"Ecological studies on the dispersal of newly emerged masu salmon fry, Oncorhynchus masou."

"Experiments on newly emerged masu salmon fry were undertaken in artificial and natural streams to elucidate physical and biological factors that affect dispersal of masu salmon fry, its ecological significance, and to propose a hypothesis to explain dispersal. In the artificial channel experiments, masu salmon fry exhibited both up- and downstream movements; downstream movements were typically nocturnal, but upstream movements were predominantly diurnal. The number of up- and downstream migrants was affected by changes in water velocity; high velocity promoted downstream movement and decreased upstream movement. Moreover, larger fry tended to move upstream or remain in the spawning area and feed more than small fry. Masu salmon dispersal was concluded as follows; Dispersal after emergence mainly commences at loss or recovery of positioning with change in light intensity at sunrise and sunset. Quantitative relationships between upstream migrants, residents and downstream migrants are controlled by individual swimming ability associated to flow intensity in stream and social interaction. More wild fry in the artificial channel migrated downstream than upstream, in contrast domesticated fry tended to migrate upstream or remain in the channel. In the natural stream experiments, however, most masu salmon fry, irrespective of origin, moved downstream or remained near the spawning areas because of high water velocity. Masu salmon juveniles at downstream sites with low density grew rapidly so that their size exceeded those in high density planting sites in summer. I discovered that more male fry moved upstream or remained in the spawning area than female fry. Early mature males were always larger than immature male in early summer, and more mature types upstream and near spawning areas than downstream. Therefore, it was concluded that dispersal of newly emerged masu salmon fry has two ecological roles; the first is to regulate population density by the spatial re-distribution between life history stages (alevin and parr) to enhance their feeding opportunities. The second role is to share the instream habitat between resident (early mature) and sea run types to contribute to accelerating the life history divergence. A hypothesis of mechanisms for dispersal including sex biased distribution and early maturation for masu salmon was proposed based on the present results and previous studies for salmon and trout. Masu salmon develop different metabolic activities that are genetically controlled. Males having the genetic potential to mature early will be included in the high metabolic group of masu salmon. More males than females remain near the spawning area because the males with high metabolic rates have high growth, and subsequently, they are greater in swimming and/or competitive ability than other fish. As high metabolic males can grow rapidly at initial feeding in spring when the daylength is increasing, when the fish are sensitive to"