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Direct Control of Fauna 6 at eye-up, or preferably swim-up, equal num- bers of eggs/fry from each family should be randomly taken to create the broodstock pool of individuals so that no family is represented more than any other-the number of eggs or animals to be taken per family will depend upon the number of families and the target number for a broodstock loc. Thus, in a single year, a culturing station can establish one line of breeding for a broodstock. A minimum of two lines are required (three to five are preferred) in order to minimize losses of genetic variability. The sources of establishment of these other lines of breeding are many. 1 In succeeding years follow the same process as described above so that in 3 years you will have established three lines; and/or: 2 Fish culture facilities exchange eyed-eggs at that time in the year when lines-of-breeding are being established. Disease control and certifica- tion'systems are necessary to make this exchange safe for recipient fish culture facilities. It is advis- able; also, to have some isolation facilities for rearing exchanged lines. It would be possible by this method to establish the minimum lines-of- breeding to maintain broodstocks in 1 year; or: 3 Use a co-operative, centralized facility to main- tain lines-of-breeding. Fig. 20.2 Rotational line crossing (after Kincaid 1977. Rotationallme crossing 393 Once a captive broodstock is established in a fish culture station, in addition to the above pro- cedures for the establishment of a stock, a breed- ing scheme is required which will prevent the mating of siblings or half-sibs. Thus inbreeding is minimized through successive generations. The technique is known as rotational line crossing (Kincaid 1977). At the time a broodstock is established, three distinct lines are created from a single year class, or three lines are created in succession with three separate year classes. The three lines, which may be called A, B and C, must be held in separate units or marked by fin clips, branding or some other means by which they can be distinguished one from another. All three lines may be used for producing eggs for production purposes in any given year, but when the time comes to produce the next gener- ation of broodstock, a specific breeding scheme must be followed. For example, the females of line A are crossed with males from line B; the females of B are fertilized with males from C; and the females of C are fertilized with line A males. This creates three new lines, AB, BC and CA. The next generation is then produced from those three lines, again by crossing females and males from different lines. The creation of three a, e, c, Az ~ ~ eZ ~~