How Do Fish Reproduce?

This is the fourth edition of our 'I want to know about...' series! This week it's all about: Fish Reproduction! 

There are around 32,000 known species of fish in our oceans - and likely more considering how little of the ocean we have actually explored! All fish are connected in that they share two major common traits; they live in water and they have a backbone (ie they are vertebrates). However, they are not all the same in how they reproduce. In the whole world, marine and terrestrial, 99% of the known species lay eggs - the majority of this percentage being fish. 

In fish, fertilisation occurs internally and externally. We covered some of the internal ways in our How Do Sharks Reproduce blog, however in this one we will focus mainly on fish that lay eggs and have them fertilised externally. The development of the embryo can similarly happen internally/externally and sometimes it changes at various stages - it all depends on the species of fish. 

Fish tend to be broadly separated into 1 of 3 categories:

  • Guarders

Obviously, this when the fish will guard their eggs after fertilisation. Usually guarders will make a nest, and the male protects them by chasing away predators until they hatch and leave.

  • Nonguarders 

This is the opposite to guarders - they usually scatter their eggs which makes them nearly impossible to guard. The majority of freshwater fish will not guard their eggs, as they may lay thousands! 

  • Bearers 

These types of fish will carry their eggs/young as they develop - either inside the female or in the mouth of either parent (this will depend on the species). 

Although most fish will lay eggs and fit into one of the 3 categories mentioned above, within these 3 categories their breeding and nesting strategies overall can contrast.


Nonguarders tend to also fall into 2 categories - pelagic spawners and benthic spawners. Put simply, pelagic spawners will spawn towards the surface of the water in open water, whereas benthic spawners will spawn on or near the bed of the water. 

  • Egg Scatterer

The scattering method is used by a lot of fish species. This method is when the female will scatter her eggs in high quantities and the male then spreads his milt (fish semen) in the same area in the hopes of fertilising them. This method does not take much energy from either parent. Due to the eggs being scattered, there may be decreased chances of a predator finding them and eating them in (in comparison with a nest), however on the other hand this method has a lower success rate than others because the male will not be able to fertilise all the eggs when they are in different locations. They are also open to predation as they are not guarded or specifically hidden. 

  • Open Spawning 

This is very similar to the scattering method, however many more fish will be involved as opposed to just 2 parents. Usually a method used by schooling fish, they will enter a sort of frenzy and the eggs and milt are expelled all at once in a large group. Due to the frenzied chaos, they are open to predation during this method. However, it does allow for high levels of genetic variation within the species - many males have been observed to be aggressive during this time too, which means that the strong male’s genes will be the most likely to fertilise the eggs. 


  • Nest spawners 

This method is highly competitive between fish, and it is probably where the most territorial behaviour is seen. This type of fish will actively clean out a site they deem suitable, and then make a nest there. Many fish use the process of building a nest to attract a mate, and once the eggs are fertilised they will guard the nest until the eggs hatch. 

  • Bubble nest builder

This strategy is quite remarkable. It begins in the mating ritual stage - the male will create a bubble nest on the surface of the water by spitting small bubbles towards the surface. The making of the nest is sometimes audible and usually a frantic occurrence triggered by the presence of a female. The male’s nest will vary in size and thickness, mostly it depends on the size, territory and personality of the fish - a large confident male will make a larger nest. After attracting a female, spawning and fertlising the eggs, the male will pick the eggs up in his mouth and spit them into the bubble nest. It is his responsibility to defend the nest until the eggs have hatched. The newly hatched fry (fish babies) will be cared for by the parents until they are independent (this time frame varies depending on the species of fish - it could be one day, it could be several weeks).

  • Substrate Spawner

Substrate spawners are very similar to nest spawners, but there are very small differences. Although substrate spawners will clean a chosen area suitable for laying their eggs (and maintain this area), they do not however actively build a nest in this area. 

Brood Hiders

Brood hiders tend to be categorised between guarders and nonguarders. This is because whilst they create a nest for their eggs to be placed, they do not guard the nest once it is made nor do they partake in any parental care. They tend to be benthic spawners that bury their eggs to keep them safe. A species of fish called bitterlings have a remarkable brood hider strategy, as they deposit and fertilise their eggs between the gill filaments of mussels. The female may do this to multiple mussels, ensuring that her eggs are not all concentrated in one place. After 3-4 weeks once the larvae have hatched, they will swim away from the mussels to start life on their own. 


  • Mouth Brooders 

Mouth brooders are a type of external bearers. There are two types of mouth brooders. The first are called ovophiles. Ovophile females will dig a pit in which to lay their eggs, and then the female will suck the eggs into her mouth. The eggs will be fertilised whilst they are in the female’s mouth. The eggs will hatch in her mouth and the fry will stay there until they are ready to leave and care for themselves. An example of an ovophile fish would be a Mbuna Cichlid. 
The other types of mouth brooder fish are called larvophiles. These types of fish will lay their eggs and guard them until they hatch, and then the newly hatched fry are carried in the mother’s mouth until they are ready. 

  • Livebearer 

This subcategory includes viviparous fish, obligate internal bearers and facultative internal bearers. We covered this in our [HOW DO SHARKS REPRODUCE]. 

Because there are so many species of fish in our oceans, it isn’t possible that they all conform to these types of breeding. There are a few outliers and anomalies that don’t fit into any of the categories that we discussed here, so check out our part 2 to this blog about those fish!

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