Introduction to Salmon Scale Reading

• Terminology

• Common scale / growth patterns

• “Problem” scales

• Erosion

• Spawning marks

• Smolts

• Warning

• 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 scaleVisible 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.

Focus

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

Transition from 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.

focus

River zone

First summer at sea. Widely

spaced circuli.

Winter band of

narrower-spaced circuli.

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 :-2.1+

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 or Salmon

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 !!

Common scale / growth patterns

River Patterns

2 River Winters, with a

straight and abrupt

transition from river to

sea

One River Winter and a

gradual transition to

marine growth = “Run

Out”

This can be written as

“+” growth for the River

phase, e.g.

1+. 2+. etc

Run out

Juvenile 21

Two River Winters

and Run-out

= 2+.

Run out

A “Summer Grilse” – with Summer growth at the edge

x.1+o (o = open)

1

+

o

Sea growth patterns

1SW + and

Closed,

Winter

growth at

the edge

An “Autumn Grilse” – with Winter growth at the edge

x.1+cl

1

+

cl

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

1

2

3 closed bands, but without a date could be an

Autumn Salmon ( x.2+cl) or a Spring Salmon (x.3)

1

2

3 or cl ?

Not all scales conform to the patterns we have just seen.

• What follows are a couple of examples of “problem” scales.

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?

?

?

?

Erosion

• When salmon return to fresh water they stop feeding

• To provide energy, material is reabsorbed from body tissues, including the roots of the scales

• 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

29/09/2007

4.5lb 61cm

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

September,

100% erosion makes

this scale impossible

to read. 2.NR- the scale

is no longer complete to

the edge

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, early July

Same fish, 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.

Spawning marks

• 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 “SM”

04/09/2007 9.25lb 72cm.

2+.1+SM+o

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 “+”

1SW

SM

+

+

Erosion & Spawning Marks

SM

Not SM

Loch Ness 35lbs, female, 26-03-1931 2.3.SM.1

Smolts

• Scales can be collected from smolts and be read in the same way as other life cycle stages

16 April 2007 131mm

2+. With a lot of + growth (= “run-out” on the adult scale)

1

2

+

16 April 2007 127mm

2. With no + growth (or very little) but an average sized smolt

1

2

16 April 2007 101mm

2. With no + growth (or very little) but a small smolt (101mm)

1

2

+

11 April 2007 85mm

1+. :with a lot of+ growth but a very small smolt

1+

River Dee

A hatchery smolt : two years old

Warning

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 your data.

One scale to the tune of another!

However, you can use clearer scales from the same population of fish to interpret more difficult ones

- 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 lower Tweed.

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 single band.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

? ?

1

1

?

?

The problem fish: two possible bands for the second Sea Winter

The other fish, much the same size, caught on the same day

2

?

This shows that the inner of the two bands is the Sea Winter – with this other fish, the outer band is faint.

One scale to the tune of another!

However, you can use clearer scales from the same population of fish to interpret more difficult ones