マダイ

The present report aimed to study the fecundity of sea bream, Pagrus major (Ternminck et Schlegel), by obtaining the basic information on the number and maturation process of its ovarian eggs and comparing the number of spawned eggs by the cultured fish with the fecundity of the wild ones. The materials used in this investigation were collected from the coastal waters of Chikuzenkai region of northern Kyushu, during the period from March, 1969 to July, 1970. Here the study is principally based on the histological observations of the ovarian eggs, related with their diameters as well as their numbers. The ovaries preserved in 10 per cent formalin were weighed, and the egg counts and diameter measurements were made under 50 times magnification using the projector. Egg number was obtained by computing the number in a small portion of the ovary for its total. Histological sections of the ovary were prepared by the paraffin and the celloidin method, and stained with Delafield haematoxylin and eosin. The peak spawning season is during the period from late April to late May. From the egg diameter distribution curves, the growth of the ovarian eggs for maturity were discussed, and the multiple spawning by individual sea bream was suggested by the multiplicity of modes in the distribution curves (Fig. 1). There are shown in P1.1, 2 and 3 the general histological features of the ovary and the ovarian eggs of sea bream in each maturation stage. The eggs in the various maturation stages, newly formed empty follicles and the old ones were found simultaneously within the ovary of a mature sea bream. Therefore, the spawning is thought to occur in many times for long period. Maturation stage and size range of the eggs, maturation phase and type of the ovary were arranged in Table 1 based on the observations of the sections and the egg diameter measurements. Relationship between the number of ovarian eggs developed over peripheral nucleolus stage and fork length of the fish by ovary types is shown in Fig. 2. No significant differences are observed between wild fish and cultured one in the number of ovarian eggs corresponding to the fork length in different areas. Fig. 3 shows the relationship between the number of eggs developed over yolk vesicle stage and fork length in each ovary type. All these eggs increase in number with the increase of the fork length of the body. The total number of spawned eggs during a spawning season by cultured fish is about twice as many as the number of ovarian eggs developed over peripheral nucleolus stage and about quadruple as many as the number of eggs developed over yolk vesicle stage. It may be concluded that the immature eggs are continuously developed into the mature ones throughout the spawning season. Comparing the number of the eggs in the most matured batch with those of spawned by the cultured fish in the same size, it may be estimated sea bream ovulate their eggs into the ovarian cavity from one to four times during a day and spawn once at a time a day.