Effects of amino acid mixtures on upstream selective movement of four Pacific salmon
The homing migration of salmon has been thought to be controlled mainly by their olfactory functions. Behaviour experiments were conducted to investigate whether amino acid mixtures have attractive effects on upstream selective movement of mature male chum salmon (Oncorhynchus keta), masu salmon (O. masou), sockeye salmon (O. nerka), and pink salmon (O. gorbuscha) using the two-choice test tank consisting of two water inlet arms and one pool. Either the artificial natal stream water, containing amino acid mixtures, or natural lake water was added to the water inlet of the left or right arms. The fish movement was monitored and the number of fish moving to each arm was counted. The two test pairs of natural and artificial solution were used: (1) both natural lake water, and (2) artificial natal stream water and natural lake water. In pair (1), all species showed no selectivity for either arm. In pair (2), percentage of upstream movements of 4 salmonids was 82.3, 63.6, 53.3, and 46.7 in pink, chum, masu, and sockeye salmon, respectively. In contrast, percentage of upstream selective movement in the arm with artificial natal stream water containing amino acid mixtures was 57.1, 85.7, 75.0, and 71.4, and in pink, chum, masu, and sockeye salmon, respectively. These results indicated that the artificial natal stream water containing amino acid mixture had different effects on upstream selective movement among the four species of Pacific salmon. Pink salmon showed the highest upstream movement and the lowest accurate selectively to the artificial natal stream water. It is interesting to note the evolutionary relationship between the olfactory functions and the homing accuracy among four Pacific salmon species.