The demand for fish for food, recreation, and ornamental aquariums is steadily increasing. Natural fish populations have declined during the last several decades because of environmental degradation and over-fishing.
This has resulted in an increased effort in the development of techniques for hatchery production of fish. Traditional aquaculture species such as trout, catfish, common carp, golden shiner, and goldfish reach sexual maturity and spawn in hatcheries or ponds, when conditions are appropriate.
However, a number of fish species that have or potentially have great economic significance for aquaculture do not reproduce spontaneously in captivity. Many of these fish spawn in environments that are nearly impossible to simulate in a hatchery.
Hormone-induced spawning is the only reliable method to induce reproduction in these fishes. Hormone-induced spawning of fish has been used for almost 60 years.
Surprisingly, the same procedures, with only minor modifications, have been used to spawn an entire range of fishes from the ancient sturgeon and paddlefish to carp, catfish, salmon, sea bass, redfish, snook, and mullet. In addition to breeding other desirable fish species, induced spawning can be used to:
- produce hybrids that are different from the parent species;
- produce sterile polyploid fish (for example, sterile triploid grass carp for aquatic weed control);
- synchronize reproduction of large numbers of fish for simultaneous spawning, thereby simplifying production and marketing of the fish;
- produce fry outside the normal spawning season for maximum hatchery production and to provide fish when the price and market demand is greatest; and
- maximize survival of fry under controlled hatchery conditions.
Proper fish handling:
The physical injury and physiological stress of capturing, handling, transporting, injecting, and holding brood fish can have a greater detrimental effect on spawning success than almost any other factor. Fish must be handled carefully and optimum water conditions must be maintained to minimize stress.
The importance of proper handling and water quality cannot be overemphasized. Female brood fish ready for spawning are in a particularly delicate condition. When female fish are stressed or injured, they may undergo rapid physiological changes that can result in the breakdown (resorption) of eggs in the ovary.
Crowding, dissolved oxygen depletion, rapid changes in temperature, and osmotic imbalance are well known causes of stress and must be avoided. Suboptimum conditions, while not immediately lethal, may stress brood fish, resulting in delayed mortality or failure to spawn.
Reducing stress and injury to brood fish can greatly increase the success of hormoneinduced spawning.
Determine sexual maturity:
The external appearance of brood fish has long been used to assess the stage of sexual development. In some species, males change in appearance during the spawning season. These physical changes make it relatively easy to identify sexually mature males.
However, secondary female sex characteristics such as plumpness of the abdomen and redness of the vent are extremely subjective and can be misleading. Sampling the eggs and sperm of the brood fish eliminates the guesswork in determining the stage of sexual development.
Milt can usually be stripped from males of most species when they are ready for spawning by applying gentle pressure to the abdomen between the pelvic fins and the vent. Sperm viability usually can be determined by observing motility with a microscope.
Several methods are available to determine the developmental stage of the eggs in the fish’s ovary. The diameter and appearance of the egg and the position of the nucleus in the egg are visual indicators of development. The steroid assay procedure determines the physiological response of the eggs to hormones. Both require that an egg sample be collected.
The ovary can be sampled with either a rigid or flexible tube (catheter). An egg sample can also be taken by making a small incision along the belly or side of the fish. This technique is commonly used for sturgeon and paddlefish.
An understanding of sperm viability and egg stage development will greatly improve the success of hormone-induced spawning of fish.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
