シロウオ

The ice goby, Leucopsarion petersi Hilgendorf, that is a small gobioid fish about 50mm in total length is caught mainly with a fishweir, lift net and landing net at mouths of several rivers in Japan on the season when fish ascends to river for a spawning. In the river Muromigawa which runs in Fukuoka Prefecture in the northern Kyushu, the catches of this fish that were large in the past have recently decreased in quantity. Therefore, it is necessary to propagate the ice goby for fisheries. And the life history of this fish has not been described yet in detail. In the present report, the life history of the ice goby was studied on the basis of morphology, distribution, and anadromous and spawning ecology mostly at the river Muromigawa and the Hakata Bay. Furthermore, a plan of the preparation of spawning area was examined on the basis of these ecological informations in this river, and a method for the propagation of the ice goby was proposed. The ice goby is widely distributed in shallow bays from Kagoshima to Hakodate in Japan and southern Korea and ascends to spawn at the rivers flowing into them. There were morphological differences among fishes with rivers flowing into different bays, and this has led to conclusions that the ice goby in one bay belong to the same population and the reproductive isolation has occurred among fishes in different bays. The anadromous male and female is about 46 mm and 50 mm in total length, respectively. In the Muromigawa, after the ice goby approaches to the mouth of the river in the early February, the ascending migration to its lower part begins immediately and lasts to early April. The sex ratio during the ascending season is one to one. But, it is not always definite by the period, because male ascends to river earlier than female. The high period of the ascending quantity to a river varies between the late February and the late March with a place throughout Japan and begins to ascend earlier in the southern Japan than in the northern Japan, i. e. in Jan.-Feb. in the southern Kyushu, southern Shikoku and southern Kii Peninsula regions, in Feb.-Mar. in the northern Kyushu, Inland Sea and Tokai regions, in Mar.-Apr. in the northern Chugoku, northern Kinki, northern Chubu and Kanto regions, in Apr.-May in the Tohoku and the Hakodate regions. Environmental conditions under which the ice goby ascends to the river were examined. The water quality of the river is as follows: BOD lower than 2.3-2.7 mg/l, DO higher than 7.7-8.8 mg/l, NH,-N lower than 0.14-o. 78 mg/l, NO_2-N lower than 0.015-o. 04 mg/l, PO,-P lower than 0.068-o. 11 mg/l. In the Muromigawa, the average water temperature in the early period of the ascending migration to river is about 7℃for the past ten years, 1975-1984, when the sea and the spawning area become almost equal in water temperature, and in the same way, the water temperature at the high and the late periods of the ascending migration to river is about 8 and 11℃, respectively. The ascending migration to river is active at night in ebbing tide on the flood tide at the mouth of river and in flowing tide at the fishweir site. The ascending quantity to river increases when the water temperature rises and the amount of flowing water of river decreases more than in the preceding day, and the duration of sunshine is more than 5 hours in a day. The maturation of the ovary progresses rapidly from the late March and the fish spawns between the late March and the late April in the Muromigawa. The water temperature at the spawning period is 11-14℃. It is found by the examination of its alimentary contents that the ice goby doesn’t feed in the river. The spawning area in the Muromigawa is situated at the river bed of 1.3-2.4 km up the river from its mouth. This area is a tidal compartment where salinity is variable between the low and high value. The main spawning areas are formed at three sites in this area. Environmental conditions suited for the spawning were examined. The river bed adapted for spawning is well washed and is less than 90cm in height of ground which is not dried up even at low tide, and its bottom material is well pervious to water and many fist-sized stones for the spawning are present in its place. In the laboratory, forming process of nest and spawning habit were observed. A male in an aquarium taken out sands and gravels from the underneath of the stone with his mouth during most of a day. After the entire procedure of nest preparation is accomplished by the male, he puts out his tail from the entrance of his nest and weaves it quickly. As soon as the female enters the nest to respond to his actions, the male closes the entrance by gravels with mouth. Weaving of the tail in the male seems to introduce the female into the nest. The female spawns 380-650 adhesive eggs in a layer mainly on the ceiling of nest. Considering the number of matured eggs in an ovary and developmental stage in an egg mass, these eggs in the nest appear to be spawned at a time by a female. After the spawning by the female and nursing of eggs to hatching by the male, the parents end their lives in a year. Developments of eggs were observed, and their tolerance to water temperature and salinity were examined. The relationship between the mean hours required for 50% hatching (H) and water temperature (0) is shown by the following equation; H=1408.1 exp (-0.0756 0). In the Muromigawa, hatching hour from the spawning is calculated at 245-360 hours, because the water temperature at the spawning season is 11-18℃. The optimum salinity to develop and hatch is lower than 8.4‰. The newly hatched larvae show a positive phototaxis and rheotaxis, and come out to be induced by light and flow out to the sea from the later April to the early May. The larvae grow, feeding mainly on copepoda, at the lower layer of the river mouth and the inner part of the bay until the next February when the fish ascends the river to spawn. In February, the ice goby attained 42-43 mm in total length retains still larval form. Morphological changes and flextion points in relative growth to total length appear at 6-7, 10-12.5, 37-38 and 42-43 mm in total length. Only at 42-43 mm in total length with which the fish enters the river for maturing and spawning, the ecological changes are recognized. On basis of these results, the developmental process of the ice goby is divided into two stages; the growing stage with larval form, and the maturing and spawning stages with larval form. According to the present studies on the life history of the ice goby, in order to propagate the ice goby resources, it has been clarified that it is necessary to conserve the environment in the sea where this goby spends the growing stage, and to prepare the spawning area in the river. The values of the environmental condition to be conserved in the river and the sea were indicated. Furthermore, through the experiment on the preparation of spawning area in the Muromigawa, it has been found that removal of mud over the height of ground where is dried up at the low tide and setting of fist-sized stones in the river bed are effective methods for preservation and propagation of the ice goby resources.