中賢治，的場洋，伏木省三．1976．人工河川河口域のコアユの遡上に関する研究−〔III〕 標識放流魚の遡上について．滋賀県水産試験場研究報告 28: 1-6　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
田沢茂，水谷英志．1976．人工河川における産卵親魚の放養適正量について〔II〕．滋賀県水産試験場研究報告 28: 7-13　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
水谷英志，田沢茂，大野喜弘，的場洋，中賢治，伏木省三．1976．アユの産卵から流下仔魚までの生残率について〔II〕．滋賀県水産試験場研究報告 28: 15-20　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
水谷英志．1976．各種魚類による流下アユ仔魚の食害〔II〕 実験人工河川河口域と姉川のアユ産卵場附近に棲息する魚類の胃内容物について．滋賀県水産試験場研究報告 28: 21-28　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，中賢治，滝克典．1976．産卵アユ親魚の河川放流後の湖中降下について．滋賀県水産試験場研究報告 28: 29-31　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，的場洋．1976．電照方法によるアユの成熟促進．滋賀県水産試験場研究報告 28: 33-36　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
中賢治，的場洋．1976．入れ墨および注入法によるアユの標識方法の検討．滋賀県水産試験場研究報告 28: 37-42　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
山本淳之．1976．姉川人工河川による河口噴流実験．滋賀県水産試験場研究報告 28: 43-52　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)

中賢治，伏木省三，滝克典．1975．人工河川河口域のコアユの遡上に関する研究−〔II〕 標識放流魚の遡上について．滋賀県水産試験場研究報告 27: 1-9　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三．1975．人工河川におけるヨシノボリ，ウツセミカジカの遡上防止施設．滋賀県水産試験場研究報告 27: 10-11　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
田沢茂，水谷英志，伏木省三．1975．人工河川における産卵親魚の放養適正量について−〔I〕．滋賀県水産試験場研究報告 27: 12-17　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
大野喜弘．1975．人工河川におけるアユの産卵環境〔II〕 小型親魚に対する産卵床の適正な環境について．滋賀県水産試験場研究報告 27: 18-22　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
水谷英志．1975．各種魚類による流下アユ仔魚の食害〔I〕 実験人工河川の河口域に棲息する魚類の胃内容物について．滋賀県水産試験場研究報告 27: 23-25　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，中賢治．1975．琵琶湖産春アユの漁況予報に関する考察．滋賀県水産試験場研究報告 27: 26-30　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)

中賢治，八木久則，伏木省三．1974．人工河川河口域のコアユの遡上に関する研究−〔I〕 標識放流魚の遡上について（予察）．滋賀県水産試験場研究報告 25: 1-6　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，八木久則，中賢治．1974．産卵終期に実験人工河川に遡上したアユについて．滋賀県水産試験場研究報告 25: 7-11　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，八木久則．1974．実験人工河川に遡上した雑魚について．滋賀県水産試験場研究報告 25: 12-14　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
田沢茂，水谷英志．1974．人工河川におけるアユ産卵親魚の適正量について（予察）．滋賀県水産試験場研究報告 25: 15-19　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
大野喜弘，伏木省三．1974．人工河川におけるアユの産卵環境（1） 大型養成親魚に対する産卵床の砂礫の適正な大きさについて．滋賀県水産試験場研究報告 25: 20-25　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
水谷英志，田沢茂，大野喜弘．1974．アユの産卵から流下仔魚までの生残率について．滋賀県水産試験場研究報告 25: 26-30　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
西田陸，伏木省三，中賢治，水谷英志，田沢茂．1974．びわ湖のアユの天然産卵場および産卵群について．滋賀県水産試験場研究報告 25: 31-45　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
伏木省三，田沢茂，八木久則．1974．滋賀県におけるアユの産卵期ならびに成熟について．滋賀県水産試験場研究報告 25: 46-51　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
中賢治，水谷英志．1974．びわ湖におけるコアユの浮上群泳（マキ）についての研究−〔I〕 マキを形成するコアユの体長組成，成熟度，摂餌状態について．滋賀県水産試験場研究報告 25: 52-62　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
中賢治，田沢茂，水谷英志，八木久則．1974．コアユの群形成に関する池中実験．滋賀県水産試験場研究報告 25: 53-68　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
水谷英志，西田陸，田沢茂，伏木省三．1974．コアユ産卵場におけるヨシノボリ・ウツセミカジカのアユ卵（アユ仔魚）食害について．滋賀県水産試験場研究報告 25: 69-78　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)
添付資料　人工河川試験設備図面．滋賀県水産試験場研究報告 25: --　==> 本文PDF (滋賀県，ｵｰﾌﾟﾝｱｸｾｽ)

1) 1993年から4ヶ年，相模川において，アユの遡上生態調査を実施した。また，過去の資料も用いて，海産アユの資源増大のための方策を検討した。
2) 遡上は，年別に変化が見られるが，4月が最盛期で，過去の調査と比較すると，1ヶ月早まっていた。
3) 各年の最初の大量遡上日は月令8日，23日の上弦，下弦に見られ，遡上が月令に影響を受けていることが示唆された。
4) 海産種苗の採捕量(C)と遡上量(A)には正の相関が認められ，両者の関係は
A=exp (1.17・10-6・C+11.18) (γ=0.824)
の回帰式で表わされた。 また，2〜4月の平均河川流量が少なく，10t/s以下の年を除くと
A=exp (1.35・10-6・C+11.39) (γ=0.971)
と，さらに高い相関が得られ, 河川流量が遡上に影響を与えていることが示唆された。
5) ふ化仔魚降下量(E)と翌年の海産アユ種苗採捕量(C)には正の相関が認められ，Beverton-Holt型およびRicker型の再生産曲線に回帰させたところ，
C=0.01757/7.193・10-9+1/E (γ=0.651)
C=0.0485E・exp (-5.79・10-9・E) (γ=0.670)
となった。
また，仔魚降下量(E)と翌年の遡上量(A)には，
A=exp (6.02・10-9・E+11.563) (γ=0.668)
と，指数的な関係が認められた。
6) アユの資源増大には遡上時期の河川流量の維持と降下量の増大が重要であると考えられた。

1994年から2005年の3月から5月もしくは6月にかけて，茨城県久慈川の下流域においてアユの遡上様式を調べた。その結果，アユは早い年では3月上旬に，遅い年では4月中旬に遡上しているのが確認され，遡上個体数が多い時期は，主に4月下旬から5月中旬にかけてであった。遡上期間中の体長組成の変化をみたところ，遡上期初期には大型群が，後期には小型群が遡上する傾向にあった。また，大型群から小型群への移行は毎年一定ではなく，遡上群が徐々に移行する年と急激に移行する年および遡上群の移行が一定ではなくモザイク状の年が存在することがわかった。河川と海洋の平均水温に基づき調査年をクラスター分析した結果と遡上群の移行状況の関係を検討したところ，大型群もしくは混合群から小型群への移行が順に移行した年は全体的に平均河川水温が低く推移する年に該当し，逆に遡上群の移行がモザイク状であった年は，全体的に平均河川水温が高く推移する年に該当していた。さらに，大型群がみられた年は平均海水温が10°Cを下回る旬が5旬以上みられた年であった。以上のことから，平均河川水温と平均海水温の推移を調べることで久慈川におけるアユの遡上様式を推定できる可能性が示唆された。

he annual stock fluctuation of ayu, Plecoglossus altivelis altivelis, was investigated through their larval and juvenile stage from November to May in the 1999, 2000 and 2001 year groups both in the coastal waters and in the Hidaka River of the central Wakayama Prefecture. The catches of juvenile ayu in coastal waters and the number of ayu migrating upstream the Hidaka River increased with the year group from 1999 to 2001. Thus the 2001 year group was considered to be a bumper year. Ayu larvae in the 2001 year group occurred abundantly in the surf zones around the mouth of the Hidaka River, and their sizes were larger than those of the other two year groups. These findings suggest that the amount of larval recruitment and the following larval growth rate in surf zones should correspond to the level of the stock of ayu population. We suppose that high larval mortality of ayu in the sea could occur before recruitment to surf zones, and extent of the mortality would be an important determinant for the year-class strength.

