Fish Breeding and Larval Rearing
Fish Breeding
Ovoviviparous vs. Viviparous
Fertilization, as with viviparous fish, is internal for ovoviviparous fish. However, the eggs are retained within the female until hatching. The developing fish are nourished by the egg yolk, not directly from the mother.
In viviparous fishes, fertilization is also internal. Unlike ovoviviparous fish, each embryo receives nutrients directly from the mother’s body, not from an egg reserve. These fish give birth to fully formed offspring.
Internal Fertilization
Fertilization occurs internally. In some species, like the Pecile, the male has a modified fin that acts as a copulatory organ. In other species, secondary sexual characteristics like color, body size, or fin shape and size are used to attract mates.
Selection of Good Breeders
Identifying Ready-to-Spawn Fish
Carefully select fish that are ready to spawn. Look for these characteristics:
- Males should release a few drops of milt when their abdomen is gently squeezed.
- Females should have a swollen, lumpy, red/pink genital opening and a well-rounded, soft abdomen, indicating developed gonads.
Maturation of Eggs
Ovulation Period
The time required for eggs to mature (the period of ovulation) is measured in degree-hours (dh). This is calculated by adding the water temperature (°C) at the end of each hour.
Egg Development and Incubation
How Fish Eggs Develop
Egg development begins immediately after fertilization and contact with water. It progresses through several stages until the hatching of fish larvae. This is the incubation period.
Note: The adhesiveness of eggs in some species (e.g., common carp, African catfish, tilapia) develops upon contact with water, reaching its peak after 30-60 seconds.
Three Main Phases of Egg Development
- Swelling of the Egg Stage: Fertilized eggs absorb water, developing the perivitelline space. The animal pole sits atop the yolk. The eggs swell and become larger.
- Division and Development Phase of the Germ: The animal pole undergoes cell division, forming a multi-layered blastoderm. This includes the blastula and gastrula stages.
- Stage of Embryonic Development: The embryo develops around the yolk, forming head, tail, and visible eyes. Movement increases until hatching.
Types of Incubators
- Simple Incubators
- Double Compartment Incubators
- Gutter Incubators
- Cube Incubators
- Incubators in Plastic Bottles
Larval Rearing
From Larva to Fry
Larval rearing encompasses the period from hatching until the larva:
- Inflates its swim bladder with air
- Begins to swim
- Starts to consume external food
At this point, the larva becomes a fry.
Development and Growth
During this period, larvae primarily develop their food and respiratory organs. The yolk sac provides nutrients until it is fully absorbed. The duration of this period depends on the initial yolk sac size and the larval development rate, which is influenced by water temperature. The larval rearing period is measured in degree days (dd).