A STUDY ON THE GROWTH AND REARING METHODS OF THE FRY OF BLACK PORGY, MYLIO MACROOEPHALUS (BASILEWSKY)

Abstract

Hitherto, many attempts have been made to rear the larvae hatched from the pelagic eggs of various kinds of marine fishes. However, notwithstanding the efforts made by many biologists and fish-culturists, no satisfactory method to raise the fry and to maintain their survival at a high percentage has yet been achieved. The authors made three trials in the artificial fertilization of the black porgy, Mylio macrocephalus. These three trials were carried out at the Iride fish market, situated on the wes-tern coast of Hamana Bay, on the 30th of April, the 7th of May and the 23rd of May, 1959. Rearing experiments were carried out at the Ikawazu Fisheries Laboratory, Aichi Pref. to which the fertilized eggs were transported from there. The eggs hatched out in 40-50 hours and 60-70 hours at water temperatures of 19°C and 17°C respectively (Table 1). As food for the hatched out larvae, substantial amounts of several kinds of protozoa were previously cultured and given to them from a few days after hatching. It was observed that the fry, shortly before the yolk sac was entirely absorbed, namely at the age of two to three days after hatching, began to feed on fine Dino-flagellata, Oxyrrhis sp. (10-30μ×8-15μ). Subsequently, at fry measurements of 3-3.5mm. in total length, a species of Spirotricha, Stylonchia sp. (50-100μ×30-40μ) were largely fed on by the fry. Later, as the fry grew, they were observed to take in various living food organisms of gradually larger size in direct relation to their growth. For instance, in our experiments it was observed that the fry took the nauplius of copepoda or barnacle at 4-4.5mm. in total length, ie. 9-12 days after hatching; the nauplius of brine shrimp at about 5mm. in total length, ie. 15 days after hatching; young mysid shrimp at about 7mm. in total length, ie. 3-4 weeks after hatching and finally the fry was able to feed on chopped fish when they measured about 15mm. in total length, ie. 6-7 weeks after hatching (vide Fig. 2). As to the number of fry that survived, we obtained 638 young porgies (13% to the number of the hatched larvae) after 52 days in the first round of rearing, 614 fry (17.5%) after 45 days in the second trial and 102 fry after 60 days in the last trial. In each trial to raise the larvae, a considerablly high mortality occurred at the age of 6-8 days after hatching. It is observed that these days are the period when the fry change their diet from protozoa to larger food organisms. Further study is necessary so as to make it possible to improve the survival rate of the fry especially during this critical period when the fry changes its diet from protozoa to larger living organisms. Once this critical period was crossed it was observed that the fry grew rapidly, reaching about 4.5mm. after two weeks, 10.1mm. after a month and 32.0mm. (1.03gm. in body weight) at the age of two months (Fig. 3).