まとめ：びわ湖のアユの集団構造と陸封化による変異性の特徴を明らかにしようとした研究の一部として，本報では，発育初期から集団の分化が明瞭になるまでの時期における，分布様式の変化と体形の変異をとりあつかった.形態的特徴を基準にして鑑別された発育段階区分(phase)に基づいて，各種の採集方法で湖のいろいろな場所から得られたアユを分け，各phaseの分布を整理するとともに，phase別体長組成から発育と生長との関係を検討した.さらに，この関係を体形の変異と結びつけて考察した.得られた結果を以下に要約する.
1.孵化仔魚(phase A₁・A₂)は秋に流入河川の産卵場から湖へ向って夜間降河する.親魚の繁殖様式の多様性を反映して，降河時期や仔魚の大きさに変異が認められる.
2.phase B₁・B₂・C₁の仔魚の分布密度は，湖流に対応して湖の西岸に高く，湖内全域および流出河川へもおよんでいる.このことから，その分布様式は遊泳力の未発達による半ば受動的な分散を特徴としている.
3.phase C₂・C₃・D₁の仔魚は，沖合から沿岸部の第2湖段〜湖段崖に生活の場を移す.またこの段階から流水型と止水型の体形分化がはじまる.
4.群れ形成とともに体形分化が顕著になるphase D₂では，生活様式の分化がはじまり，流水型の集団は湖棚部まで分布域を拡張し，止水型の集団はそのまま湖内深所にとどまる.phase E₁・E₂になると，前者は2〜3月頃からそ河回遊を開始し，後者は5月頃から再び沖帯部に分布する.
5.冬季には，湖内の同じ水域内に種々の体形のアユが混在するが，群れ形成が進む早春頃から，生活様式や体形の異なった集団が別々の場所に分布するようになる.
6.体形の変異を発育と生長との関係から検討した結果，phase C₂以前の初期段階では，相対的に発育の進んだものの体形は流水型に，相対的に生長の進んだものは止水型になり，それ以後の段階では，その閨係が逆転することが示唆された.

まとめ：群れ形成後のびわ湖のアユを魚体の大きさと生活様式から，一応そ河集団と湖中集団とに区分し，各集団の諸特徴の変異（食性.体形•鰭条数）を手がかりにして集団相互の関係を検討した.
1.そ河集団は，そ河回遊量や体長組成の時期別変化からみて，晩春をさかいにして，早期そ河群と晩期そ河群とに分けられる.
2.湖中集団のうち晚春から夏にかけて沖合に出現する矮小形の、"沖アユ"は動物プランクトン食を続け，湖内の岩礁部で付着藻類を主食にする、"大形アユ"と区別できる.
3.湖中集団は典型的な止水贺体形を示すのに対し，早期そ河群は典塑的な流水型の体形をもっている.他方，晩期そ河群は両者の中間的な体形を呈する.
4.背鰭と尻鰭の条数を4つの集団で相互に比較した結果，早期そ河群は両鰭条数とも多く，晩期そ河群はその逆であった.また湖中集団では背鰭に多く，尻鰭に少ないという結果を示した.標本数の少ない"大形アユ"を除く 3集団の間の差異は統計的に有意であった.
5.びわ湖アユの生活様式の多様性と湖の環境条件との関係および形態的諸変異と発生初期の環境条件との関係について若干論議した.

まとめ：本報では性成熟期における各集団の繁殖様式のちがいを，性成熟や産卵の過程，産卵行動の比較をつうじて検討するとともに，成魚段階でみられる諸形質の変異について整理した.
1.びわ湖の産卵アユは産卵場への来遊の経路と時期からみて，そ河産卵集団と降河産卵集団とに一応大別できる.
2.そ河産卵集団は，8月下旬から9月上旬にかけて湖から短時間のうちに流入河川の下流域に来遊し，雌雄とも魚体のよくそろった小形アユが高密度の産卵コロニ一をつくって河口に近い浅瀬で産卵する.9月上旬頃を産卵盛期として時期とともに魚体が小形化する.
3.降河産卵集団は，前者よりも遅れて産卵場へ降河し，9月中旬頃から産卵を開始する.来遊経路•性成熟過程および体長組成は前者より多様である.産卵場所は前者よりやや深く，また産卵コロニーは形成しない.
4.背鰭条数においてそ河産卵集団は降河産卵集団よりも条数が多く，その差は高い有意水準を示す.
5.両産卵集団と成熟前の生活様式の異なる集団とのつながりを背鰭条数の比較に基づいて検討した結果，そ河産卵集団が"沖アユ"に，降河産卵集団が晚期そ河群に由来することが示された.早期そ河群に由来する産卵群は標本数が少なくて，この方法では追跡できなかった.また湖内の"大形アユ"は8月下旬に高い群成熟度を示したが，繁殖様式は不明である.
6 .各産卵集団の抱卵数と卵径•背鰭前部長•總把数は，いずれも魚体の大きさ一各集団の生長速度のちがい一と関係して生じた連続的な変異としてあらわれ た.

まとめ：1.今までの3報の結果を総括して，びわ湖のアユの.内部構造をA〜Dの集団のグル一ピングによって示し，各集団の諸特徴を整理した.
2.グル一ピングの根拠のうち各集団の鰭条数や体形の変異が発育初期の異なづた生活諸条件のもとで生長と発育との関係に規定されて生じており，発育のかなり初期から準備されていることを考察した.
3.びわ湖めアユの特徴を海アユとの比較によって整理した.その形態的特徴は全体として生長速度の遅滞に基づいていること，産卵期や卵数および卵径など繁殖に関係した特徴も海アユとは異なり，性成熟の早期化と抱卵数の相対的増大の方向に変異していることを明らかにした.
4.びわ湖めTユめ陸封にともなう矮小化やそれに関連した諸変異が，アユ以外の陸封魚においても広ぐ認められる変異性の一般的特徴であることを論じた.
5.びわ湖における陸封型ブユの起源と湖内での種内分化について，びわ湖の地史との関係から考察し，生態的分岐が各集団の隔離の要因になっている面についても論及した.

The larval development and growth of Plecoglossus altivelis are described based on a series of 268 reared specimens, ranging between 6.1 and 88mm standard length (SL). Development of morphological characters are described, with special reference to fin formation.
Transformation from the larval to juvenile stage was observed when fish attained about 27mm SL, 80 to 90 days after hatching. Fin rays completed segmentation at 33mm SL, and branch-ing at 42mm SL. Larval growth for 140 days was expressed by linear regression. Relative length of preanal length to total length changed drastically during yolk-sac larva to shortly before transformation, then became constant after 45mm TL. The importance of organogenesis and morphometric characters with respect to the ecological aspects of early life stages is discussed.

