溪流区に放流したマス稚魚の生残に関する研究-I

Abstract

For the estimation of the survival rate of fingerling trout stocked in a brook, a method through visual observation, by means of water-glass, was adopted. Some sample pools were taken for the observation, within the brook section of experiment (divided into Section I and II), and both the upper and the lower ends of a sample pool were blocked by setting screens to prevent the fish comming in or out the pool. The number of fish dwelling in a sample pool was counted by observation and denoted as N', then, the fish in the pool was collected under an utmost effort by means of a circling seine, the number symbolized by C. Followed by the seining the fish remained still in the pool was sought by observation, deriving the number as (N-C)'. N denotes the true value against N'. Then the rate of discovery, defined as r=N'/N may be calculated following the formula (1) found on page 430 of the text, and the rate of fishing, f=C/Nfrom (2) on page 430 could be established. The symbol C' denotes the number of fish observed which were assumed to have been mingled among the caught fish, therefore, the rate C'/C also carries the meaning of rate of discovery. The fraction C'/N' bears again the meaning of rate of fishing. Considering the combined population of these sample pools, the author can estimate the synthesized discovery rate which represents that of total section surveyed from which sample pools were taken. Apparent survival rate s1' which could be defined as the rate of apparent population size N2' measured by observational method at time t2 against the apparent initial population size N1' at time t1, could be corrected to the true survival rate s1' after the formula, s1=r1/r2•s1'. As for apparent survival rate (s1)e which is defined as the rate of apparent population size N1' at time t1 against the initial stocked population size P at time of planting tp, could be corrected to the true survival rate (s1)p as follows-(s1)p=1/r1•(s1)e. Finally the two corrective coefficients were induced, one kpt, which corrects the apparent survival rate at a given time tn against initial stocked population size to the true survival rate, while the other ktt corrects the apparent survival rate at a given time tn, against initial apparent population at the time tn-1 to the true survival rate. Thus, the author has arrived, 1. Discovery rate, rj, at a pool, j, having population size less than about 100, was estimated to be between 0.6 and 1.4, and consequently, 2. The size of accompanied error of estimation of population size shown in proportion to the true size, or the size of relative deviation, Ej=1-rj, showed the range of 0.4>Ej>-0.4.