The body components of female rainbow trout (Salmo gairdneri) reared in an experimental fish farm were studied during sexual maturation. For a period of 13 months, carcass, liver, gut and ovaries were sampled every month and their fat and non fat constituents analysed. In the trout, lipids are mainly stored in the carcass and to a less extent in the gut and ovaries. The distribution ranged from 68 to 73%, 31 to 6% and 0.1 to 20%, respectively from the beginning to the end of sexual maturation (March-November). Whatever the stage, carcass and visceral total lipids (TL) were mainly composed of neutral lipids (NL). The ovarian NL relative to TL content decreased from 90% during the first gonadal slow growth phase (SG. I from March to June) to 60% at ovulation in October or November while the proportion of phospholipids (PL) increased during the second slow growth phase (SG. II from July to August) and the rapid growth phase (RG from September until ovulation in October or November). This observation suggests that trout preferentially incorporate fatty acids into phospholipids in the oocyte during late ovarian growth.

During sexual maturation there was a large mobilisation of carcass and visceral lipid reserves, but non fat dry matter (NFDM) was only slightly changed. In our experimental fish (mean weight 1200 g) the loss of lipids averaged 76 g (42 and 34 g, respectively from the carcass and gut) versus 33 g for NFDM (from the carcass mainly). Loss of visceral lipids began during SG. I and ceased at ovulation in November, whereas loss of carcass lipids seemed to start during the RG phase and finished 1 month after ovulation.