Steve Fitzjohn·
Fishing 101
·4 min read

How To Know The Sex Of A Fish You Have Caught?

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Ever wondered if the fish you have caught is male or female and wondering how to tell? In fishing, it can be hard to determine and understand what are the major differences in the fish’s gender, especially when you are out fishing. 

When you love going fishing, your relationship with the species you are targeting can become quite personal and you want to know more about them. Fish gender is often at the top of the list. 

Join me as I run through everything you need to know about the sex of a fish and how to easily know if the fish you caught is a male or female. 

Are Fish All Male Or Female? 

Yes, pretty much every species of fish has a male and female which spawn together to create their offspring. 

Some fish species are also hermaphrodite fish, meaning they can be both female and male at the same time, or switch genders as required. But, these fish species are quite rare. 

How To Know The Sex Of A Fish 

Knowing whether a fish is a male or female is often very difficult but sometimes incredibly easy. This often depends on the species. 

In general, a fish has to be mature for you to be able to see any physical or behavioral signs that you can use to determine sex. 

But, how easy it is to determine the sex of a fish depends on the species of fish. Some fish species are sexually dimorphic, meaning males and females look different making them easy to ID. While other fish species are sexually monomorphic, meaning males and females look the same from the outside, and you need to observe behavior and shape during spawning to tell the sex.  

Let’s take a look at four of the main indicators that you can use to determine whether a fish is male or female. 

Overall Size

In some fish species, the female is a lot larger than the male. If we take blue and black marlin as an example, males will only grow to around 300 lbs and females can grow up to 2000 lbs. 

So, if you catch a blue or black marlin over 300 lbs, chances are it is a female. Of course, a fish of 200 lbs could be a young female or an old male, but the big ones are definitely female.  

Fin Size 

Fish hold their reproductive organs on their underside behind their stomach. In a lot of species, the anal fin (on the bottom and the back of the fish) is longer for males and shorter for females.

In some species, the dorsal fin is larger in the males than the females and the fins in general are often more colorful too.

For example, a Tench is quite easily determined, with males having prominent curved, and spoon like pelvic fins on their belly, and female tench have longer triangular shaped pelvic fins.

Fish Shape

If we take dorado, aka mahi mahi, as an example, the males and females are identical except for the shape of the head. 

Bull dorado (males) have a square shaped head whereas females have a rounded shaped head. 

See also

Some male fish of a certain species will also have a nuchal hump on their forehead which is a large bump. This makes it very easy to determine whether the male is female. 

Overall Visual Comparison

If you lay a male and female of equal size next to each other, determining the difference is much easier. As you look more carefully the female will typically be the one with the larger and round belly area.  

Summing Up

In most fish determining their sex is easy as looking at the telltale areas such as the fins, the shape, and their size. In some species, it can be quite difficult from a visual aspect.

During the spawning season, it is often easy to see the belly of a female fish being more round and plump. Females will be larger and rounder, and their vent area will be slightly swollen and puffed out, compared to the males which are all tucked in. 

During the non-spawning time, the male’s torso is usually smaller and thinner than the female’s. 

In summary, it is not always clear whether the fish you have caught are male or female. The easiest time to tell the difference is during the spawning season. However, overall size, fin size, the fish shape, and an overall visual comparison will more than often give you the answer.

If you have several fish of the same species together it becomes far easier to establish which one is male and which is female. 

You can read more articles on establishing the age of a fish and also how the length and weight vary within the seven species of Salmon to get more insights into the science of fish.

Steve Fitzjohn
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