Angler’s guide to
The purpose of scale reading is to interpret the age of an Atlantic salmon
consistently from features on its scales including determination of
river-age and sea-age.
Definitions used in scale reading
The following definitions based apply to scale reading:
- Focus - the point on the scale that is in the centre of the concentric
lines; synonymous with nucleus.
- Circuli (singular circulus) - circuli appear
on the surface of the scale as dark concentric lines; synonymous
- Band - a concentric region of the scale which is formed during
a particular time of year.
- Winter band - a dark band thought to be associated with slow
growth during the cold period of the year.
- Summer band - a light band thought to be associated with rapid
growth during the warmer period of the year.
- Winter check - up to 3 wide-spaced circuli occurring within
a winter band.
- Summer check - narrow-spaced circuli, generally fewer in
number than a winter band, occurring within a summer
- Annual zone - a concentric region of the scale referring
to a complete year of life.
- Annulus - the theoretical boundary between two successive
- Plus-growth - region of wide-spaced circuli which
may follow the last annulus of the sea zone signifying
that a full
year of growth has not
- Erosion - reabsorption of the edge (and sometimes the
surface) of the scale.
- Spawning mark - erosion associated with the spawning
|Part 3SW salmon scale
The examination of an adult salmon scale reveals 2 distinct parts which
can be defined as:
1. River life: the period spent in freshwater up to last river annulus.
Salmon parr populations in different areas experience a very wide range
conditions and thus exhibit large differences in patterns and rates of growth.
The mean age at which salmon smoltify, therefore, varies from between 1 and
2 years in some southern populations in Europe and North America to 7 years
in Labrador and northern Norway.
2. Sea life: the period from the onset of sea growth; it may include time spent
in freshwater as an adult.
During the sea phase of scale growth, 3 types of dark 'band' may be
1. Winter band.
2. Summer check.
3. Closing at the edge of the scale: this is not necessarily associated with
winter growth but may occur in response to eg maturation, homeward migration
There are also 4 types of light band, which are:
1. Summer band.
2. Winter check.
3. Spawning mark.
4. Plus growth.
Winter check comprises no more than 3 wide-spaced circuli in a winter
band. Where this is seen, the annulus is assumed to be located at the
end of the whole winter band.
On some scales there is a third
distinct part which occurs between the River and Sea Zones, this is referred
to as run-out: a period of intermediate growth between the last river
annulus and the start of the sea growth.
Notation for describing salmon age
Examples: the age of a fish is given as 5.1+ and that of another fish
is given as 3+.1+SM1+. The first figure is always the number of annuli
in the river zone. Where this is followed by a plus sign, this signifies
run-out on the scale. The stop indicates the transition to sea growth.
The next figure is the maiden sea-age of the fish and if followed by
a plus sign indicates that plus-growth was present. A spawning mark
is indicated by SM and further annuli and plus-growth described as
for maiden sea years.
In many cases it will be possible to identify winter bands clearly. An example
of a typical scale pattern for a 3 sea-winter (3SW) fish is shown above. The
focus is designated F and the end of each winter band is identified and numbered.
is designated R and sea growth is designated S.
A slowing down in growth will produce the formation of narrow-spaced circuli.
These may occur at the edge of the scale. In some cases, these will signify
that the fish has been captured before the start of the next growing season
and the closing, therefore, signifies the beginning of a winter band. However,
fish, especially one sea-winter fish, may be caught in late summer and show
narrow-spaced circuli at the edge of the scale. In such cases, the date of
capture is essential information if a correct interpretation of the sea-age
is to be made as these one sea-winter fish will appear to have the same sea-age
as 2 sea-winter fish caught earlier in the year.
During a long sojourn in freshwater, material is reabsorbed from the margins
of the scale and sometimes from the surface, this is termed erosion. When scales
from salmon which have spent some time in freshwater, especially kelts, are
this re-absorption of material can be seen as erosion of the scale margin and,
sometimes, of the surface detail of the scale. In severe cases, especially
when the fish has spent many months in freshwater, complete bands may be lost
as a result of erosion.
|Area for gathering scale sample
Collecting scales for reading
The importance of choosing the best area for sampling scales is self-evident.
Scale samples should be taken from the area shown on the diagram. If scales
are absent from this location then the equivalent position on the right side
of the fish or alternatively just ahead of the preferred area on the left side
should be used.
Prior to sampling, excess mucus should be removed from the area using the back
of a knife. The knife should be cleaned before the scale sample is removed
and between each sample. The scale sample (about 20 scales) should be placed
inside the scale envelope and allowed to dry slowly before being stored. Under
no circumstances should scale samples be placed in plastic bags. Clean white
paper may be used.
Information recorded with each scale sample includes when possible
the weight of the fish, fork length, sex, date and where
captured, together with any remarks e.g. tag number (if a recapture), colouration
if taken in freshwater, fin conditions and any other useful information. Also
note whether or not scales were removed from the recommended area.
Aquaculture and hatcheries
Fish from artificial origins may be identified by the following characteristics:
The fins, particularly the caudal and dorsal, of reared fish tend
to be deformed and some fins may be missing. This should be noted on
the scale packet
along with any other remarks which might be helpful in identifying the origin
of the fish.
The amount of growth made in the river zone tends to be greater than that
of equivalent wild fish from the same stock. Furthermore, the growth pattern
on the scales is more irregular than that shown by the majority of wild fish.
If ageing using accepted scale reading criteria produces over-aged fish for
the particular area from which the samples came, then the possibility of
been reared for at least part of their lives should be considered.
It is generally recognised that the age determination of hatchery-reared
fish presents a number of problems not encountered when dealing with wild
When it is suspected that hatchery-reared fish are present where scale samples
are being collected, this should be noted.
For a detailed methology for scale reading see the ICES report
of the Atlantic Salmon scale reading workshop Aberdeen Scotland 1984