The ayu, Plecoglossus altivelis altivelis, migrates from the sea to the river in spring. Horita investigated on the water temperature of Mikawa Bay and Umedagawa River and pointed out that the reason why the ayu migrates is coincidence of water temperatures in the river and sea. Wada pointed out that the water level at the sea greatly affects on the migration number. However, a collective view has not been obtained. In this study, the water level at upstream and downstream of the estuary barrage, water temperature, DO, COD, electric conductively and turbidity when the ayu migrates and those when the ayu does not migrate were compared. The ayu wants to migrate when discharge of Guide-flow Fishway, time variation of downstream water level and water temperature are high. On the other hand, the ayu does not want to migrate when the turbidity and COD are high. If the discharge of Guide-flow fishway is large, sometimes difference of water level is high. If time variation of downstream water level is high, time variation of electric conductively becomes high. Besides, if turbidity is high, electric conductively becomes high.

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Gonadal maturation of female ribbon prickleback, Lumpenus sagitta, collected in Funka Bay, Hokkaido, from September 1991 to Mayebruary 1994 was examined histologically. Oocytes from the chromatin nucleolus stage to the migratory nucleus stage were observed in the ovary. Based on the developmental stages of ovarian eggs, the maturity of female of L. sagitta was divided into five stages: the yolk vesicle stage, the oil drop stage, the early yolk formation stage, the late yolk formation stage and the migratory nucleus stage. In January, 91.6% of the femals had oocytes at the yolk vesicle stage and 8.4% at the oil drop stage. In the same period, postovulatory follicles were found in the ovary. Percentages of oocytes at the oil drop stage increased to 80% in May. In the middle of September, all of the females were in the late yolk formation stage. In Novemver, many oocytes at the migratory nucleus stage were observed. These observations, indicate that L. sagitta spawn from December to January in Funka Bay.

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Rainbow smelt Osmerus eperlanus mordax, were collected during May 1993 to April 1995 from Funka Bay, Hokkaido, and the Yamazaki and Rukotsu rivers, which flow into the western side of the bay. Ovarian eggs from females were examined histologically. Oocytes from the chromatin-nucleolus stage to the ripe stage were found in the ovaries. From May to August, females had immature oo-cytes in the yolkless stage and yolk vesicle stage. Postovulatory follicles were also found in the ovaries of females collected from the bay in May. After November, a bimodal distribution of egg diameter was observed. More advanced oocytes developed from the primary-yolk stage in November to the migratory-nucleus stage in April. These observations indicate that group-synchronous oocyte development occurred.
All oocytes in the migratory-nucleus stage from females collected by set nets in the coastal area off Mori were found in late April. From late April to mid-May, oocytes of females collected from the two rivers were in the ripe stage. These results suggest that rainbow smelt inhabiting Funka Bay spawn from late April to mid-May in the rivers flowing into the western coast of the bay. Visual observations of adults in the rivers and the developmental process of oocytes indicate that rainbow smelt return to the sea after spawning.

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2000年から2003年に5月から7月にかけて，北海道東部の別寒辺牛川支流大別川で20種類の淡水魚類が採集された．トミヨ属淡水型とジュズカケハゼは，調査期間を通して数多く採集されたことから，大別川付近に定住する優占種であると推察された．それに対して，イトヨ太平洋型，キュウリウオやアメマスなどの遡河回遊性魚類は大別川を産卵遡上の際に，一時的に生活の場として利用すると考えられた．これらのような淡水魚類の出現パターンの差異は，生息する魚類が多様な生活史を有することを反映していると考えられる．そして，別寒辺牛川の広大な汽水域では，魚類が利用できる多様な生息場所が維持されており，さまざまな生活史を持つ魚類の生息が可能となっていると考えられる．

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It has been considerably investigated on the factors that ayu are triggered migration from ocean to river. However, there are few studies that observed migration characteristics of ayu in the river. There is no collective view of migration characteristics of ayu in the river, because contradict observation results were obtained in the fishway. One of the reasons may depend on the school population, because ayu has a tendency to follow the swimming front ayu. In this study, the behavior of fish school of ayu in running water was analyzed. It was found that staying tendency increases with an increase of school population, so that the migration rate decreases with an increase of school population. The distance between each ayu decreases with an increase of school population. Further, it was found that the fluctuation of streamwise ground speed in upstream migration type is high and also that the fluctuation of spanwise ground speed in downstream migration type is high.

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At Nagaragawa Estuary Barrage, a quantitative monitoring survey has been made on the anadromous population of juvenile ayu since about 10 years at fishways of the Estuary Barrage and in mid and down streams of Nagara River. In this paper, the migrating upstream characteristic of juvenile ayu in the Estuary Barrage fishways and the midstream area was considered, using the data of the anadromous population of juvenile ayu. In addition, a study was made on rationalization of the frequency of measurements and estimation of the total anadromous population from the anadromous population of juvenile ayu in some fishways at the Barrage. From the results of study, findings on the migrating upstream characteristic of juvenile ayu in Estuary Barrage and midstream, rationalization of the frequency of measurements and a method of estimation of the anadromous population of juvenile ayu were obtained.

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三重県宮川の河口から約 30 km 地点にある長ケ逆調整池堰堤において採捕した天然遡上アユ 300 個体に PIT タグを挿入して放流し，宮川の支流である大内山川にある滝原堰堤の魚道内に設置した PIT タグリーダーによ って遡上を記録し，一般化加法モデルを用いた遡上要因の解明を行った．2018 年 4 月 25 日に放流された標識魚のうち，PIT タグリーダーに記録されたのは，5 月 3 日から 7 月 5 日にかけての 24 個体（8％）であった．一般化加法モデルによる要因解析の結果，アユの遡上には滝原堰堤の取水量と宮川の水位が関係していることが示唆された．このこととアユの遡上特性から，遡上魚の一部は宮川本流を大内山川との合流点を過ぎてそのまま上流へ遡上し，三瀬谷ダム直下で滞留していることが予想された．三瀬谷ダムに魚道は付設されていないため，これらのアユの多くは資源的に利用されていないと推察される．これらのアユの宮川上流や大内山川への人為的な汲み上げ，あるいは大内山川への遡上の誘導など有用な資源利用方法の考案が望まれる．

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　本研究は，欧米で用いられている，河川水の電気抵抗の変化から魚の通過数を計測する手法（魚カウンター）を用い，日本の魚道を利用する魚類を計測することを目的としている．魚カウンターのセンサー部を，水理条件等の魚道の設計指針を満足するように，日本の魚道の形状に合わせて後付で構築し，産卵期に魚道を遡上するシロサケ（サケ）の数を計測した．センサー部をビデオカメラにより計19時間撮影した画像から計数したサケの遡上数と魚カウンターによる計測結果を比較したところ，魚カウンターによるサケの計測精度は96％であり，十分な精度で遡上数を計測できることが示された．魚カウンターの精度を低下させる可能性のある要因，センサー部の後付での構築方法及び調査コストについて整理，考察した．

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