Reproduction in fishes is regulated by external environmental factors that trigger internal mechanisms.
The final event of the reproductive cycle, the release of eggs and sperm resulting in spawning, can be controlled by either placing the fish in an appropriate environment or by changing the fish’s internal regulating factors with injected hormones or other substances.
The internal mechanisms that regulate spawning are similar for most fishes. The external environmental factors that control reproduction, however, vary considerably among species.
For this reason, more is known about the internal regulatory mechanism of fish reproduction than the specific environmental requirements for spawning each species.
Environmental factors that have been shown to play a significant role in the reproductive cycle are:
- photoperiod
- water temperature
- water quality (e.g., dissolved oxygen, pH, hardness, salinity, alkalinity);
- flooding and water current
- tides and cycles of the moon
- weather cycles (e.g., atmospheric pressure, rainfall)
- spawning substrate (e.g., aquatic plants, sticks, gravel, spawning caverns)
- nutrition
- disease and parasites
- presence of other fish
These factors do not function independently of each other, but are interrelated. The internal mechanism that regulates the process of reproduction in fish is the brain hypothalamuspituitary- gonad chain (Figure 1).
Hormone-induced spawning techniques influence this sequential mechanism at several levels, by either promoting or inhibiting the process. The primary substances used for hormone- induced spawning have been: (1) pituitary extracts and (2) purified gonadotropin to stimulate the ovaries and testes; (3) LHRH analogs (LHRHa) alone or in combination with (4) dopamine blockers which enhance the potency of LHRHa to stimulate the pituitary; or (5) steroids to stimulate the gametes directly.
The appropriate hormone preparation should be selected on the basis of the species to be spawned and the availability y of the hormones.
Preparation of hormones:
The hormones must be mixed and stored properly to prevent contamination and preserve potency. The proper dosage must be calculated for the brood fish, and the optimum injection schedule must be used for best results. To calculate the proper dosage, (1) the recommended dose, (2) approximate weight of the brood fish, and (3) the desired volume of the injection must be determined.
The quantity of hormone to be injected can then be calculated from the weight of each individual brood fish and the appropriate injection schedule.
Taking the spawn:
The eggs and milt of fish can be taken by several different methods: (1) tank spawning; (2) hand stripping, and (3) surgically removing the eggs. The method of choice depends on the fish species, hatchery facilities, experience and skill of the hatchery staff, and the desired manipulations of eggs, sperm, or fertilized eggs.
Tank spawning is the simplest method for obtaining a hormoneinduced spawn. Brood fish of both sexes are placed together in the spawning tank following injection( s). The female ovulates when she is physiologically ready. The males stimulate the female to release the eggs and fertilize the spawn.
Hand stripping is commonly used for taking the spawn of many species of fish. Brood fish are separated by sex prior to hormone injection to prevent spawning in the holding tank. It is important to determine the exact time of ovulation when hand stripping.
In many species, egg quality can deteriorate rapidly if the eggs are not taken shortly after ovulation. For most species, ovulation can best be verified by checking the female to determine when eggs flow freely from the vent. To strip the eggs, the fish is held slightly on her side, tail down; gentle hand pressure is applied to the abdomen, moving toward the vent. The stream of eggs is directed into a clean, dry bowl positioned so that water from the fish does not drip onto the eggs.
It is important to insure that no water comes in contact with the eggs until after the milt is added and mixed. Water activates the sperm and also causes the opening through which the sperm enters the egg to close. Because the internal anatomy of fish vary greatly, hand stripping may be difficult in some species.
Sturgeon and paddlefish have no ovarian sac; the eggs are released into the abdominal cavity during ovulation. The best method for taking the spawn of these species is to surgically remove the eggs. For delicate species that seldom survive the rigors of hand stripping, humanely killing them and surgically removing the eggs may be the best option. In addition, more eggs can usually be obtained by this method than by hand stripping.
Fertilizing the spawn:
The eggs obtained by hand stripping or surgical removal are usually fertilized with fresh milt. Males are captured, wiped off, and held belly down over the bowl containing the eggs. The portion of the abdomen posterior to the pelvic fins is gently massaged to extrude the milt onto the eggs.
Milt can be collected from males and stored up to three weeks prior to stripping eggs. Ovulated eggs of many species such as white bass, sturgeon, paddlefish, and common carp become sticky after water is added. During natural spawning, this stickiness causes the eggs to become attached to rocks, sticks, or aquatic plants. Catfish eggs are connected by a sticky matrix that holds the eggs together in a mass.
In the hatchery, this stickiness causes problems during incubation. Siltclay, Fuller’s earth, or bentonite suspension, urea and salt solution, and tannic acid solution are preparations commonly used to deactivate the sticky layer of fish eggs. In addition, the gelatinous matrix of catfish egg masses can be dissolved with sodium sulfite so the eggs can be incubated in hatching jars.
Induced hatchery spawning of fish requires a continuous series of decisions, any of which if improperly made, can diminish or completely obliterate the success of the project. There are many ways to fail at each step and only a very few that are productive. Consistent performance requires strict attention to detail.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
