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シロウオの生態と増殖に関する研究

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概要 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 o...n 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.
シロウオの形態,分布,河川への遡上及び産卵,海域での成育生態について,主に室見川と博多湾において研究し,これらの知見をもとに本種の増殖方策について取りまとめを行った.1.わが国におけるシロウオの遡上は,鹿児島から函館までの内湾に注ぐ河川に広くみられた.地域間で形態を比較した結果,同一湾内の河川に遡上するシロウオは,形態上に有意な差は認められないが,異なった内湾の河川に遡上するシロウオでは体部比と計数形質などに有意な差を認めた.本種は内湾を成育域とし,これに注ぐ河川を遡上して産卵し,異なった内湾間の交流はほとんどなく,大きな移動を行わない.2.河川に遡上したシロウオの大きさは,雌で平均全長約50mm,雄で約46mmを示し,雌が大きい.遡上期間の後期になるに従い雌雄とも全長が大きくなるが,体重は雌で減少する傾向があり,場所別には河口部より上流の産卵場のものが成育した傾向が認められ,遡上期間におけるシロウオの成長と成熟が考えられた.性比は遡上期間を通してみると1:1であるが,雄の遡上が雌に比して早期に始まり,初期には雄が50%以上を占め,遡上盛期以後に雌が50%以上を占める.3.室見川における遡上期は2月上旬から4月上旬で,その盛期は2月下旬から3月下旬である.全国のシロウオの遡上盛期は,南九州,南四国及び南紀地方の1~2月,北部九州,瀬戸内海及び東海地方の2~3月,北部中国,北部近畿,中部及び関東地方の3~4月,東北及び函館地方の4~5月で,南から北へ遅くなる.4.遡上と環境条件との関係を検討した.まず遡上する河川の水質は,年間平均値でDO7.0mg/l以上,SS14mg/l以下,全窒素量1.8mg/l以下,全燐量0.2mg/l以下,BOD3.6mg/l以下で,遡上期の水質もほぼこの範囲である.遡上の始まる室見川の水温は,海域と河川が等しくなる約7℃で,盛期となる水温は8℃,終期は11℃である.潮汐との関係をみると,大潮時の夜間に遡上量が多く,河口部では下げ潮時に,やな場では上げ潮時に最も多い傾向がある.また,水温の上昇時や日照時間の長い日及び河川流量の減少時に遡上量が増加し,水温の下降時や流量の急増時には減少する傾向がある.5.遡上期におけるシロウオの塩分濃度に対する選好性を求めた結果,23.8‰S以下の汽水を好み,低塩分の河川水を求めて遡上する.6.海域のシロウオは,かい脚類を中心に摂餌するが,成熟,産卵のため河口域に入るとほとんど摂餌しなくなる.しかし,淡水中でも水槽内で餌生物を高密度に与えると産卵床形成に関与していない雌は摂餌するようになるが,雄はほとんど摂餌しない.7.成熟度指数や卵巣の組織学的観察によると,室見川におけるシロウオの成熟は,肥満度が低下しはじめる3月上旬から徐々に始まり,3月下旬の急激な進行後,産卵は3月末~4月下旬に行う.卵巣内卵は,卵径分布からみると,1回産卵されると考えられ,その卵数は約530粒である.産卵は11~14℃で行われる.8.室見川におけるシロウオの産卵場は,河口から1.3~2.4km上流の常時低塩分である感潮域に形成され,産卵場の位置は流量に影響され,年によって多少変化する.営巣や産卵は産卵場内で上流部から順に行われ,下流部ほど遅い.産卵が行われる河床の底質は,シルトや粘土が少なくて,通水性が良く,硫化物量が0.01mg/g以下で,地盤高は大潮の干潮時にも干出しない90cm以下であり,しかも産卵基盤となる石が多い場所である.9.水槽内で営巣及び産卵行動を観察した.雄は産卵基盤となる石の下の砂粒を口にくわえて搬出し,石の下に出入口の口径0.5~0.8cm,奥行3~5cm,幅2~3cm,高さ約1cmの産卵室を作り,雌を巣穴の中から誘い込む.雌は右の下面の産卵床に約500粒の付着卵を一層に産み付け,巣外に出て斃死する.雄も孵化まで卵を保護した後,死亡する.産卵床は底からの深さ20cmまで認められるが,深くなると死卵が多くなる.河川内でシロウオを捕食する魚類は,ウナギ,ビリンゴ,チチブとマゴチの4種であったが,この内,ウナギは明らかに卵保護中のシロウオと卵を捕食する.10.卵内の発生経過を観察した.水温18~23℃で高い孵化率を示し,その孵化時間は245~360時間を要し,高水温ほど短時間である.水温(θ)と孵化時間(H)との間にはH=1408.1exp(-0.0756θ)の関係がある.産卵初期の3月末~4月上旬における室見川の水温は約11℃であり,産卵末期の4月下旬における水温が約18℃であるので,この関係式から室見川におけるシロウオ卵の孵化時間は360~610時間を要することが認められる.また塩分は8.4%S以下で正常に孵化するが,発生が進んだ発眼卵では,かなり塩分濃度の高い汽水でも孵化に到る.孵化仔魚は正の走光性と走流性を示し,室見川では4月下旬~5月上旬に孵化直後から海へ流下する.11.博多湾と大海湾において,稚魚網,底曳網及び地曳網でシロウオを採集し,海域における成長と生態をみると,海域へ流下した仔魚は全長6.1mmで卵黄を吸収し,河口部から湾奥部のアマモ場を主な生息場とする.昼間は底層に生息するが,夜間になると表層にも出現し,かい脚類を中心とした摂餌を行う.翌年の2月まで海域で成長し,稚魚の形態で全長42~43mmに達した後,河川を遡上する.12.体各部の相対成長と一般形態の変化から,シロウオの形態は全長6~7mm,10~12.5mm,37~38mm及び42~43mmで大きく変化し,生態的には生息域が変わる全長42~43mmで大きく変化する.この結果,シロウオの発育期は,形態と生態の変化が同時に生ずる全長42~43mmを境に,海域における幼形型成育期と河川における幼形型成熟産卵期に区分される.13.シロウオの生活史に関する本研究の成果をもとに,室見川におけるシロウオ資源の増殖方法を検討した.成育場としての海域及び産卵場としての河川における水底質などの環境条件の基準を示し,さらに室見川では,5カ所の試験区を設定して,産卵場の造成試験を行った.その結果より産卵場の造成における河床の削土と投石による増殖方法を提案した.
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更新日 2015.12.07