Abstract
1. Estimation of population size was made directly through observation with water-glass in an experimental area settled in a mountain brook. 2. The research period was divided into four periods owing to the occurrence of principles having influence on the errors of observation. 3. The flood was defined newly. 4. With use of errors of observation measured, a deviced procedure of correction of apparent survival or mortality rate was actually applied. 5. Some adjustment were applied tb apparent observed data including the population size of naturally propagated fingerlings of rainbow trout together with that of stocked rainbow trout fingerlings. 6. The change of behavioristic character made a cause to change the method of adjustment. Change of fishing method involved also made a cause to change the method of adjustment. (However, as the fishing method is subordinate to the change of behavioristic character of fingerlings, the actual principle should be taken to be the latter.) 7. Floods caused a large mortality showing the measures of 50.4 to 62% in rainbow trout fingerlings, and 67.9 to 83.7% in brook trout fingerlings at each flood. 8. Floods had scouring effect on mortality, and of decreased fingerlings 75.3 to 95.5% with rainbow trout and 99.0 to 100% with brook trout were estimated to be swept away from the experimental area, though some of them were expected to remain over some downstream area. 9. The intervals of measuring date in survivor survey before and after the occurrence of flood was thought to have some influence on the measure of scouring effect as well as on that of mortality caused by flood. In this case, the dispersion of fingerlings made on the days of usual flow was considered to be the actual agency to reduce the apparent impression of violence of flood, the action of which emerged in mortality of fingerlings and scouring effect.
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