The Spawning and the Early Life History of the Engraulid Fish Coilia sp. Distributed in Ariake Sound

The life history of the Engraulid fish Coilia sp. was studied. The problem whether the fish should be identified as C. ectenes, C. mystus or C. nasus was discussed in the previous paper (TAKITA, 1967). This fish is distributed in Ariake Sound and the downstream of the Chikugo River which finds its way to the head of the sound, and its distribution in any other locality in Japan has not been reported. The spawning season seems to extend from the latter part of May to the early part of August. In this season, the fish enters the Chikugo River and spawns eggs in the fresh water at the down stream of the river. The time and area of spawning seem to be limited. According to the author\u27s investigations which was carried out during July 24 to 27, 1963, the fish seems to spawn mainly around 5:00 p.m. in the area about 15km up the mouth of the river. The developing egg sinks to the bottom in the standing fresh water, for the specific gravity of the egg is slightly higher than that of the river water, but the one in the river is floated by the movement of the water and drifted down the river. The hatching of the egg in nature seems to take place in the mouth area of the river where the river discharges itself into Ariake Sound after the egg is floated down the river for some distance from the spawning ground. Accordingly, the larvae and juveniles are more abundantly distributed in the mouth area of the river than in the spawning area. It is likely that most of them are not carried away by the river water to the sea, but retained and concentrated in that area. On July 5, 1966, artificial insemination of the fish was carried out at Jojima Town which was situated about 15km up the mouth of the Chikugo River. The egg is spherical in shape, measuring 0.97～1.13mm in diameter. The yolk bears faint blue and is segmented. Usually the egg has only one large oil globule, measuring about 0.6mm in diameter, but sometimes there are some eggs that have several small oil globules in addition to the large one in early developmental stages. The perivitelline space is narrow. The hatching took place 19 hours after insemination at the water temperature of 24°～26℃. The newly hatched larva, measuring 2.52mm in total length, had 39+17=56 myomeres (This fish has 71～80 vertebrae). One large oil globule is situated at the anterior part of the yolk sac. The anus is opeposteriorly and accordingly the tail length is one-seventh of total length. This larva has no chromatophore. The 2 days old larva, being 4.61mm in total length, had 39+38=77 myomeres and the eyes became black. The melanophores appeared on the ventral part of the abdomen and the dorsal part of the intestine. The 3 days old larva was 5.23mm in total length and the mouth opened. Though the yolk had been consumed, the oil globule, measuring 0.55mm in diameter, remained yet. The melanophores appeared on the ventral margin of the tail. The prolarvae possessing an oil globule, the post larvae and juveniles of this fish were collected with larval net in the downstream of the river during July 24 to 27, 1963. In the prolarva, 7.2mm in total length, an oil globule remains yet, and an air-bladder appeared above the alimentary canal. In the post larva, 13.8mm in total length, rays of each fins except the ventral ones are formed. In the post larva, 19.8mm in total length, the snout is prominent and the maxillaries begin to be prolonged. The tail begins to be elongated and the notochord is curved upward. Rays of the ventral fins are formed. In the post larva, 26.9mm in total length, the free pectoral fin-rays are filamentous and the upper lobe of the caudal fin is longer than the lower one. The scutes are formed on the ventral ridge of the abdomen. The melanophores are on the dosal part of the tail. The juvenile, 35.6mm in total length, resembles in shape to the adult. The tail is longer than the length from the tip of the snout to the anus. The pectoral fin-filaments reach the insertions of the ventral fins.有明海および筑後川において有明海産エツの産卵生態および稚仔魚の生態,生活史について調査を行い,次のことを明らかにした. 有明海産エツは有明海の奥部と筑後川の下流域に分布しているが,その棲息地は成長段階および季節によって変る.本種の産卵は5～8月上旬に筑後川下流の一定水域において行われるが,1963年7月の調査によれば,河口より15km上流の地点よりさらに少し上流で午後5時前後の比較的短い時間内にいっせいに産卵が行われたと思われた.本種の人工受精を行った結果によれば,受精卵の卵径は0.97～1.13mm,油球は直径0.58～0.64mmの大型のもの1個がある.卵黄は淡い青緑色を帯びており,泡沫構造がある.受精卵は産卵が行われる水域の水(淡水)の中では水が動いていない場合は底に沈むが,天然では河水の動きによって水の表層より底層に浮上して流れている.また,受精卵は死ぬと浮力が強くなり,静水中でも表層に浮上する.受精卵は水温24°～26℃において約19時間でふ化した.ふ化直後の仔魚は全長2.5mm,色素は全くない.仔魚はふ化後約3日で全長5.2mmとなり,卵黄は全部吸収されたが,油球はまだ大きく残った.後期仔魚および稚魚の観察は筑後川下流において稚魚網を用いて採集した標本によった.全長8.5mmの仔魚には油球がまだ痕跡的に残っている.この時期の仔魚には背鰭,臀鰭と尾鰭の原基が現われ,鰓が形成されている.全長27mmの仔魚では吻端が突出し,主上顎骨,胸鰭の上部遊離鰭条と尾部が伸びて,力タクチイワシ科魚類の特徴が明確となる.全長36mmで各鰭は完成し,体型は成魚とほぼ等しくなって稚魚期にはいる.筑後川では下流域のうちの比較的上流部で産卵が行われ,受精卵は下流部へ流されて河口近くでふ化し,稚仔魚は河口域に集積される傾向がみられる.河口域では稚仔魚が大きさによって別-の群をつくり,潮汐に従って上流と下流との両方向へ移動をくりかえしながら成長するものと思われる