Induced Spawning in Fish Farming: Hypophysation and Hormonal Induction
How hatcheries make captive fish spawn on demand: why it is needed, hypophysation with pituitary extract, modern GnRH analogue plus dopamine antagonist hormones, latency, and stripping.
Overview
Induced spawning is the use of hormone treatment to make mature fish ovulate and spawn on demand in a hatchery. Many important cultured species, including the major carps and several catfishes, ripen their eggs in captivity but do not complete the final maturation and ovulation without the natural environmental triggers they would receive in the wild. Hormone induction supplies the missing signal so the hatchery can produce fry predictably.
Why it is needed
In ponds, many cultured fish fail to spawn reliably. FAO notes, for example, that common carp does not spawn well in ponds that hold other fish, especially predatory species, and that natural spawning exposes eggs and fry to numerous small enemies. Inducing spawning under controlled conditions lets the hatchery time reproduction, collect and fertilize the eggs cleanly, and greatly increase the survival of the resulting fry.
Hypophysation with pituitary extract
The classic method is hypophysation: injecting an extract of fish pituitary gland, traditionally carp pituitary, which contains the gonadotropin that triggers ovulation. The injected hormones perform the same role as the fish's own hormones at spawning. The technique was established for Indian and Chinese carps in the 1950s and remains widely used. FAO describes a two-step dose for carp: a small priming dose, about 10 percent of the total, followed 18 to 24 hours later by the larger decisive (resolving) dose.
Modern hormone preparations
Today many hatcheries use purified or synthetic hormones instead of crude pituitary. Human chorionic gonadotropin (hCG) and gonadotropin-releasing hormone analogues (GnRHa) are common. In carps and other cyprinids, GnRHa is combined with a dopamine antagonist, because dopamine inhibits gonadotropin release in these fish; blocking it lets the GnRHa stimulate the pituitary to release luteinizing hormone (LH). Commercial combined products include Ovaprim and Ovopel. The LH surge in turn triggers the ovarian maturation-inducing steroid that leads to ovulation.
Latency period
The latency period is the time between the final injection and ovulation, and it depends strongly on temperature: it is shorter in warmer water and longer in cooler water. FAO reports that for common carp at an optimum of about 20 to 26 °C, ovulation occurs roughly 9 to 11 hours after the decisive injection, corresponding to about 200 to 220 degree-hours. The fish must be checked frequently as the expected time approaches so that ovulated eggs are collected promptly.
Stripping and fertilization
After ovulation the eggs are obtained either by allowing controlled natural release onto egg collectors or by hand-stripping, where gentle abdominal pressure expresses the eggs into a dry bowl. Milt is stripped from males, mixed with the eggs by the dry method, and water is then added to activate fertilization. FAO reports a typical success rate of about 60 to 70 percent for common carp hypophysation. Fertilized eggs are transferred to incubation jars or hatching units.