Inhibitory effects of testosterone on downstream migratory behavior in masu salmon, Oncorhynchus masou

The effects of testosterone (T) on downstream migratory behavior of yearling masu salmon, Oncorhynchus masou, were studied during its downstream migratory period using artificial raceways. In experiment 1, 22 and 19 smolts were implanted with a medical silicone tube capsules containing 500 mug of T or vehicle only. These groups were transferred together to the upper pond of artificial raceway which was connected to the lower pond through fish-way. Downstream migratory behavior was then observed for 1 week. In experiment 2, T500 mug-, T50 mug-, and T5 mug-treated smelts, control smelts, and precociously mature males were transferred to the upper pond of the raceway and downstream migratory behavior was observed for 2 months. In experiment 3, 40 smelts were implanted with capsules containing 500 mug of T or vehicle only. These groups were transferred separately to the upper ponds of raceways and downstream migratory behavior was observed for 3 weeks. In each experiment, injection of T caused increases in plasma T levels within physiological levels. In experiment 1,frequency of downstream migratory behavior was 89.5% in the control group and 31.8% in the T500 mug-treated group. In experiment 2, the frequency was lower in the T500 mug- and T50 mug-treated groups than in the control group. T5 mug implantation failed to inhibit downstream migratory behavior. Precocious males were not observed to go down the raceway. In experiment 3, frequency of downstream migratory behavior of the control group and the T500 mug-treated group was 100 and 40%, respectively. In each experiment, plasma levels of T in T500 mug-treated smelts which did not show downstream migratory behavior was higher than those of migrants. These results indicate that the downstream migratory behavior of masu salmon smelts is inhibited by physiological levels of T.