Induced Reproduction in Fish
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; and
- 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.
Background: Reproduction in Nature
In fish, the reproductive process involves three basic steps:
- maturation - the development of the gametes (eggs and sperm) to a point where fertilization can occur;
- ovulation - the release of eggs from the ovary; and
- spawning - the deposition of eggs and sperm so that they can unite.
In fish, as with all higher animals, hormones play a critical role in the reproductive process. Hormones are chemical messengers released into the blood by specific tissues, such as the pituitary gland. The hormones travel through the bloodstream to other tissues, which respond in a variety of ways. One response is to release another hormone, which elicits a response in yet another tissue. The primary tissues involved in this hormonal cascade are the hypothalamus, pituitary gland, and gonads (Fig. 1).
Fish have evolved to reproduce under environmental conditions that are favorable to the survival of the young. Long before spawning, seasonal cues begin the process of maturation. In many fish, this can take up to a year. When the gametes have matured, an environmental stimulus may signal the arrival of optimal conditions for the fry, triggering ovulation and spawning. Examples of environmental stimuli are changes in photoperiod, temperature, rainfall, and food availability. A variety of sensory receptors detect these cues, including the eye, pineal gland (an organ in the dorsal part of the forebrain that is sensitive to light), olfactory organs, taste buds, and thermoreceptors.
The hypothalamus, located at the base of the brain, is sensitive to signals from sensory receptors and releases hormones in response to environmental cues. Principal among these hormones are gonadotropin releasing hormones (GnRH), which travel from the hypothalamus to the pituitary gland. The pituitary is responsible for a wide variety of functions, including growth and reproduction. Certain cells of the pituitary receive GnRH and release gonadotropic hormones into the bloodstream. The gonadotropic hormones travel to the gonads, which synthesize steroids responsible for final maturation of the gametes.
Maturation of the egg is a long process that involves complex physiological and biochemical changes. One important step, vitellogenesis, is a process in which yolk proteins are produced in the liver, transported to the ovary, and stored in the egg, resulting in tremendous egg enlargement. The yolk is important as a source of nutrition for the developing embryo.
Also critical are germinal vesicle migration and germinal vesicle breakdown (GVBD). Before it migrates, the germinal vesicle, or nucleus, is located at the center of the egg in an arrested stage of development. At this stage, the egg is physiologically and genetically incapable of being fertilized, even though it has the outward appearance of a fully mature egg. When conditions are appropriate for final maturation, nuclear development resumes, and the germinal vesicle migrates to one side. Finally, the walls of the germinal vesicle break down, releasing the chromosomes into the cell.
The maturity of eggs can be determined using biopsy techniques. Eggs are removed from the ovaries, cleared with a prepared solution, and viewed under a microscope. In mature eggs, the migration of the germinal vesicle to the side of the cell will be complete.
After the egg has matured, a class of compounds called prostaglandins are synthesized. These stimulate ovulation, which is the rupture of the follicle cells that hold the egg. The egg is then released into the body cavity or ovarian lumen, where it may subsequently be released to the outside environment. Following ovulation, the viability of the eggs can decrease rapidly.
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.
There are two main strategies used to induce reproduction. The first is to 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.
The second strategy is to inject the fish with one or more naturally occurring reproductive hormones or their synthetic analogs. This is only effective in fish that are already in breeding condition and have mature eggs in which the germinal vesicle has migrated. Often the two strategies are used sequentially: the first to manipulate maturation, then the second to induce ovulation.
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 Analog 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. In recent years, synthetic analogues of LHRH, referred to as LHRHa, have been developed that are far more effective. Because they are purer and are not rapidly metabolized by fish, they remain active for longer periods.
Under natural conditions, there is a feedback mechanism in the fish that limits the release of gonadotropin. This mechanism uses a chemical called 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 for this. HCG offers three major advantages over the pituitary extract: 1) it is much less expensive, 2) it is more stable and thus has a longer shelf life, and 3) it comes in a purified form.
The details of inducing spawning differ from species to species. They will also differ according to the goals and means of the farmer. It must be emphasized that the following techniques do not apply to all situations. Farmers should always thoroughly research the techniques that have been developed for their species of fish and select those that best fit the circumstances.
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 is released when the abdomen is pressed gently.
- Secondary sexual characteristics are evident.
Brooders must be handled with extreme gentleness. They are physically very vulnerable during spawning and may die if dropped or roughly handled. Often, a good deal of the farmer’s resources has gone into developing the broodstock line and keeping the fish healthy. The potential return from this investment can easily be eliminated through sloppy handling.
When the brooders are kept in ponds or pens, all water quality parameters, such as temperature and oxygen, should be kept at optimal levels. A satisfactory amount of high quality, well balanced feed should be provided. If the fish are moved from one set of conditions to another, time must be allowed for acclimation. When handling the fish, gentle firmness should be the rule. Covering the fish with wet burlap or cloth, or covering their container to cut off light, will help to keep them calm. Tranquilizers may be used when fish are to be injected or stripped.
