These periods have been termed “Summer” and “Winter” growth respectively, though the

correspondence with these seasons is not absolute. Some populations are known that make

their best growth in Winter, when their food is seasonally abundant.

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

The ridges on

one side of

the scale

mean that

impressions

can be taken

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.

Some scale

readers prefer to

read the actual

scales rather than

impressions.

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


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.

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.

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.

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.

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

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.

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

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.

However, there was a continual decline of numbers of Grilse over the latter half of the

19th century, though MSW numbers were maintained. This gave the impression of the

fish getting bigger – showing that protection of kelts “worked”

It was scale-reading that eventually showed what was really happening.

sfcc introduction to scale reading - 2017

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