Development of brood stock
Although brood stock for propagation can be taken from natural open waters or from culture installations, most of the rainbow trout brood stock is obtained from farms. In Europe, brown trout brood fish are caught from open waters when they start migrating upstream for spawning. Many trout farms have develop their own strains of trout with specific characteristics for rapid growth, better food conversion, early or late maturity, larger egg size, etc. Scientifically planned selective breeding of rainbow trout has been carried out in a number of areas, as a result of which strains have been developed through several generations, with high percentages of spawning at an age of two years, and increased egg and fingerling production. Fast growing strains with large eggs are now available for culture. The progeny of crossbred salmonids are often sterile, but in some cases the males or females only are infertile. Fully fertile cross-breds are usually obtained when crossed with closely related species. With the development of trout farming in sea water, interest in late-maturing, fast growing strains or sterile hybrids has increased, and considerable research is now under way in this direction. A recent development is the use of sex-reversed all-female brood stock, in order to produce all-female progenies that grow faster. Functional males are produced by oral administration of the male hormone 17-methyl testosterone through starter feeds containing 3mg per kg of feed, at the fry stage. Higher levels of hormone may cause sterility, and lower levels may result in low percentage masculinization.
A farmer has the option to carry out his own selective breeding or depend on specialized breeding centres to obtain eyed ova of the desired strains for incubation and rearing. Eggs of early or late spawning fish can be obtained, in order to spread the hatching time, grow-out period and attainment of market size over as long a period as possible.Trout hatcheries in the southern hemisphere, where climatic patterns are reversed and consequently also the breeding seasons, are a major source of egg supply to hatcheries in the northern hemisphere during the off-season. When eggs are purchased, strict regulations pertaining to infectious and communicable diseases are observed, to prevent introduction of diseases.
If the fish culturist decides to carry out propagation in his own farm, a suitable stock of brood fish is reared in special brood ponds. A density of about 8000/ha is recommended in small ponds with a current of water. It is considered best to feed them on natural food, but if artificial feeds have to be given, the quantity of feed is gradually reduced before spawning and the fish transferred to holding tanks before propagation.
Although two-year-old trout start spawning, females are seldom used for propagation before they are three or four years old. Males of two to four years are considered to be the best for breeding. The quantity of eggs or milt increases with increase in size of the brood fish. Larger females have larger eggs and hatch into larger alevins (hatchlings, sac-fry).
The number of brood stock required naturally depends on the number of fry or finger-lings needed. The number can be calculated by extrapolation backwards, based on the expected survival rates of alevins to fry or fingerling stage, fertilization and hatching rates and the fecundity of the parents available. There are differences of opinion about the proportion of males to females required, but one male to three females is generally considered
satisfactory. The males and females are usually held in separate ponds or tanks. The state of maturity is examined at regular intervals, so that as soon as a fish is ripe it can be removed for stripping.
In a commercial fish farm it is not easy to prevent inbreeding while building up a brood stock. As will be described later, when the eggs are fertilized by the dry method the largest number are fertilized with the milt from the first two or three males (in descending order, the first one fertilizing the largest number) and the later ones have a lesser chance of finding any unfertilized eggs. To avoid this and to reduce inbreeding, it has been suggested that a mixture of milt from a number of fish should be used for fertilization. Milt from a number of male fish is collected, avoiding any admixture of water, mixed together, and kept in a cool dark place. The mixture is viable for several hours. The required amounts are removed by pipette for fertilization after the eggs have been stripped from the females.
Another means of reducing inbreeding is by using cryopreserved sperm of fish from selected sources. If such preserved sperm is available from dependable sources, the need to maintain male brood stock will be minimized. There is also the possibility of cryopreservation being carried out in the farm itself and of using a mixture of cryopreserved milt for fertilization.
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