Fishes are oviparous organisms. This means that they do not give birth to young fishes, instead, they lay eggs which further develop into young fishes.

The eggs that they lay are unfertilized. The female fishes lay unfertilized eggs and the male fishes produce sperms. Both these gametes float in water. When a sperm comes in contact with the eggs, there is a fusion of nuclei between the two. Hence fertilization occurs. As this fertilization takes place outside the body of the organism, it is called external fertilization.