Triploid hybrids between female rainbow trout (Oncorhynchus mykiss) and male arctic char (Salvelinus alpinus), together with triploid and diploid rainbow trout controls from the same dams, were tested in freshwater farming from fry stage up to the age of 3 years. When farming was operated in cold water, the survival rate of the hybrids was as good as those of the diploid and triploid rainbow trout. Hybrid growth was noticeably hindered relative to that of diploid controls at the beginning of the juvenile period, but this was partly tempered later on by the effect of diploid sexual maturation; at 3 years of age, the hybrid weighed 20% less than the diploid rainbow trout and did not differentiate significantly from the triploid rainbow trout. The impairment of sexual maturation was very much the same in the hybrid as in the triploid rainbow trout, and dressing traits were therefore quite similar, despite a slight difference in perivisceral fat losses of immature individuals, to the advantage of the hybrid. These results allow one to consider the suitability of this hybrid for large-sized fish production in cold water farming.

Oncorhynchus mykiss alevins originating from a monosex female population were treated with methyltestosterone incorporated in the diet (0.5 or 3 mg/kg) for 60 or 90 days from the beginning of first feeding. At sexual maturity the percentage masculinization achieved was high (89-98%) whatever the concentration or duration of treatment. However, the hormone concentration was the major factor in determining the frequency of functional males obtained. Of the 2-year-old neomales produced by the treatment with 0.5 mg/kg diet for 60 days 82% were able to emit their sperm under abdominal pressure compared to only 36% of those produced with 3 mg for 60 days. The increase in the duration of treatment also resulted in a decrease in the number of functional males from 82% (0.5 mg/60 days) to 72.6% (0.5 mg/90 days) and from 35.6% (3 mg/60 days) to 19.2% (3 mg/90 days). It is therefore recommended to use a low dose of hormone (0.5 mg/kg) for only 60 days to get a high proportion of functional males.

Survival and growth performances of autotriploids in rainbow trout (Salmo gairdneri) and three types of triploid hybrids between female rainbow trout and brown trout males (Salmo trutta,) coho salmon males (Oncorhynchus kisutch) and brook trout males (Salvelinus fontinalis) were analysed relatively to the parental species in freshwater farming conditions. At the end of the immature period, autotriploids exhibit moderate but significant depression relative to the control for both survival (about 25% depression) and growth (10 to 15% depression). Most of this depression is established during the first three months of life, triploids appearing very similar to the control later on. At the onset of sexual maturation at 2 years old, relative value of triploids becomes higher than that of the control, for both survival and growth. Long term evaluation (2+ or 3+) balances in favor of triploids with particular advantage for triploid females. Triploid hybrids exhibit much more important depression in early survival (up to 60%). Later on, their survival remains always lower than those of parental controls. Their growth performances are intermediate between their two parental species. Despite a relative advantage for these groups during the onset of sexual maturation in control groups, especially for growth, long tem evaluation is in favor of diploid rainbow trout, but in some specific cases.