大岡川を対象にアユの分布と産卵場選択について解析した。調査は2009年10月から2010年2月、2010年9月から2011年2月の2箇年、下流約2kmの範囲を上流D1からD10区間に分けて行った。繁殖期間におけるアユの生息密度は、産卵場あるいはその周辺で10月、11月に高い値を示した。卵は、2009年が10月から12月、2010年は10月から2011年1月まで確認した。産卵場は2地点、最戸橋一最戸下橋のD5区間が主な産卵場、他の1地点は、11月だけであった。卵ありと卵なしの瀬の物理的環境要因の比較では、貫入度、基質組成の砂、ゾW樂で有意差を示し、卵ありの瀬で貫入度、ゾW樂が高い値を示した。D5区間の産卵場は橋下の暗所、水深21cm、流速43cm/s、基質性状が/W樂、浮石状態であった。D5区間には長さ30cm、幅23cm、深さ4cmの産卵床が形成され、礫には付着卵が多く確認された。卵付着基質は5から10mmの小礫が多かった。以上、大岡川の主な産卵場は狭い範囲に限定されており、今後、在来アユ集団を保全、再生するための方策が望まれる。
キーワード：アユ 分布 産卵場 物理的環境 大岡川

Annual changes in the catch of marine juvenile ayu were examined with special reference to the cause of the rapidly decreasing catch in recent years.
The influence of various factors such as environmental changes on ayu resources used to be small in the days when good and poor catches of ayu were repeated naturally. Even when an influence was found, it was corrected in a short period. It seems that the abnormal shortage of water in the autumn of 1984 caused a negative effect on the reproduction of ayu and such a situation subsequently resulted in the rapid decrease in the catch of marine juvenile ayu in the spring of the following year. In addition, a positive correlation was found between the amount of precipitation during spawning season and the catch of marine juvenile ayu in the following spring.

Sexual dimorphism in the anal fin of ayu Plecoglossus altivelis was studied by serial sampling of ayu. Changes in the shape of the anal fin were triggered by sexual maturation in both sexes; however, secondary sexual characteristics developed drastically in males while they did so incon-spicuously in females. The anal fin base became larger in both sexes during sexual maturation, but grew much faster in males than in females, and consequently males have larger anal fin bases than females. Secondary changes in the anal fin height occur only in males, and the change is negative, and consequently females have larger anal fin height than males. The relationship between the shape of the anal fin and the sex hormone levels in diploid and triploid ayu suggested that the development of secondary sex characteristics in the anal fin of ayu is controlled by androgens.

The spawning pattern and relationship to body size at maturation of amphidromous and land-locked forms of ayu, Plecoglossus altivelis, was investigated. Ovarian eggs of prespawning females showed a multimodal size distribution, regardles of body size, indicating that all the females were potentially a batch spawners. The spawning schedules of individual fish were described under simulated spawning conditions. Most large-sized females died soon after their first spawning, whereas approximately half of the medium and small-sized females successfully completed a second spawning some two weeks after the first. Such a size-specific spawning pattern was common to both forms, but development of a third batch of ovarian eggs after the second spawning occurred only in the land-locked form. Spawning tactics in ayu were related to offspring survival and adult mortality during spawning season.

Spawning downstream migration of ayu Plecoglossus altivelis in the Chikuma River was investigated by using catch data obtained from fishweirs, and several ecological aspects were revealed. Following by the finish of the sexual maturation, lager sized fish migrated downstream earlier than smaller sized fish. The number of migrating fish showed diel change, and the migratory activity was rather diurnal in relation to water temperature. During the migratory season, frequency of migrating fish changed daily with some peaks at which steep increase in the number occurred. The peaks often corresponded to rainfall，suggesting that some disturbances in environmental factors caused by rainfall worked as a trigger for the simultaneous downstream migration.

Landlocked ayu stocks in the Biwa Lake system were investigated using a mitochondrial DNA marker to evaluate their genetic structure. Samples examined consisted of spring migrants (o-ayu), fall migrants (ko-ayu), and juvenile residents in the lake at a predifferentiated stage. Compared to amphidromous fish, the landlocked fish, overall, were characterized by a lower genetic diversity that has resulted from lower haplotype diversity and a higher similarity in nucleotide sequences between haplotypes. Some nucleotide diversities that were unique to the samples showed that the landlocked ayu are not genetically homogeneous, suggesting the occurrence of a single metapopulation throughout the Biwa Lake system. The pairwise fixation index inferred different respective local populations for the o-ayu and ko-ayu ascending the same stream in the same year class, as well as the isolation by distance among local populations of ko-ayu. For conservation practices during transplantation with fish from non-native stocks, it should be taken into account that the Biwa Lake stocks have a high probability of not being genetically uniform.

Studies on population structure of ayu (Plecoglossus altivelis) were reviewed to evaluate the present status of our task concerning genetic structure of the species. More detailed description has been allowed according to the technical progress in analysis. At first, phenotypic markers including morphological, ecological and behavioural characteristics were successful in distinguishing landlocked Lake Biwa form from amphidromuos form. Allozyme analysis was used to describe new subspecies as P. a. ryukyuensis, and provided genetic basis on the divergence of landlocked Lake Biwa form. Mitochondrial DNA analysis observed metapopulational structure between insular and mainland local populations, while genetic structure within a Japan'samphidromous form was remained suggestive but unclear. Our study proved effectiveness of microsatellite DNA as a genetic marker, and successfully detected genetic structure within a Japan's amphidromous form. Application of microsatellite DNA can contribute to establish conservation unit and/or resource management unit.
Key words: amphidromy, phenotypic characteristics, allozyme analysis, mitochondorial analysis, microsatellite analysis, gene flow, metapopulation

Drifting larvae of amphidromous ayu, Plecoglossus altivelis must encounter seawater at the river mouth where adaptability to salinity has an influence over their early survival. Salinity tolerance of newly hatched larvae of ayu was investigated employing two different tests. One represented a traditional way, in which survival rates of larvae held in 120 % artificial seawater were compared between treatments at 18 or 26°C ×for 24 or 48 hours. Judging from the outcomes that fluctuated according to the conditions, this test was found to be less appropriate as a measure of salinity tolerance. Another approach was rather novel, using larvae exposed to 180% artificial seawater at 26°C. The mean survival period was successful in separating clutches by their salinity tolerance ability. In addition, time spent for the test was comparatively short, which could set larvae free from the limitation of energy expenditure. In case of evaluating the salinity tolerance of yolk-sac larvae with focusing on the performance of chloride cells, we propose the acute test based on individual survival periods because of its convenience.

As to the sex ratio of the Ayu crowding together at the spawning ground, it has been known that in the early period of the spawning season males predominate, while, on the contrary, towards the end of it females predominate. But no attempt has been made to investigate thoroughly the matter.
The present study was undertaken to study this problem. For this purpose, observations were made, on the one hand, on the seasonal changes of the sex ratio in connection with those of the maturity, population size and quantity of eggs spawned at two well-known spawning grounds of Sagami River in Kanagawa Prefecture. On the other hand, the spawing behaviour was observed in detail at artificial spawning beds which were prepared in experimental ponds.
The results obtained were as follows:
1) The proportion of females to the population exhibited marked seasonal changes. The number of males increased gradually from the early period of the spawning season to the height of it, where reaching its maximum. Thenceforth it gradually diminished (Fig. 1).
2) Parallelisms and also high correlations were found to exist between the proportion of male and population size or maturity (Fig. 2, 3). Furthermore the amount of eggs spawned showed a tendency to be largest when the proportion reached the maximum (Table 1).
3) Males, as the gonads matured, crowded together densely at the spawning beds. By this prespawning behavior silts laid on the gravel of the beds were removed. A few attendant females joined the male shoals, thus depositing their eggs. From these observations it would be assumed that the great proportion of males in the height of spawning was ascribed to such a spawning behaviour.
4) During the actual spawning process, much gravel of the beds were moved by fish, thus a circular hollow being formed (Table 3). A high correlation between the amount of the movement of gravel, which seemed to indicate the intensity of the spawning activity, and the quantity of eggs spawned was observed to exist (Fig. 7).