Injecting the Fish
There are two common places to inject hormones into a fish. An intraperitoneal (within the body cavity) injection is given through the ventral (bottom) part of the fish behind either the pelvic or pectoral fin (Fig. 2). Intramuscular (within the muscle) injections are commonly done on the dorsal (upper) part of the fish above the lateral line and below the anterior part of the dorsal fin (Fig. 3). In either case, it is important to place the needle so that it slides under the scale rather than through it.
Two dosage levels are commonly used: a preparatory dose and a decisive, or final, dose with a time gap generally of 12 to 24 hours between the two injections. The preparatory dose brings the fish to the brink of spawning, and the decisive dose induces ovulation. In general, the preparatory dose is about 10 percent of the total dose. For some fish, several preparatory doses may be necessary.
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, fish “slime”, feces, 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.
The following procedures provide examples of a variety of techniques used to induce spawning.
Some fish can be induced to spawn simply by providing an appropriate environment. An example is furnishing spawning substrate for golden shiners, which naturally lay their eggs on vegetation. There are two common ways of doing this. The first is to plant natural vegetation around the shore. The second is to place spawning mats in the pond. The advantage of spawning mats is that once the eggs are laid, the mats can be removed and placed in another pond for grow-out. One common method for constructing these mats is to sandwich Spanish moss or a similar material inside steel-welded wire. A piece of 2” by 4” steel mesh is folded in half so that the dimensions of the mat are about 20” by 40”. The Spanish moss is placed inside the fold, and hognose rings are used to hold the bottom together. The following is adapted from Third Report to the Fish Farmers, published by the United States Fish and Wildlife Service.
- Twenty to forty pounds of brood stock may be stocked per acre.
- Plankton blooms can inhibit spawning. If heavy blooms occur, the pond should be flushed with fresh water.
- If grass is to be planted, the water level in the brood pond should be lowered during early spring to allow growth along the shoreline. Once the grass has been established, the ponds should be refilled.
- If spawning mats are to be used, they should be placed in shallow water, about one inch below the surface with one side at the edge of the pond. Several mats may be placed end-to-end.
- As each mat fills with eggs, it should be transferred to a rearing pond for grow-out. Care must be taken so that the mats do not become over-populated with eggs, because they may eventually smother each other or be attacked by fungus.
- If spawning activity diminishes quickly, a rapid rise in the water level may extend activity.
In nature, trout build their nests in gravel and require cold, flowing, aerated water to reproduce. Changes in photoperiod are critical for bringing the fish to spawning condition. The spawning season varies with locality, temperature, and the species or strain of fish. Brook and brown trout are fall spawners, while rainbow trout spawn in the spring. The following steps, summarized from Fish Culture Manual, published by the Alaska Department of Fish and Game, illustrate one method for spawning trout by stripping females and mixing the eggs with milt. Many other methods also exist.
- Anesthetize the fish. Placing them in MS-222 at a concentration of 130-260 mg/l (assuming temperature between 4¡C and 15¡C) is common. The solution should be within 5¡C of the holding or rearing containers, and the pH should be between 6 and 8.
- Holding the caudal peduncle with one hand and supporting the head with the other, dip the fish in clean water to wash off the anesthetic.
- Strip the female’s eggs into a dry, shallow container. The fish should be held belly down in a horizontal position with the head slightly higher than the tail. Gently press upward near the base of the pelvic fin to start the flow of eggs. Slowly stroke the belly, moving the hand from front to rear using constant pressure. Repeat this procedure until no more eggs are obtained.
- In the dry method of fertilization, sperm from the males is poured onto the eggs. Water should not come into contact with the mixture for it will inhibit fertilization by the sperm. The sperm, eggs, and ovarian fluid should be gently mixed with a feather.
- After 30 seconds, fertilization will have taken place. Add enough water to cover the eggs and mix again.
- Let the fertilized eggs soak for 30 seconds.
- Rinse the fertilized eggs in clean water.
- Water harden the eggs. This is a process whereby the eggs absorb water and grow about 20 percent. Place the eggs into a container that will accommodate them as they grow. Add water, then do not disturb for an hour.
- Put into incubators.
Temperature is a determining factor in the control of catfish reproduction. Spawning activity begins after water temperatures stay consistently above 70¡F. It drops off when temperatures fall below 70¡F or rise above 85¡F. There are a variety of methods commonly used for spawning catfish, including spawning in ponds, spawning in pens, and hand stripping. In many, though not all cases, hormone injection may be desirable. The following procedure from Third Report to the Fish Farmers provides an example of spawning induction with hormones followed by hand stripping.
- Anesthetize the fish with MS-222 at a concentration of 80 mg/l.
- If the fish are kept in a holding tank, loosely stitch their mouths closed to prevent them from biting one another. The thread must be loose enough to allow respiration.
- Using an 18-gauge needle, inject intraperitoneally with: a. 3.5-4.0 mg of acetone-dried pituitary gland extract per kilogram of fish for the female and about 2 mg per kilogram for the male, or b. 500-800 international units of human chorionic gonadotropin (HCG) per pound of fish.
- If necessary, inject again 48 hours later. Usually, the female ovulates within 16-20 hours after an injection.
