2 Ways to Determine the Sex of a Trout

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Here are two ways to determine the sex of a trout.

Over the years, I’ve found that the majority of my clients have a hard time determining whether a trout they catch is a male or female. Below are two ways to quickly identify the sex of a trout.

1. Look at the mouth

One of best ways to distinguish the sex of a trout is to examine the mouth. Female trout all have a short rounded nose or upper jaw, while male trout have a more elongated snout. If your trout has a lower jaw with a kype, it’s a male for sure. Although the mouth of a female trout will grow larger as it ages and increases in size, the mouth will never grow a kype (hooked lower jaw). Upon becoming sexually mature, male trout will begin to grow a pronounced kype. At first, it will just be a tell-tale sign, but as a male trout ages, its kype will become more pronounced. It’s important to point out that even for trout that aren’t sexually mature, an angler can look at the mouth of a trout and see either a uniform mouth with a short rounded nose (female), or a elongated snout with a slightly longer lower jaw (male).

2. Look at the anal fin


(Left) Male, (Right) Female

Sexually mature male and female trout for both rainbows and browns have different looking anal fins. A male will have a slight convex anal fin “(“, while a female trout’s anal fin will be slightly concave “)”. I’m not sure if cutthroat trout are in the same boat. I’ll have to depend on the community who regularly catch them, to provide us their insight and confirm this.

Is there a third way to Identify a trout’s sex?

When I was writing this post, I ran across a couple references that claimed a female trout’s anus will have a round shape, while a male trout’s anus will have a triangular shape. I scoured the internet and my personal book collection for a picture that would show this, but I came up empty handed. If anyone can confirm this theory, please take the time to comment on the post. Next time I’m on the water, I’m going to pay close attention to this.

Being able to determine the sex of a trout will not make you a better fly fisherman but I find  it to be really cool knowing how to tell the difference. Have a grand day everyone.

Keep it Reel,

Kent Klewein
Gink & Gasoline
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18 thoughts on “2 Ways to Determine the Sex of a Trout

  1. Very interesting post. I knew about the jaw features but not the anal fin traits. I’m going to keep my out for that trait on all the cutthroat I plan on catching this summer. I just hope they oblige me and allow themselves to get caught:)

  2. Great tip! I Never knew about the anal fin. I’ve just always relied on the jaws. That’s a great way to determine the sex for those trout that haven’t reached maturity. I’ll definitely be using this!

  3. Got into a mess of wild brookies and saw that some had there spawning colors on and others did not. Thought I only caught a few males but after looking at their anal fins I guess I hooked a bunch of males that just spawned a little early. Ya’lls posts make the internet worthwhile. Please keep doin it

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  6. If you’re lucky/skilled enough to catch one of these magnificent creatures of God, it’s good to know how to tell their sex just from an info standpoint – I only do catch and release and strongly encourage everyone to do the same, still it’s good to know.

  7. I don’t think the anal fin trick is always correct… I’d love to post a picture of a female trout with a convex anal fin or send an email to you for verification?

  8. Important to know the difference for conservation. Mature female wus carrying the eggs for maybe a 1000 new trout, leave her go. Male trout – eat him.

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