It is well known that the Ayu spawn on river beds of gravel and their eggs adhere firmly to the pebbles, but little information is available on the size of gravels on which the eggs are laid and the requirements of spawners for the size composition of gravels of spawning bed.
The present study is undertaken to these problems. For this purpose, on one hand, the size of gravels sampled from natural spawning beds in two well-known spawning grounds of Sagami River in Kanagawa Prefecture were measured and the numbers of eggs attaching to the gravels of various size were counted. On the other hand, the relationships between the size of spawners and that of gravels on which they spawn were pursued in experimental ponds (Fig. 1).
The results obtained were as follows:
1) At the natural spawning grounds, most of the eggs were laid on small gravels 10mm or less in diameter (Table 1).
2) It was commonly observed in the ponds that the spawning fish measured 13.3-15.9cm in length flocked to the artificial spawning beds made of relatively small gravels, viz., 20mm or less in diameter, and spawned there. Among them the larger spawners occasionally laid their eggs on larger gravels 10-20mm in diameter, but the smaller ones exclusively spawned on smaller gravels 10mm or less in diameter (Table 2).
3) From these observations it is assumed that the Ayu tend to gather for spawning round the stream bed riched with small gravels and spawned there, and it is more clearly observed in smaller fish. The fact mentioned above seems to relate to the spawning behaviour of the fish. The Ayu have a habit of moving gravels by their movement in spawning, accordingly it is assumed that they prefer to beds of gravels which they can move compartively easily.

On the depth of water of the spawning grounds of the Ayu, several observations have been reported, but there are noticeable discrepancies among them and the reason for the discrepancies has not yet been pursued thoroughly.
The present study was undertaken to secure information on this problem with special regards to the relationships between the size of spawners and the depth of water of the spawning grounds. The study consists of observations carried out in the Sagami River in Kanagawa Prefecture and experiments performed in rearing ponds of the Freshwater Fisheries Research Laboratory at Hino, Tokyo. In the field observations, the depth of water of five representative spawning grounds in Sagami River and the size of spawners sampled there were measured. In the experiments, artificial spawning beds of different depths were prepared in the ponds and the depth of the beds where spawners of different sizes laid their eggs were determined (Fig. 1).
The results obtained were as follows;
1) At natural spawning grounds, large spawners tend to gather about deep streams, and small spawners shallow streams (Table 1).
2) In experimental ponds, small spawners less than 13.6cm in length spawned exclusively on beds 50cm or less in depth. While large spawners more than 15.Ocm exercised no definite selection on the depth of spawning bed (though they often spawned on deep beds more than 50cm in depth).
3) Besides the depth of water, the size composition of bottom gravel has a significant effect on the spawning of the species, small spawners tend to select bottom of smaller gravel as their spawning beds than large spawners; generally in spawning grounds of the “shallow stream type” the current is less rapid and the gravels are smaller than in the case of the “deep stream type.” Accordingly it may be said that shallow streams have conditions suitable to requirements of small spawners, and vice versa (ISHIDA, 1961).

Several observations have been reported on the natural features and the distribution of the spawing grounds of the Ayu, but they seem fragmentary.
The present study was performed to obtain fuller information on these problems. For this purpose, the examination of the spawners and the eggs laid on stream beds and the to-pographical survey were done in 12 rivers.
The results obtained were as follows:
1) The location and the extent of the spawning area were very variable, i.e., in some rivers both the distance from the mouth of the river to the spawning area and the extent of the latter were less than 10km, and in others they were over 100km and 50km respectively.
2) Some certain functional relations were recognized between the average gradient (between the mouth of a river and the spawning area) of the river beds and the distance from the mouth to the spawning area or the extent of the latter. The gradient of the spawning area also changed with the gradient of the river.
But the former increased in the area of the extreme small value of the latter.
3) The gradient of the river decides the speed of the current and consequently, the composition of the gravels of the stream bed. Accordingly it is assumed that some certain ranges of the speed of the current or those of the size of the gravels are essential factors for the limitation of spawning area.
4) The spawning grounds situated at a point where the current was irregular, i. e., junction, curve, sand band and bridge.
5) As the balance between the current velocity and the size of gravels is unstable at these places pointed out above, the bed varied continually, its surface being unsettled. From the observations on the spawning behaviour of the Ayu (ISHIDA 1961), it may be suggested that unstable stream beds as mentioned above are suitable sites for the spawning of the fish.

岩木川統合頭首エの階段式魚道における魚類の遡上実態を1996年•1997年の灌漑期間の5月から9月にかけて調査するとともに，魚道内の流況•流速測定を行った。調査結果の概要は，つぎに示すとおりである。
�@ 遡上が確認された魚類は13種類で，おもな魚種としてウグイ，アブラハヤ，アユ，オイカワ，であった。また，魚類の1日における遡上数が多い時間帯は，午後から日没直後（20時前後）であった。また，翌日早朝の遡上数は少ない傾向にあった。
�A 遡上魚の体長分布を把握し，さらに平均流速を遡上した魚の体長で割った値を突進速度として表し，1日で遡上数が最も多い時間帯の体長分布，切り欠き•隔壁の平均流速から突進速度を試算した。その結果，遡上時期の体長分布や魚種により突進速度の範囲にばらつきが見られるが，体長の10倍より大きい流速で遡上する場合もあることがわかった。
�B 切り欠きのみの越流と魚道隔壁全面の越流の遡上特性を調べると，全面越流時でも産卵期を迎えた体長の比較的大きい魚体の遡上が見られたが，おおむね切り欠きのみから越流するときの方が遡上する傾向にあった。
�C 3次元ベクトル流速分布による魚道プール内の流況を把握し，切り欠きのみの越流の方が'全面越流に比べプール内の流況が良好であることがわかった。
ｷｰﾜｰﾄﾞ：頭首工事，全面越流型階段式魚道，魚類の遡上調査，水理特性

産卵保護期間を再検討するため，吉井川に遡上したアユ稚魚を対象に，採捕日ごとに耳石日周輪を解析し，遡上アユの推定ふ化日から再生産の実態を明らかにしたので，以下に報告する。

1 荒川のアユ産卵場の実態を把握するため、聞き取り調査、産卵場の形状調査及び瀬付き親魚の成熟調査を行った。
2 聞き取りによる産卵場の調査では、長瀚町2か所、寄居町2か所、花園町1か所、熊谷市2か所、吹上町1か所の合計8か所であった。
3 荒川の産卵場は、1987年には河口から74.5〜121kmに位置し、4 6.5 kmの区間にあったが、1988年には、河口から75〜112kmに位置し、その.区間は37kmと産卵水域が狭められる傾向であった。
4 玉淀ダムが設置された1964年以降に、それまで産卵場がなかった長瀞町、皆野町地先に産卵場が見られるようになり、産卵水域は、ダムが設置されたことにより拡張された。
5 現在までに存在した産卵場は30か所で、このうち、1987年には17か所、1988年には8か所の産卵場*s見られたが、砂の堆積、橋の新設等の河中築造物の影響、河川工事による河道の変化及び増水時の砂の流失等で産卵場が減少する傾向があった。
6 産卵場の水深は0. 5〜1.5 mで、深瀬型の産卵場であった。
7 産卵場の形状は産卵床は、すべて浮き石で、産卵場の上流は必ず左右いずれかに湾曲していた。
8 瀬付親魚の成熟度調査では、調査したすべての水域の親魚は成熟しており、産卵に参加していたと思われた。

