Principles of Genetics and Breeding

5.1.8.1 Viability of hybrids

1. The early stages of development (embryonic development, hatching, yolk resorption and start of feeding)

High mortalities are frequently reported during this period and this is acute in the case of intergeneric hybrids. The causes of the mortalities have not been much analysed: a high frequency of oedema in the yolk sac resulting from structural abnormalities of the pronephros has been mentioned by several authors.

2. The immature growth period

When the exogenous feeding period is reached, the survival rate of the hybrids is mostly intermediate between that of the parental species. The exact level of performance will depend on the rearing environment and on the presence of specific pathogenic agents. The hybrids appear to inherit, at least partly, the specific resistance of one of the parental species.

• The hybrid rainbow trout x coho salmon appears to be partially resistant to viral haemorrhagic septicemia, this character being of maternal origin.
• The “tiger trout” is more tolerant to sea water than the brook trout, this character being transmitted from the maternal species.