Just like counting the rings of a tree, biologists are able to gather information about a Brook Trout by looking at its scale under a microscope. We can determine the age and years when good growth occurred. Sometimes we can even see indications of when the fish spawned.

Find the core or center of the scale (It's not in the middle!). This has been labeled on the diagram. The first rings form when the fish is in its fry stage. If the water is warm and there's lots of food, the fish will grow well and the rings will be spaced far apart. This is the first summer growth. 

Next are some rings that are very close together. These grow during the fish's first winter. The water is cold and there is little food. The fish doesn't grow very much and the rings are close together. This is first winter growth.  At the end of this stage the fish was a year old.

The fish then spends another year in the river or lake. Can you find the summer and winter growth for the second year. Can you find the third summer and the third winter? The fourth summer?

How old was the fish from which
this scale was taken?

The scales of a fish are

like a book. They tell a story.

They tell how old a fish is

and if it has been eating

well. As a fish grows, rings

form around the center of

each scale. You can easily

see the rings in the diagram.

-------------------------------------------------------------------------------------------------------------------------------------------------------

Estimating the age of Coaster Brook Trout

(page 62 of report)

Age estimates can provide information on life span, mortality/survival, and growth. Brook trout ages are commonly estimated by inspecting scales, fin rays, or otoliths. Otoliths are considered to be a more accurate method for estimating the age of most fish species (DeVries and Frie 1996), however collecting otoliths requires lethal sampling. Scales and fin rays can provide a non-lethal alternative, although estimated age can be inaccurate, particularly for older brook trout (Stolorsky and Hartman 2008; Steele 1986; Dutil and Power 1977). Scales and fin rays may still be a valid method for estimating age for younger fish.

• Estimates of age using fin rays and scales are similar; however, scales underestimated the age of brook trout relative to fin rays typically by one year beginning at age 4. If estimates are indicative of true age, either method may be useful for management purposes.
• Estimates of age using scales and otoliths are not similar. Scales underestimated the age of brook trout relative to otoliths by 2 to 5 years beginning at age 1.
• Although estimates of age differed among structures, it remains unknown which structure most accurately represents true age. Further work is recommended to understand if age estimates using scales, fin rays, and othliths are indicative of true age.

The scales of a fish are like a book. They tell a story.

Tale of the Scale