1 1989年に荒川の秋ケ瀬取水堰付近におけるそ上稚アユの現存量と魚道の効果調査を4月から6月の間に実施した。
2 四ツ手網によるそ上稚アユの採捕では、1時間当たりの採捕尾数は第1回調査時の4月20日が24.6尾と最も多かった。
3 標識アユ放流後、早いもので3時間30分後から再捕された。
4 四ツ手網採捕によるそ上稚アユは平均体重では初期の大型傾向が見られた。
5 PETERSEN法による調査時のそ上稚アユの左岸側現存量は、4月1,400尾、5月500尾、6月1，300尾と推定された。
6 調査時におけるビク調査及び釣り人からの聞き取り調査から秋ケ瀬取水堰下右岸側の釣りによる漁獲量は4月7,000尾、5月6,000尾、6月2,000尾と推定された。
7 魚道の効果調査では第2回調査時の4月28日の午前6時から午後1時30分にかけて147尾のそ上が見られ、そ上する時刻別の魚体測定値には差は見られなかった。
8 魚道内で採捕したアユは第2回調査時が347尾、第4回調査時が264尾で、魚道をそ上したアユと魚道内の各プールで採捕したアユの体重組成では差は見られなかった。
9 魚道をそ上したアユと魚道内で採捕したアユから推定した現存量は、四ツ手網採捕結果の現存量の10〜17倍であり、堪下にアユの滞留が考えられた。
10 左岸側の第3洪水吐ゲートの開閉は、稚アユのそ上が活発になることから適切な放水管理は、魚道への呼び水としてそ上アユの魚道利用に効果を発揮するものと考えられた。
11 潮沒の影智については、満潮から退潮時にそ上ピークになり、張潮時にはそ上ピークは見られなかった。
12 魚道を最もよくそ上した時の堰下放流量は平均で76.43m3/secであり、また、魚道の流速は1.5m/secであった。
13 魚道のそ上ピーク時には、14.1尾/minの稚アユがそ上した。

1986年から1990年の5か年間に荒川の秋ケ瀬取水堰付近におけるそ上稚アユの現存最と魚道そ上実態調査を4月から6月の間に実施し、以下の結果を得た。
1秋ケ瀬取水堰下流のそ上稚アユの現存量は1986年5月下旬0.3万尾、1988年5月下旬0.2万尾、1989年4、5、6月0.1〜0.2万尾で、1990年が4月上旬2万尾、4月下旬7万尾、5月中旬2万尾、6月上旬2万尾と推定された。
2 魚道をそ上した稚アユと標識アユから堰下の現存Sを推定したところ、建網採捕結果から推定した現存量の1.8〜18倍に当たる。このことから、そ上してきた稚アユが堰付近に滞留していることが考えられる。
3 1990年にそ上量が多かった原因として荒川の秋ケ瀬取水堰下流の笹目橋、戸田橋、新荒川大橋のB OD値は1970年の1/2以下の値にまで改善されていることが考えられた。
4 1990年の左岸、右岸の建網による採捕状況では稚アユの採捕最、標識了ユの再捕量からそ上稚アユの主群は越流部がある右岸沿いを中心にそ上し、堰下で魚道を見つけながら滞留しているものと考えられ、釣り人の漁獲魚がほとんどアユであることからも主群は右岸に滞留していることが推察された。
5 潮汐の潮位と魚道そ上尾数については、満潮時から退潮時にそ上ピークが現れ、張潮時はそ上ピ—クは見られず、そ上の日周期は潮汐と密接な関係があるものと考えられた。

熊谷市地先の荒川における天然アユのそ上実態を把握するため、漁獲調査を行ない、以下の知見を得た。
1.秋ケ瀬取水堰と流から放流した標識魚は、熊谷市地先で漁獲され、天然アユは熊谷市地先までそ上していることが判明した。
2.放流した海産アユは、放流地点よりヒ流21.5km、下流8. 4 kmの範囲で漁獲されたが、放流地点の上流2. 0 km、下流3. 3 kmの範囲で最も多く釣獲された。
3.放流地点から上流21.5kmで漁獲された標識魚は、魚道が付設されていない明戸サイフォンを降雨による増水時にそ上したものと思われた。
4.1990年に秋ケ�P取水堰をそ上した天然アユは、推定約13万尾であったが、1991年に熊谷市地先にそ上した天然アユの数量が0.5万尾と少なかったのは、そ上途中の支流河川への分散、降水量、河川流量、水質等の要因が関与しているものと考えられた。

1 埼玉県が内水面地域活性化事業(水産庁補助事業）において1987年から1992年にかけて整備した7魚道のうち、1991、92年に整備された高麗川の鹿台堰魚道、高麗川一号堪魚道、高麗川二号堰魚道について1992、93年に利用状況を調査した。
2 鹿台堰魚道のそ上魚は、アユ、ウグイ、オイカワの3種で、降下魚はギバチであった。
3 高麗川一号堰魚道のそ上魚等は、アユ、ウグイ、ギンブナ、タモロコ、テナガエビで、降下魚等はギバチ、ドジョウ、アメリカザリガニであった。
4 高麗川二号堰魚道のそ上魚はアユ、ウグイ、オイカワ、アブラハヤ、夕モロコ、カマツカ、ギバチ、ナマズ、ニジマスで、降下魚はオイカワ、アブラハヤ、シマドジョウであった。
5 調査した魚道の構造、施設について「魚ののぼりやすさからみた河川横断施設概略点検マニュアル」をもとに評価した結果、高麗川二号堰では農業用取水ロの改修も同時に行われたため{こ、堰越流部は堰中央部側にあったが魚道設置場所は右岸側であり、堰下中央部への魚の滞留が懸念された。また、高麗川一号堰は入口の形状が変化しやすく、土砂が堆積する恐れがあるので、管理運営上、小型重機での維持管理が必要に思われた。
さらに、高麗川二号堰魚道では、河川流量が減少する時期には、魚道が施設の流れの主体であるように改善する必要がある。

1 アユの産卵保護効果を把握するために、1991年から94年までの4か年間、行田市須加地先の利根大堰で仔アユの流下実態を調査した。
2 仔アユの流下ピークは10月下旬から11月上旬であった。
3 推定総流下尾数は、1991年は9.4億尾、92年は11億尾、93年は94億尾、94年は6億尾と推定された。
4 1991年から94年までの仔アユの流下ピークは10月下旬から11月上旬で、ふ化水温から産卵日を推定すると、10月上旬から中旬になり、この期間中、産卵水域内に禁漁水域を設置することは適正な資源管理と考えられた。

岩木川統合頭首エの階段式魚道における魚介類の1995年の遡上実態を灌溉期間の6月から9月にかけて調査するとともに，魚遁内の流況•流速測定を行った。調査結果の概要は，つぎに示すとおりである。
1.遡上した魚介類は14種類で，ウグイ（413匹，37.2%)，アブラハヤ（341E5, 30.7%)，アユ（185匹，16.7%)，オイカワ（130匹，11.7%)が観察された。
2.魚類の一日における遡上が最も多い時間帯は，8月以前では昼間と夕方から夜(18時〜20時）にかけての二回あり，8月以降では14時〜16時の一回であった。また，20時以降の遡上は少なくなる傾向にあった。
3.魚道プール内の流況については，ノッチ部のみから越流する場合，水面部の流速は大きく，底面部の流速は比較的小さいことがわかった。また，ノッチ部でないプール上流部域に魚が休息できると思われる流速変化の小さい領域が形成されることがわかった。なお，魚道隔壁全面から越流する場合，プール内の流況は，プール内全域が空気混入により白濁しており，魚類が休息できないほど乱れることがわかった。このことは，この形式の魚道は，本来ノッチ部から越流させるべきものであり，画壁天端からの越流をさせるべきでないことを示している。この点，魚道の通水量管理が重要であると考えられる。

