Influence of the date and body size at smoltification and subsequent growth rate and photoperiod on desmoltification in underyearling masu salmon ( Oncorhynchus masou)

Abstract

Three experiments were undertaken using underyearling masu salmon. In the first experiment, fish at the parr stage were individually marked, date and body size at which smolt characters first appeared were recorded, and the occurrence of desmoltification (reversion from smolt to parr) and individual growth after smoltification were followed for 8 months. In the second experiment, two groups of individually marked smolts were reared under different feeding rates [1.5% (well fed) and 0.25% (poorly fed) of body weight] and individual growth and occurrence of desmolts were examined. In the third experiment, two groups of smolts were reared under different photoperiod regimes, one to transfer from a natural photoperiod of 16 h light to 8 h light and the other to remain at 16 h light; growth and occurrence of desmolts were examined. There was no difference in the rate of desmolts among the fish grouped by date of smoltification, and body size at the time of smoltification was not different between fish which desmoltified and those remaining as smolts. On the other hand, the growth of the desmoltified fish was slower than that of the fish which remained as smolts, and the fish which desmoltified early showed a lower growth rate. The rate of occurrence of desmolts was not different between well fed and poorly fed groups, in spite of the fact that the growth of the poorly fed group was slower than that of the well fed group. The rate of desmolts for the 8-h light photoperiod group (94.3%) was higher than that of the 16-h treatment (37.5%) at the end of experiment, and those transferred to 8-h light grew slower. The results indicate that endogenous growth after smoltification relates to desmoltification but feeding condition has no effect. A short photoperiod after smoltification influences endogenous growth and induces desmoltification; however, it is considered that not only photoperiod causes desmoltification.