Do Fish Have Periods?

Sometimes here at USAngler, we get a really off-the-wall question. This is definitely one of them.

People are curious about fish reproduction, and they may have seen a fish bleeding from its cloaca and assumed it was a menstruating female. 

But that’s not how fish reproduction works. The quick answer is that fish do not have periods.

If you’d like to know more about how fish reproduce, keep reading!

Related: Do Fish Bite?

Fish Reproduction 101

Most species of fish have clearly defined males and females, but there are exceptions. Grouper, for instance, can switch from female to male in a single-sex environment, allowing reproduction. And even among species that have fixed sexes, pathogenesis, the self-fertilization of an egg, is an effective reproductive strategy.

All that’s to say that fish reproduction is amazingly diverse, so let’s dive into the details.

Ovuliparity

The most common reproductive strategy among fish is ovuliparity. Scientists explain that in many fish species, “the females release eggs into the water and they are immediately fertilized by sperm from the male.”

In this video, two bass - a male ( the smaller fish) and female (the larger fish) - mate. The female releases unfertilized eggs into the water and the male releases a cloud of sperm. Fertilization occurs in the water.

The vast majority of fish species reproduce in this way, including salmon.

Viviparity

Among some sharks, like the shortfin mako, blacktip reef shark, and lemon shark, males inseminate females, fertilizing eggs that are carried by the mother until live birth.

This reproductive strategy is called viviparity, and it’s rare among fish other than some species of shark. Depending on the species, mating occurs in specific locations across the world during specific times of year.

For instance, blacktip reef sharks come together in January and February off the coast of northern Australia to mate, drawn there by instinct.

In this video, you can watch a lemon shark giving birth to live young after gestating fertilized eggs for as little as 7 months and as long as 11 months, depending on the water temperature:

Sequential hermaphroditism

In some species, fish switch sex as they mature. Some species begin life as female, others as male, but what the process of sequential hermaphroditism shares is that as fish mature and gain weight, they move from male to female (protandric) or female to male (protogyny).

Among all species that are sequentially hermaphroditic, protogyny is the more common strategy.

But snook, the popular game fish, begin their lives as male, switching sex to female as they mature. 

As Brandon Shallop, a marine biologist, explains, “Common snook are protandric hermaphrodites, meaning they change from male to female after maturation. The gonads of common snook contain both male and female sex cells. Female gonads mature directly from mature male gonads shortly after a male has concluded spawning. This transition takes place between the ages of 1 and 7 years. As a result of this process, the majority of small common snook are male and the majority of large individuals are female.”

Parthenogenesis

Perhaps the strangest reproductive strategy among fish is parthenogenesis.

In this form of asexual reproduction, a female effectively fertilizes her own egg, with no need for a male at all. This has been documented in Atlantic blacktip sharks, bonnetheads, and zebra sharks.

But this seems to be an adaptation to the absence of males since Atlantic blacktips usually mate viviparously in early summer, carrying a fertilized embryo for 10 to 12 months before giving live birth.

Final Thoughts

There are a few other rare reproductive strategies we didn't cover, mostly among skates and rays and other non-game species.

But rest assured, fish reproduction is very, very different from what we see in humans, and female fish do not have periods.

As always, we’re here to answer any questions you might have, so please leave a comment below!