Abstract Although ayu Plecoglossus altivelis altivelis did not change the abundance of inorganic nutrients such as phosphorus and nitrogen, ayu decreased the abundance of diatoms and increased the abundance of blue-green algae such as Homoeothrix. The intensive scraping of ayu also reduced the number of chironomid and tipulid larvae that mainly inhabit the upper surface of cobbles and boulders, and increased the percentage of ephemeropteran nymphs such as Ephemeroptera. Ayu had a negative effect on other omnivorous fishes such as Japanese dace Tribolodon hakonensis and pale chub Zacco platypus. In contrast, the presence of Japanese dace, pale chub and pike gudgeon Pseudogobio esocinus esocinus enhanced ayu growth by decreasing the number of algivorous invertebrates and increasing the algal biomass through trophic cascades. Interspecific relationships in stream ecosystems are thought to be very important for the management of biodiversity and ayu growth.
Key words: aquatic insect, ayu, benthic algae, stream community, trophic cascade

Downstream (seaward) migration of Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) larvae after hatching was investigated in the Yakugachi River flowing into Sumiyo Bay, Amamioshima Island, southern Japan. Larvae collected near the spawning ground and in brackish water had notochord lengths of 4.5-5.9 and 4.5-24.4 mm, respectively, larval densities in the brackish water being greater. During day time, larvae were found only in the bottom layer, but at night time were also evident in the surface layer, such behavior probably acting so as to prevent the larvae from drifting away from the brackish water area.

アユの遡上個体数は，海に下る仔魚の数と海域における稚魚の生残率という2つの要素によって大きく左右されると考えられていることから，海域への仔魚の添加状況を把握するため吉井川における産卵場及び仔魚の流下状況を調査した。

2004年および2005年の5月中旬〜8月中旬に広瀬川の名取川合流点から熊ヶ根までの6地点において10日-1ヶ月に1回の頻度で投網によりアユを捕獲し、天然魚と人工放流魚別にCPUEを求めた。郡山堰より下流では天然魚のCPUEが5〜30尾／網とかなり高密度に分布していたのに対し、上流の愛宕堰下では0〜2尾／網に低下し、さらに上流では天然魚の密度は極めて低く、2ヵ年間の調査でわずか1尾しか確認されなかった。また、2004年6月には郡山堰直下において天然魚のCPUEが100尾／網以上と多くのアユが滞留していた。以上のことから、広瀬川の中〜上流において天然魚の密度が極端に低い原因として、郡山堰および愛宕堰の両堰堤に付設されている魚道の機能が低下していることが考えられた。

The author, using a fishing implement as shown in Fig. 1., carried the study on all of the anadromous "ayu" fry which had been caught at the mouth of the River Okumo, a small river at dawn. Out river, about 8km in length, pouring into the Japan Sea, for May 1〜2, 1962. The results are as follows:
1) "Ayu" fry began to make many schools at adjacent water off the mouth of the For 9 a. m. to 10 a. m. some of them showed the anadromous activity, at midday, however, the activity was shown by few of them.
2) The anadromous activity were suddenly activated at about 4 p. m., the peak of that was observed after the sunset when both the river water temperature and the sea water one extremely approached, and the activity gradually decreased as the night goes on. And that was never seen for 9 p.m. to 3 a.m. in the next morning.
3) It was found that the anadromous activity reached to the peak for 5 p.m. to 8 p.m., amounted to 345 in caught number of fry, three quaters of the total number (453) on the day.
4) These fry caught for 3 a.m. to 9 p.m. were ranged between 4.3cm and 7.8cm in body length, and between 0.5g and 3.5g in weight. And all of the fry caught for 9 a.m. to 10 a.m. were lager size, being over 7cm in body length, them the others.

The variations in the number and the body length of anadromous ayu fry in the River Okumo were observed in 1962, and in conformity with the results obtained the suitable time and fish body length for the stock were scrutinized. Results are as follows:
1) The going up of ayu fry began when the river water temperature rose beyond 10°C in the daytime on April 1. The number of anadromous fry suddenly increased at 12°C above, and attained to the maximum at 14〜16°C for the second 10 days of the month. After the period anadromous ones gradually decreased in number, and the going up ceased about the middle of May. So it is considered that the optimum water temperature for the going up the river of marine ayu fry is ranged between 14°C and 16°C.
2) All of the fry which had gone up for first 5 days in the season were fallen into the largest size class, 6.5〜9.5cm in body length , and those of continued periods, every 10 days period, became small step by step. With approaching to the final of season the body length fell into 4.0-6.9cm, and the going up of ayu fry ceased when the schools composed of 4.5〜4.9cm fishes took preferential shares.
3) The rates of the body depth to the condition factor and to the body length are greater in the fishes caught for the beginning period , and the rates gradually fall as days goby, and that shows the fishes of corpulent habit exchanges to those of thin one through the season.
4) From various results mentioned above it is considered for stock of Ayu fry that the river water temperature is desirable to be above 10°C during from the beginning to the middle of April. Concerning to the body length , being above 6.5cm are desirable for the beginning of April and being above 5cm for the middle of this month.

The histological changes of ovaries of dwarf Ayu in Lake Biwa, so-called “Koayu”, during spawning season from August to October in 1980, were examined. And it was attempted to clarify the frequency of spawning and the generality of multiple spawning.
Many mature ovaries with empty follicles showed that the fish probably spawns twice at least. The ovaries with empty follicles were classified into three types on the basis of the histological characteristics, ovary after the first ovulation, ovary after the second ovulation and ovary after the third ovulation. A few fishes' oocytes began to degenerate after the first ovulation. However, it was presumed that the greater part of females spawned twice and a small number of fishes attained the third maturation.

The changes in number and size of eggs during ovarian maturation and ovulation of the dwarf Ayu in Lake Biwa, so-called “Koayu”, taken in 1980 were examined.
Differences of egg diameter between the Koayu and the amphidromous type Ayu became remarkable in the eggs developed over the tertiary yolk globule stage. There were no significant differences in the numbers of mature eggs and the GSI values between the ovaries at the first maturation and those at the second maturation, but those at the third maturation had a smaller egg number and GSI valve than the previous two. The number of yolk-less eggs increased after the first ovulation, but this phenomenon was not recognized in the ovaries after the second and the third ovulations. It seemed that there was no possibility of the fourth maturation in the fish because no oocytes were found in the ovaries after the third ovulation. These facts suggest that the multiplication of yolk-less eggs after the first ovulation makes the third ovulation possible and the fish uses up all oocytes in a spawning season. The mode of ovarian maturation and spawning of the fish corresponds to the annual life of the fish.

Aspects of the reproductive biology of the landlocked large type ayu Plecoglossus altivelis in Lake Biwa, "ooayu" were investigated from August to October in 1980. In more than half the number of ovaries after the first spawning, degeneration of developing eggs or collapse of the whole ovary was observed. Hence, after the first spawning, a small number of fish seemed to attain the second maturation. All the fish which attained the second maturation were skinny and half of them had lower gonadosomatic indexes and smaller number of mature eggs compared to those in the first maturation. No significant differences of egg diameter for various maturation stages of egg were indicated between the dwarf type ayu, “koayu，，，and the ooayu. The pattern of oocyte development of ooayu showed the "trimodal twice spawning type". Resting ocoytes in the peri-nucleolus stage which are usually present all the year round in the general serial spawners, were not observed in the ovary of ooayu after the second spawning. This fact suggests that ooayu have an inherent regulating mechanism of oocyte development adapted to the annual life of the fish, i.e. they discharge all oocytes in a single spawning season in spite of their serial spawning habit.

