Q.
How can you tell a male from a female trout:
specifically, rainbow, cutthroat, brown and brookie?
A.
I am assuming that you are asking about how to
distinguish the sexes by external characteristics,
rather than the obvious internal differences.
What you are asking about is called “sexual
dimorphism,” or how to males differ from females.
Here are a few external characteristics that may
help. These pertain to all trout species unless
specifically indicated.
Vent: The vent of male trout is usually
slender and almost triangular in shape, whereas
that of females is more circular and often slightly
protruding. Both of these characteristics become
more pronounced and noticeable as the fish matures
and grows larger.
Jaws: As males grow larger, they develop a
hooked lower jaw (called a “kype”). This is
especially noticeable on brown trout and least on
brook trout. Obviously, this characteristic won’t
help you much on smaller trout.
Coloration: This can be tricky, especially
if you are around streams where anadromous trout
live. Here, fish coming in from the ocean are
generally silvery, but they lose this brightness
the longer they are in freshwater. In general,
the males of most trout species develop brighter
and more intense colors during times of spawning.
However, this doesn’t help much unless you have
another specimen for comparison, especially a
known female.
Fins: It has been noted that the trailing
edge of the anal fin of sexually mature males
brown trout is convex in shape, whereas that
of the sexually mature female is concave or
falcate in shape.
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 December 6th, 2004 on Fly Anglers Online by Robert Younghanz.