An article by Dave Hoskins (E.A. Scientist).

Although, on the face of it, the reading fish scales may sound very complicated. In reality with a little practice and the aid of a microscope scales can be interpreted successfully.

Scale Collection

When removing scales always remove them from the higher side of the flank (just below the dorsal fin).

If the fish has been killed then take a good number of scales as some might well be unreadable. When removing scales from a fish that is to be released, with a pair of tweezers, gently ease the scale from its fixture point. Never remove adjoining scales as this leaves an area vulnerable to infection.

The scales should be stored in a flat envelope ensuring that they do not curl when dry.

Scale Preparation

In order to read the scales effectively, they need to be cleaned of any remaining skin.  A mild hand soap solution should be prepared in an empty film cartridge holder and the scales added and left for at least a week.

The scales should then be placed in a Petri dish containing clean water. With the use of a microscope, use a matchstick to gently remove any remaining skin.

It is preferable at this stage to select three or four of the best (complete life cycle) and using filter paper, quickly dry the scales and place them between to microscope slides and secure each end. It is also important to label the slide with as much detail as possible as it will enable later interpretation significantly.

Reading The Scale

Although scales can be read using a microscope it is usually better to use a microfiche screen. During the examination of the scales, notes should be taken of each scale from the same fish and compared later. This will prevent possible contamination by other fish scales.

Things to Look For

Although scales can be read using a microscope it is usually better to use a microfiche screen. During the examination of the scales, notes should be taken of each scale from the same fish and compared later. This will prevent possible contamination by other fish scales.

When viewing the scale you will see that a number of the bands are closer together and appear darker. This represents winter where growth has slowed due to the cold and reduction in food consumption.

When viewing the scale you will see that a number of the bands are closer together and appear darker. This represents winter where growth has slowed due to the cold and reduction in food consumption.

Evidence of previous spawning activity will also be seen on the scale (known as spawning marks). This is where the scale is partly eroded through migration to and during the spawning process. These appear as bands that crash together.

There are a number of examples to view which will clearly show this together with a directory of terms.