Q.
What makes the color on a fish? Why does it fade so quickly?
A.
Your question is concerned with fish
biology rather than stream ecology, and
it would take considerable space to answer
your question completely. Thus, I will give
you some of the basic aspects of fish coloration;
if you want to pursue this further, you can get
more information from a good fish biology text.
A fish’s iridescent hues are caused by crystals
of guanin. On the belly, they appear in their
undisguised silvery whiteness under the transparent
skin. On the back, they are overshadowed by the
thick black pigment which forms in that portion
of the skin. Guanin crystals are waste products
of the blood. Other colors are the result of one
pigment - red, orange, yellow, or black - contained
in individual sac-like structures called chromatophores.
The amount of each pigment exposed is controlled
by the chromatophore; that is why the coloration
of fish can change. Different colored species
have different amounts of the various pigments;
their combination and distribution on the fish
determine the characteristic patterns of each
species. The blue and green colors are generally
due to the reflections and interferences of light
by the colorless surfaces of the guanin crystals
and scales, acting in conjunction with the different
pigment colorations. This is similar to the blue
color in bird feathers; there is no blue
pigment - it is the reflection of guanin crystals
through the feather material.
The reason a fish loses its colors when it
dies is because the pigment contracts within
the chromatophore upon death, thus exposing
less pigmentation and more of the colorless
chromatophore.
If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com
The ‘Stream Doctor’ is a retired professional stream ecologist and
author, now living in the West and spending way too much time
fly-fishing. You are invited to submit questions relating to
anything stream related directly to him for use in this Q & A Feature
at streamdoctor@aol.com.
Originally published November 8th, 2004 on Fly Anglers Online by Robert Younghanz.