Abstract
To increase the accuracy of estimating reproductive parameters, a method of examining ovarian egg size frequency distributions of Japanese anchovy is reexamined. An improved version of Tanaka's graphical method for analyzing polymodal distributions using computer graphics is proposed. Comparisons of frequency distributions by five egg size indexes verify the validity of the major axis as a reasonable fit to a normal distribution and measurement cost. The effect of sample size on the analysis is tested by a sampling experiment from a simulated trimodal population.
Ovaries taken in 1982 and 1984 were analyzed using this method. Major egg size frequencies were separated into two normal distributions and small frequencies into three. The monthly mean diameter of large and small egg groups in the cold year of 1984 are larger than those in the same month in 1982, a normal year. The mean diameter of the large egg group decreases, and the proportion of large eggs increases seasonally from April to June. Temperature-dependent changes of size observed in planktonic eggs is also recognized with compensating control of the mixing proportion in the large ovarian egg group.
