2 Ways to Determine the Sex of a Trout

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Photo by Louis Cahill

Photo by Louis Cahill

Lets talk about sex!

One of the things I love most about being a part of Gink & Gasoline, is educating our readers. Today’s post, talks about 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 fish “(“, 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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9 thoughts on “2 Ways to Determine the Sex of a Trout

  1. Looking through my pictures of catches over the past year, I see that all of the anal fins i can see are concave? Maybe im only getting pictures of the females butts, lol. Surely i couldnt have caught all female fish in the past year

  2. Thanks, Kent. It is fun to know these things. I knew about the snout and kype, but not the anal fin. You learn something new every day (especially if you subscribe to G & G).

  3. Pingback: Tippets: Behind the Lens, Yellowstone Cutthroat Reintroduction, Telling Trout Apart | MidCurrent

  4. Pingback: Figuring the Sex of a Trout: Gink & Gasoline | The Ozark Fly Fisher Journal

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