烏魚人工繁殖試驗(第五報)

Abstract

The grey mullet, Mugil cephalus Linnaeus, is one of the important commercial fishes in Taiwan. From December to January, schools of grey mullet are found along the middle part of the western coast; they then move slowly southward for spawning. Most of them are 4-5 years, old, measuring 42-45 cm long') . Grey mullet are captured mainly during this spawning migratory period for their roe, which after removal from the fish is carefully dried for export mainly to Japan. In Taiwan for stocking fresh and brackish-water ponds, fingerlings 2 to 3 cm long are caught from river mouths and estuaries along the west coast during the period from December to March. The annual demand for such fingerlings is estimated at 6 to 7 million, but unfortunately due to unpredictable changes of oceanographic and meteorological conditions the natural supply of grey mullet fingerlings fluctuates to such a great extent that fish farmers suffer loss from the uncertainty in establishing a profitable pond stocking system. In each of the last five years a team of workers was organized by the Taiwan Fisheries Research Institute, Taiwan Fisheries Bureau and the Institute of Fishery Biology of Taiwan University to carry out a program of research on the artificial propagation of grey mullet with the objective to achieve constant supply of mullet fingerlings, but no satisfactory result had been obtained. This difficulty lies principally in the rearing of the mullet fry, for in 1967 only one fry out of millions survived up to 23 days. In 1968 the same program was repeated in Tungkang Shrimp Culture Center with some improvement in technique and equipment. As a result two fingerlings 1.0 to 1.1 cm long survived up to 30 days. The procedure and results of the present study carried out from November 1938 to February 1969 are summarized as follows: (1) From November to January, 30 spawners were obtained (body weight 1.7-3.0 kg) . Among those 30 spawners, 2 died from injury in the viscera, the sex of 2 were wrongly selected for hormone injection, 7 failed to spawn and 19 did spawn after hormone treatment (2.5-4 pituitaries of mullet combined with 10-35 rabbit unit of Synahorin), but only the eggs of three females were successfully fertilized and hatched. (2) The effect of hormone treatment had some relation to the fishing season. Injection at the peak of the fishing season was more effective. Healthy eggs were obtained 40 to 50 hours after the first pituitary injection. (3) At a salinity of 32.4-32.75‰ and water temperature of 21°C, fertilized eggs took 60-65 hours to hatch, but higher water temperature up to 24°C shortened the hatching time to 44-50 hours. The hatching ratio showed no marked difference between the running water system and the still water system. (4) The newly hatched larvae measured 3.09 mm in body length(Plate I , Fig. 1) with mouth not yet open. It could not swim but lay at the bottom of the still water. On the third day, mouth formation was complete, and on the fourth day, it began to feed on minute organisms such as oyster larvae., When yolk sac was absorbed on the fifth day, it could feed on rotifera. The 19 day old fry measured 5.0 mm and fed only in the daytime. Scales began to develop on the 23rd day and body color became darker. Soft-rayed fins appeared on the 25th day and the body was covered completely with scales on the 27th day when it measured 1 cm long (Plate II, Fig. 8). (5) The larva shoWed phototaxis under 600-1400 lux, an instinct favorable for raising the survival rate in rearing. (6) Mullet larva could not tolerate salinity lower than 17‰ and did not appear to adapt to frequent changes of salinity. (7) In running water the larva survived for 9 days only. In still salt water in a larva rearing vessel which was gradually diluted with brackish pond water, two mullet fry were reared to 30 days measuring 1.0 and .1.1 cm long.

於民國57年(1968)11月至翌年2月，烏魚群作產卵洄游路經台灣南部近海時，選購圍網捕獲之種魚，給予賀爾蒙處理，進行其人工繁殖試驗，獲得下述結果： (1)試驗期間共獲種魚30尾，施以賀爾蒙處理（2.5~4個烏魚腦下腺，混合10~35家兔單位生殖腺刺激賀爾蒙），結果排卵19尾佔63％，又其卵能孵出幼魚者有3尾佔10％。 (2)賀爾蒙處理效果與漁季似乎有關，在漁季盛期所得之效果似乎較為顯著，而第一次賀爾蒙處理後在40~50小時內排卵者，其人工繁殖成功之機會較高。 (3)受精卵在鹽分濃度32.4~32.7‰之下，水溫21℃上下約須60~65小時孵化，但水溫若高至24℃，則只須44~50小時即可孵化，而孵化率在流水式及止水式間似無顯著之差別。 (4)幼魚生存之鹽分濃度範圍甚廣，其低鹽分濃度之耐度可低至17‰，不過，對其變化之適應性則似乎不強。 (5)幼魚在照度600~1,400 lux 之下，具有趨光性，而此性質或可應用於飼育上而達提高生存率之目的。 (6)此次孵出之幼魚在漸漸加入魚塭水而淡化之飼育水中，培育30天共養活2尾魚苗其體長為1.0及1.1公分。