- Strip the eggs and the milt into a 0.3 percent solution of common salt.
- Very gently mix the eggs and milt. Fertilization should occur in 2-5 minutes.
- Place the egg mass in a suitable incubator for hatching.
- After 10-12 hours, stop the water flow and treat with a 0.3-0.5 percent solution of alkaline protease enzyme for 2-3 minutes while gently stirring. This will dissolve the sticky layer of the eggs and allow them to float free.
- When the eggs begin hatching, they should be taken out and placed in a flat tray.
Many species of fish will not readily reproduce under certain culture conditions. Others will, but not necessarily when the farmer desires. In these cases, induction of spawning can be of great value.
Two techniques are commonly used, sometimes in conjunction with one another. The first is manipulation of the culture environment to mimic some important quality in the fish’s natural environment. The second is injection of hormones to stimulate spawning. The hormones may be natural hormones taken from fish or other animals, genetically engineered from bacteria, or synthetic analogs of naturally-occurring hormones.
Methods vary from species to species and situation to situation. However, at least two generalizations can be drawn. First, brooders are very vulnerable to rough handling. Care should always be used to avoid damaging these valuable animals. Second, a fish that does not have mature gametes will not produce viable eggs or sperm no matter how many times it is injected with hormones. Ripeness is the result of environmental factors working over a period of time, leading to maturation of the gonads and production of viable eggs.
Many procedures have been developed for inducing fish to undergo the last steps of spawning. Farmers should thoroughly research the procedures that have been developed for their species of fish through experimentation, and select those that best suit the circumstances. In addition, once the fish have spawned, there are many techniques involved in incubating and caring for the eggs, and caring for the hatched fry. These too must be thoroughly researched.
References and Recommended Readings
- Fish Culture Manual. 1983. Staff, F. Alaska Department of Fish and Game, Division of Fisheries Rehabilitation, Enhancement and Development.
- Third Report to the Fish Farmers. 1984. Dupree, H. K. and J. V. Huner, eds. United States Department of the Interior, Fish and Wildlife Service.
- Fish Hatchery Management. 1989. Piper, R. G. et al. United States Department of the Interior, Fish and Wildlife Service.
- Hatchery Manual for the White Sturgeon Acipenser transmontanus RICHARDSON with Application to Other North American Acipenseridae. 1988. Cooperative Extension, University of California, Division of Agriculture and Natural Resources.
- A chemical that inhibits the release of hormones from the pituitary and thereby blocks the pituitary’s response to injected LHRHa.
- Dopamine antagonist:
- A family of drugs that block action of dopamine.
- Sperm and eggs.
- An organ in which either sperm or eggs are produced; general name for either the testes or ovary.
- A pituitary hormone that controls the production by the gonads (testes and ovary) of sperm and eggs.
- Gonadotropin Releasing Hormone (GnRH):
- A hormone produced by the hypothalamus that stimulates the pituitary to release gonadotropin.
- A chemical formed in endocrine glands that affect the functions of specific tissues.
- Human Chorionic Gonadotropin (HCG):
- A commercially available, semi-purified hormone that is used to induce ovulation and spermiation, i.e., egg and sperm production.
- A part of the brain that controls many internal body functions and the activity of the pituitary gland; produces gonado-tropin releasing hormones.
- Intraperitoneal injection:
- Injection into the abdominal cavity.
- Leutinizing Hormone Releasing Hormone (LHRH):
- Mammalian gonadotropin that has been used to induce the reproductive cascade in fish.
- The cavity of a tubular organ.
- The growth and development of the gametes to a point where they are ready to fuse to form a fertilized egg.
- The release of eggs from the ovary.
- Pituitary extract:
- An aqueous, alcoholic, or acetone extract of the pituitary gland used for artificial induction of spawning.
- Pituitary gland:
- An endocrine or hormone-producing gland found on the underside of the brain just behind the eyes.
- A class of compounds that, among other things, stimulate ovulation. Spawning: The deposition of eggs and sperm such that they unite to form a fertilized egg.
- Any of a large group of hormones, some of which are involved in final maturation of gametes.
- The male reproductive organs.
- A steroid hormone produced by the testes; along with ketotestosterone, it is responsible for the development of male secondary sex characters.
- A stage of egg development in which the yolk is stored, resulting in tremendous enlargement.
Funding for the Minnesota Aquaculture Development and Education Program was approved by the Minnesota Legislature (ML 1989, Chapter 335, Art. 1, Sec. 29, Subd. 11(P)) as recommended by the Legislative Commission on Minnesota Resources from the Minnesota Future Resources Fund. The funding ran from July 1, 1989, to June 30, 1991. It was a cooperative effort between the University of Minnesota, state agencies and the private sector. Researchers and extension staff of the University of Minnesota Department of Fisheries and Wildlife and Minnesota Sea Grant College Program designed and ran the program. Cooperators from the private sector provided the facilities and volunteered their time for day-to-day management of the fish. Other groups involved included NRRI Business group, and the Minnesota Extension Service.
by Jeff Mittelmark and Anne Kapuscinski