This study aims to propose a new approach of river flow management to sustain healthy nverbea condition by utilizing the grazing effects of freshwater fauna. We performed experiment to investigate the functions of grazing of ayu (Plecoglossus altivelis) and pale chub (Zacco platypus) on the composition of periphyton. The results of this study are as follows: i) The grazing of ayu restrains and reduces deposition of fine sediment and blooming of filamentous green algae, which were pointed out the inferior quality of riverbed condition. However, the grazing of pale chub cannot reduce them.ll)Feeding activities by not only ayu (Plecoglossus altivelis) but also pale chub (Zacco platypus) contributed to a decrease in the Autotrophic Index and an increase in the AFDM (%) and the ratio of living algae for the periphyton assemblages which were dominated by diatom.
Key Words : periphyton, grazing, Plecoglossus altivelis, Zacco platypus, riverbed, algae

相模川における、アユ降下仔魚量推定の精度を高めるため、採集時間間隔、採集ネットの設置位置、採集時間帯について検討した。
2時間間隔と1時間間隔の採集から推定される降下仔魚尾数には、大きな差がなかったので、降下仔魚尾数の推定は、2時間間隔での採集で可能と考えられた。
岸部と流心部で河川流量が大きく異なる場合、仔魚は河川流量の多い流心部を多く降下するので、複数の地点で仔魚を採集する必要がある。
相模川の神川橋下流では、17：00〜5：00の夜間に70% 以上の仔魚が降下する。また、23：00〜5：00に概ね60%以上の仔魚が降下している。1日の降下仔魚尾数の推定を行う際には、17:00〜5：00の時間帯を調査した方がより正確であるが、23：00〜5：00の採集でも推定は十分可能である。

Before river improvement work, it is necessary to conduct the environmental assessm^it. One oi the most famous environmental methods is PHABSIM. A lot of researchers used this model to evaluate the habitat suitability of fish in the present river. There are little studies which investigate on whether a new habitat area can be made or not. In this study, a numerical simulation using a horizontal 2-D model was carried out with changing shape of cross section in Gokasegawa River. The suitability of spawning for ayu is predicted. As a result, it was conformed that a new spawning beds are made in some cases. The most suitable cross section is chosen with considering both of flood control and suitability of spawning for ayu.
Key Words : Ayu, spawning, PHABSIM, suitability index, 2-D numerical simulation

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.
Key Words : ayu, migration, environmental factor, Nagaragawa Estuary Barrage

Ayu Plecoglossus altivelis of 12cm in body length were raised in concrete tanks at four different water velocities (4cm/s, 25cm/s, 35cm/s, and 45cm/s) for 60 days. Rearing of the fish in water velocities 35 and 45cm/s exerted favorable growth and feed efficiency. Rearing in current water, however, failed to depress lipid content of muscle, liver, and intraperitoneal fat body when com-pared to that in the standing water (4cm/s).
Following the feeding experiment, the fish were starved for 13 days in indoor tanks. The body weight loss was suppressed in the fish reared in water velocities of 35 and 45cm/s. Rearing in high water velocity (<45cm/s) could improve lipid metablolism, especially in activation of utilization of lipid reserves for energy requirements.
In ayu culture, water velocity in the rearing tank should be controlled to 45cm/s or more.

久慈川は，本県におけるアユの約り場として軍要であり，約り開禁になると，県内はもとより，近県からも多くの遊漁者が集まる。そのため，同河川漁業協同組合でも毎年多くのアユを放流して，アユ資源の維持，培養につとめている。しかし，本河川での，天然そ上アユや放流後のアユの生態についての調査は少ない。1978年度から，本河川にアユの保護水面が設けられ，その調査管理事業として，同水面における産卵，ふ化仔魚等の調査を行なった。また，それと平行して.河川内のアユの生態調査を行ない，成長及び成熟について，多少の知見を得たので，報告する。

那珂川の両側回遊型アユについて, 河口付近における冬の積算海水温, 栃木県における遡上初認日・遡上観察群数・秋の平均体重の4要因の関係を解析した。その結果, 冬の積算海水温が高い年ほど遡上初認日が早く, 遡上観察群数も多いという有意な傾向が認められた。また, 遡上初認日が早い年ほど遡上観察群数は多く, 遡上観察群数が多い年ほど秋の平均体重は小さいことが明らかになった。以上の結果から, 遡上初認日は冬の積算海水温から, 遡上観察群数は冬の積算海水温と遡上初認日からそれぞれ予測できると考えられた。

河川の濁りとそこに生息する魚類との関係は，河川改修など一時的かつ程度の強い泥濁水の発生時に悪影響を及ぼすことが指摘されてきた．しかし，通常に生じる濁りが及ぼす影響についての詳細かつ継続的な調査例は見当たらない．そこで，2000-2002年に高知県下の９河川でアユを採取し，その胃内容物中の砂泥分の占める割合を調べるとともに，採取時における河川水の濁度を測定した．濁度と胃内容物中に占める砂泥分との関係は，濁度値が高くなるとともに砂泥分の占める割合も高くなる傾向が見られ，通常でも軽微な濁りが生じている河川では，それがアユの餌料の質に影響を与えている可能性が示唆された．

The spawning habits of the dwarf Ayu-fish, Plecoglossus altivelis, in Lake Biwa were studied in the autumn of 1972-1974. Spawning grounds were mainly located in shallow riffles of lower reaches of rivers flowing into the lake. As is the case with the amphidromous type, the principal physical characters indicating an area as the spawning ground of the dwarf type seemed to be the type of substrate and water velocity. General conditions of the spawning grounds were unsettled sandy-pebble bottoms, 10-30cm in depth and 30-70cm/s in water velocity. Spawning usually occurred in the nighttime. Spawners crowded together in the spawning grounds. Prior to extruding eggs, one of the males followed and nudged a female. The female then pushed her snout to the bed of the river. Simultaneously the male dashed and swam in parallel with the female, and both fishes quivered closer together.
As to the sex ratio, males predominated in the spawners crowded in the spawning ground, while females predominated in the pool near the spawning ground in the nighttime. In contrast, in the daytime a greater proportion of males was recorded in the pool as compared with the spawning ground. Gonad conditions and stomach contents of the Ayu-fish taken from the spawning ground differed from those in the pool. These facts suggest that spawners acting in the nighttime move to the pool in the daytime. Whereas, immatures resting in the pool in the nighttime move to the riffle and feed on algae attached to stones in the daytime.

Variation of 11 enzymes and the muscle protein presumably controlled by 19 loci was analyzed by electrophoresis in 506 individuals of Ayu, Plecoglossus altivelis, from 9 natural populations. Six loci (32%) were polymorphic in one or more populations. The number of alleles per locus in the average population was 1.13. The average proportion of polymorphic loci and the average heterozygosity were 9.1% and 0.6%, respectively. These estimates of genic variability in Ayu are lower compared to those for other species.
At polymorphic loci, landlocked populations in Lake Biwa were homogeneous in allele frequencies, and amphidromous populations were similarly homogeneous on the whole. But between these two forms of Ayu, there were significant differences in allele frequencies at two loci (Me-l and Pgm).

