Following hormone injection, the eggs and milt of fish can be taken by several different methods:
- tank spawning,
- hand stripping, and
- 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.
Taking the eggs:
Ovulation is the final phase of normal egg development. The time between the final or resolving dose of hormone and ovulation is referred to as the latency period. This is usually dependent on the species of fish, water temperature, and hormone preparation used, It is especially important to know the latency period when hand stripping or surgically removing the eggs. Check the literature for the latency period for the fish species you are spawning.
During ovulation, the connection between the female fish and the eggs in the ovary is eliminated. In warm-water fishes, egg quality can deteriorate rapidly if eggs are not taken shortly after ovulation; they become “overripe” and can no longer be fertilized. In general, the eggs of tropical and sub-tropical species of fish become overripe more quickly than those that spawn at cooler water temperatures.
The eggs of cold-water species remain viable for several days after ovulation. Table 1 presents the reported maximum period between ovulation and the deterioration of egg quality for some species of fish commonly spawned by hormone injection.
Tank spawning:
Tank spawning is the simplest method for obtaining a hatchery spawn. Brood fish of both sexes are placed together in the spawning tank following injection(s). Brood fish should not be disturbed and subdued lighting is recommended.
The female ovulates when she is physiologically ready. The males stimulate the female to release the eggs and fertilize the spawn. Better fertilization occurs if males are accustomed to the tank, and have been injected with a preparatory dose of hormone several days prior to and again at the same time as the female. Males can be used for several tank spawns, week after week, until their milt flow diminishes.
Unless the males are aggressive toward each other, it is advisable to put two or three males for each female in a tank to ensure fertilization. If the spawning tank is of sufficient size, more than one female may be spawned in the same tank.
The presence of other individuals may help stimulate fish that are mass spawners. However, too many breeders in a small tank might be disruptive to the spawning process. A round tank is advantageous for species with non-sticky, floating or semi-buoyant eggs that spawn in a river or estuary.
The circular flow simulates the current in which these fish naturally spawn. The vigorous swimming action of the female in the swift water current is believed to assist in emptying the ovaries. The eggs are carried with the drain water from the spawning tank to a screened collector (Figure 1). Eggs are then transferred to an incubator.
Nest breeders and substrate spawners can also be tank spawned if suitable nesting sites or spawning material are provided. When tank spawning species that scatter sticky eggs, it is advisable to place spawning mats or brushes on the bottom of the tank.
The eggs will attach to the substrate. Brood fish are removed from the tank after spawning, unless they provide parental care to the eggs. Fertilized eggs are usually incubated in the spawning tank.
Tank spawning has both advan- tages and disadvantages. Advan- tages are:
- skill on the part of the hatchery staff in predicting the exact time of ovulation or checking females to verify ovulation is unnecessary
- rapid deterioration of eggs in the ovary after ovulation is not a problem
- there is less potential of injury to the brood fish because it is unnecessary to check and strip the fish; and
- less labor is required.
Disadvantages are:
- a screened egg collector or suitable spawning substrate is required;
- dirt and debris may be mixed with the eggs, potentially causing problems during incubation
- some females may not release all their eggs
- it is more difficult to accurately estimate the number of eggs
- the surface area of substrate in tanks is insufficient for large species that release large quantities of sticky eggs. The-eggs clump together, resulting in fungus problems and poor hatch;
- this method cannot be used if induction of polyploidy or other manipulations of eggs, sperm, or fertilized eggs are desired.
Authors:
R.W. Rottmann, J.V. Shireman, and F.A. Chapman
