Fish - Inducing Reproduction - Anatomy & Physiology
Among the most significant advancements in the field of aquaculture during recent decades is the development of techniques to induce reproduction in fish. These techniques have allowed farmers to profitably breed and raise species that do not naturally reproduce in captivity, and to manipulate the timing of reproduction to suit production cycles.
Some species will not readily breed in captivity due to environmental or culture conditions that are different from those found in nature, such as water temperature or substrate type. These conditions may cause stress or may not provide the cues needed to complete the reproductive process.
Fish in captivity may not always reproduce at the most advantageous time, and alteration of the spawning cycle may be desirable. This allows a farmer to:
- Obtain fish outside of the normal spawning season either to lengthen time for grow-out or to produce hybrids with other species.
- Improve efficiency by getting fish to spawn on a predetermined date.
- Maximize survival by fertilizing and incubating eggs under hatchery conditions.
Where successful, techniques for altering the spawning cycle of fish have become a valuable tool.
There are two main strategies used to induce reproduction. Often the two strategies are used sequentially, the first to manipulate maturation, then the second to induce ovulation.
1. Provide an environment similar to that in which spawning occurs naturally.
- Catfish, for example, like to spawn in enclosed spaces such as hollow logs. A farmer can simulate this by putting milk cans in a pond.
- The presence of vegetation and an increase in temperature will usually work for goldfish. *
- Changing the photoperiod in a hatchery can accelerate or delay maturation and ovulation in many salmon and trout species.
2. Inject the fish with one or more naturally occurring reproductive hormones or their synthetic analogues.
- This is only effective in fish that are already in breeding condition and have mature eggs in which the germinal vesicle has migrated.
Induction using Hormones
Numerous hormones have been used to induce reproduction. Two methods have emerged over the past few years that seem to offer the best chance for success at the least expense. They are injection of a GnRH analog with dopamine antagonist, and injection of gonadotropin.
GnRH Analogue with Dopamine Antagonist
- Leutinizing Hormone Releasing Hormone (LHRH) is the name of a mammalian hormone that has been employed successfully to induce the reproductive hormonal cascade.
- Recently, synthetic analogues of LHRH (LHRHa) have been developed that are more effective.
- More pure and are not rapidly metabolized by fish.
- Remain active for longer periods.
- Under natural conditions, there is a feedback mechanism in fish that limits the release of gonadotropin.
- This mechanism uses dopamine, which inhibits the action of LHRH.
- When dopamine is present in the fish, even LHRHa will have only limited success.
- A dopamine antagonist is often used to limit the effects of dopamine.
- When LHRHa and a dopamine antagonist are used in conjunction, reproductive success dramatically increases.
Two types of gonadotropin extracts have been used to induce ovulation in fish: Human Chorionic Gonadotropin (HCG) and fish pituitary extract.
- Pituitary extracts are made by removing the pituitary from a fish and extracting the hormones, which may then be injected into another fish.
- Carp, catfish, salmon and other fish have been used.
- HCG offers three major advantages over the pituitary extract:
- Less expensive
- More stable and thus has a longer shelf life.
- Comes in a purified form
Fish with gametes that have not yet been released by the gonads are called “green”. The term “ripe” is used to describe fish with gametes that have been released from the ovary into the ovarian lumen. Ripe fish can be stripped, green fish cannot.Brooders should be fast growing, disease free, and sexually ripe. There are several indicators of ripeness:
- The abdomen is rounded and soft.
- The genital opening is swollen, protruding, and reddish.
- The anus is often also swollen and reddish.
- Secondary sexual characteristics are evident.
- Milt (seminal fluid of fish) is released when the abdomen is pressed gently.
- Secondary sexual characteristics are evident.
Stripping (removing) the eggs or fry will decrease the time in which is takes for the parents to spawn again, thus increasing the stock of fry.
- Stripping should be carried out as soon after ovulation as possible.
- Fish should be anesthetized and examined 6 to 12 hours after the final injection.
- Before stripping, both the male and female should be cleaned and dried around the vent with a soft towel because residual anesthetic will kill sperm.
- Water, faeces and blood should not be allowed to mix with the gametes as they are stripped.
- To strip the fish, the female should be held around the caudal fin with one hand, while applying slight pressure to the abdomen with the other hand.
- If ovulation has occurred, a stream of eggs will emerge.
- If only a few appear, the female is still green and should be returned to the holding tank.
- If there is a stream of eggs, the abdomen should be messaged from front to back to strip out all the eggs.
- Grasp the male and gently squeeze the abdomen.
- Strip the milt into a container with water.
- Mix the milt with the eggs stripped from the female.