Salmon scale reading for the salmon angler
- its nice to know how

atlantic salmon scale reading for anglers

Angler’s guide to scale reading

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:
  1. Focus - the point on the scale that is in the centre of the concentric lines; synonymous with nucleus.
  2. Circuli (singular circulus) - circuli appear on the surface of the scale as dark concentric lines; synonymous with rings.
  3. Band - a concentric region of the scale which is formed during a particular time of year.
  4. Winter band - a dark band thought to be associated with slow growth during the cold period of the year.
  5. Summer band - a light band thought to be associated with rapid growth during the warmer period of the year.
  6. Winter check - up to 3 wide-spaced circuli occurring within a winter band.
  7. Summer check - narrow-spaced circuli, generally fewer in number than a winter band, occurring within a summer band.
  8. Annual zone - a concentric region of the scale referring to a complete year of life.
  9. Annulus - the theoretical boundary between two successive annual zones.
  10. 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 been completed.
  11. Erosion - reabsorption of the edge (and sometimes the surface) of the scale.
  12. Spawning mark - erosion associated with the spawning migration.
typical atlantic salmon scale
Part 3SW salmon scale

Salmon scales

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 of environmental 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 seen:

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 and cessation of feeding.

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.

Typical Scales
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. River growth 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 viewed under magnification, 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.

scale sample for salmon
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 these fish having 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 smolts. 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

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Copyright 2007 Alastair Gowans AAPGAI and FFF Master and THCI, APGAI. All rights reserved.