Anatomy

Characteristics that make the Brook Trout unique

lateralline

The Brook Trout is a distinctive looking fish with its unique colouration and body patterns. The back is usually drab olive green, fading to a lighter shade on the sides. The lower sides (below the lateral line) are more yellow/orange and this fades into a white belly. A most distinct feature is the vermiculation pattern on the top of the back and head that appears like ‘worm-like marks’. The dorsal, caudal and adipose fins also have the same vermiculated pattern. The adipose fin is a vestigal fin of unknown function. The lower fins are orange in colour and has a leading edge of white piping with a smaller stripe of black between it and the rest of the orange fin. The white tipped fins are so distinctive that they are easily identifiable as the fish swims in the water. That gets my heart pumping!

Another distinct feature are the spots scattered along the sides of the fish. There are numerous yellow spots intermingled by pink spots with blue halos. The scales are often overlooked. They are cycloid scales, characteristically thin, small and particularly imbedded in the skin, typical of fast moving fish.

 

Releasing a female

underwaterfemale

This female brook trout is being revived and released. You can clearly see the lateral line and nasal receptors.

Brook Trout have an acute sense of smell and can use a series of nasal receptors to sense the particular odors of the streams in which they live. A series of cells arranged in a linear pattern along both sides of the fish called the lateral line allows fish to sense vibrations and disturbances in the water. The lateral line is somewhat analogous to both the sense of hearing and a motion detector.

Note the shape of the male's head (right photo) in comparison to the smoother, rounder head of this female.

A fall male

fallmale

This fall male brook trout displays the typical fall orange belly and has developed a prominent kype of the lower jaw, presumed to attract the females.

The male’s maxillary (upper) jaw is extended further than the female jaw. Unlike other chars, the brook trout jaw extends posteriorly for a longer distance, giving the appearance of a larger mouth.

Although fall colouration intensifies for both sexes, males are more dramatic. The male also develops more of a hump on the back during the fall courting season. All these features quickly fade after spawning.

 

the broad tail creates power

tail

The broad, flat tail is another distinctive feature of the brook trout. Names like “square-tail” and “Mr. Broad tail” are often are used to describe a fish that uses its powerful tail to fight the strong current it prefers.

“Salvelinus” is another name for "Char" and “Fontinalis” means “living in springs”.

"Say...Ahh"

vomer

Brook Trout are members of the Char family which are characterized by a boat-shaped structure of the vomer, a bone in the roof of the mouth. This bone has a patch of small teeth on its head or crest, while the extension or shaft is depressed and lack teeth. Teeth are visible to the naked eye on the tongue and back of the throat, but none on the roof of the mouth. There is an outer row of teeth on the lower jaw and an inner and outer row on the the upper jaw.

 

Estimating the age of Coaster Brook Trout

agechart

 

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.

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

Scale

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?

It is possible to count them with a magnifying glass. There are approximately 230 scales along the lateral line...and “NO” I didn’t count them, some other individual with a serious mental illness must have done it.

“One a rather deep and thin fish with very red sides, especially next to the fins,
and the other variety much rounder and fatter, not so deep, with a rather light bluish colouration on the sides”

Edward R. Hewitt, 1891

 

I’ve noticed two body shapes to the Nipigon brook trout similar to the 1891 quote. One is more ‘torpedo’ shaped similar to other members of the trout family, suited to a lake environment. The other brook trout appears more “football” shaped adapted to living in the strong current of the river. However, the river does host both body shapes, but the lake appears to have only the longer adaptation.

the "torpedo"

Briantorpedo

This spawning coloured male from Lake Nipigon,
measured 25 x 13.5 @ 5.3 pounds

the "football"

Alfootball

This "football" from the river measured 21" in length
with a girth of 15" and weighed 5.6 pounds

 

holdphoto