アユにはいくつかの変異の存在が示唆されているが，地理的変異の全体像はまだ明らかでない.そこで，日本•琉球列島各地から得た集団の分子，形態および繁殖形質を調査した.18-28遺伝子座が支配する酸素分子の電気泳動分析および29の形態的形質の分析結果より，この地域のアユ集団は，これらの形質において明瞭に異なる日本列島グループと琉球列島グループに大別されることが明らかになった.びわ湖の陸封アユ集団と日本列島産両側回遊型集団との分化程度は，上のそれに比べると小さいものの，びわ湖アユはいくつかの特異性を有していた.とくに，両側回遊型集団に見られた北方ほど卵数が多いという傾向からはずれ，その卵数は著しく多かった.このようなびわ湖集団の特異性は，びわ湖に特殊な環境への適応の結果と思われる.日本列島の両側回遊型集団間において，分子形質はほとんど均質であったが，形態形質や卵数にはかなりの変異が認められた.形質間に見られるこうした地理的変異パタ一ンの差異の一部は，分子レベルと表現型レベルで進化様式が異なることを反映したものと考えられる.

1. 東京湾の河口から約35Km上流にある秋ケ瀬取水堰の下流で稚アユの採補を刺網、投網、四つ手網を用いて行った。
2. 調査は4月に2回、5月に2回の合計4日（延日数12日）行い、第2回調査（4月下旬）より稚アユが採捕された。
3. 稚アユの採捕は、四つ手網のみで、調査全期間で26尾採捕された。
4. 5月22日脂鰭を切断した標識アユ1,642尾放流し、22〜24日にかけて8尾が採捕され、これをもとに3日間の遡上量を推定すると2,189尾となった。

1) 1986年4月〜5月にかけて秋ヶ靡取水堰扣設置されている魚道でアユの遲1上調査を行った。
2) 4月22日よりアユの遡上が見られ、5月24日までの調査期間中に4 0尾が遡上し、魚道を利用していることが分かった。
3) アユ以外の遡上魚として、ウグイ、ブラックバス、ニゴイ、ヒガイが、魚道を利用していた。
4) 魚道内に滞留していた魚種は、アユ、ニゴイ、ウグイ、オイカワ、ヒガイ、ナマズ、ヨシノボリの7種であった。

1 1987年及び1988年の両年度に荒川の秋ケ瀬取水堰に設置されている魚道でアユのそ上調査を行った。
2 両年度とも魚道をそ上する稚アユは極端に少なかった。
3 最も数多くそ上した時の水温は16,0〜16.5°Cで、魚道入水ロの流速は、0.943m/secであった。
4 魚道入水ロの流速の変動と堰下流の放水量の増減が稚アユのそ上に影響を及ぼしていると考えられた。

1 東京湾の河口から約3 5 km上流にある荒川の浦和市地先秋ケ瀬取水堰の下流で、天然そ上稚アユの採捕を四つ手網を用いて行った。
2調査は1987年と1988年の2年間、4月下旬〜6月中旬に行った。
3 調査期間で採捕された稚アユは1987年が16尾、1988年が3 6尾であった。

1980年〜1983年の4ケ年久慈川について流下するアユ仔魚の採集と，1981年〜1984年に遡上魚を採集した結果から，仔魚流下数と遡上の状況.それに海況が遡上に与える影#についてまとめた。
(1)流下仔魚数は1シーズンに�h億〜8億尾と推定され4ケ年では1983年に最も多く仔魚が流下した。
(2)遡上の最も多かったのは1982年であった。
(31流下仔魚数と遡上魚採捕総数とは，一定の相間関係はみられなかった。 (4)12月から月までの沿岸生息期間中の累積日水温（毎日10時の那賀湊定地水温を累積)と相対的遡上率との間には，ある一定の関係がみられ，累積日水温の1,500〜1,600を境にして，これより高くなると相対的遡上率が急激に良くなり，逆に低くなると相対的遡上率は漸次減少する傾向がみられた。
(51累��n水温が小さくなる時は.親潮系水の南下と接岸が強勢となり，これがアユの南下移動を促すのか減耗が大きくなるのか不明であるが，遡上を悪くする要因と推定された。

We examined the distribution, age, growth, and feeding of ayu larvae and juveniles in the coastal waters around the mouth of Miya River, Mie prefecture, Japan. The larvae and juveniles were also genetically distinguished between amphidromous and landlocked Lake Biwa forms to determine the possibility of recruitment of larvae originated from stocked Lake Biwa ayu. After drifting into the sea, the larvae shifted their distribution from the coastal areas near the river mouth to the surf zone as they developed into the white-bait larval stage (20mm, 30 days old). They subsequently occurred in the estuary after they reached 30—40mm (90 — 130 days old), and then they began their upstream migration. The earlier born and larger size juveniles tended to go upstream earlier in spring. The ayu juveniles fed mainly on copepods such as Paracalanus spp., Acartia spp., etc. which were major components of zooplankton fauna of the research area. DNA analysis of ayu larvae and juveniles clearly suggested the mortality of ayu larvae with Lake-Biwa genes just after drifting into the sea.
Key words: amphidromous ayu, land-locked ayu, migration history, growth

Ten samples of amphidromous-form from Japan Islands, Ryukyu Islands, and Korean Peninsula, and three samples of landlocked-form from Lake Biwa were used for the population analysis of ayu Plecoglossus altivelis. Among 23 loci examined, 6 loci were polymorphic (lower than 0.95 in major allele frequency) in Japanese populations, 2 loci in Korean populations and 4 loci in Lake Biwa populations. The polymorphic locus of Ryukyu's population was only one, but the allele substitution was observed at five loci comparing with the other populations. Average heterozygosity ranged from 0.039 to 0.061 (0.051 on the average) in Japanese populations, 0.045-0.061 (0.054 on the average) in Lake Biwa, 0.027-0.039 (0.032 on the average) in Korean, and 0.011 in Ryukyu. Gpi-l100 and Mpi100 allele frequencies of Korean populations were slightly higher than those of Japanese populations. However, the level of genetic distance between them was very low (D=0.0012). On the other hand, the genetic distance between amphidromous and landlocked forms was relatively large (D=0.0197). The population of Ryukyu Islands was re-markably different from Japanese and Korean populations (D=0.2812).

島根県西部の主要河川である高津川では，近年の夏季から秋季の小雨傾向と，河川構造物による砂利供給量の不足により，下流部のアユ産卵場の環境は年々悪化している．そこで，アユ産卵場としての機能回復を「造成」によって図り，さらにそこでの産卵状態を検証した．造成は虫追の瀬および長田の瀬で行い、造成面積は5,651m2，産卵面積の割合は86%であった．また，産着卵の埋没深は虫追の瀬及び長田の瀬で10cm以上に達し，造成の効果があったと判断できた．

この章では，矢作川の漁業者と矢作川漁協とが，アユを媒介として矢作川と取り結んできた関係の変遷をおってみたい．

天竜川では，土砂吸引バイパスによる佐久間ダムから下流域への土砂供給が計画されている。本研究では，土砂供給がアユの産卵環境にどのような影響をもたらすのかを予測するために，航空写真から産卵場適地を推定する方法について検討を行った。既往研究と現地調査により，礫底で比較的最近に土砂が堆積した瀬がアユの産卵に適しており，砂州と瀬の位置関係がアユの産卵適地の重要な条件の一つであることが推定できた。この情報をもとにアユの産卵に関するSIモデルを作成し，産卵適地を航空写真上で抽出した。さらに，過去の航空写真を用いて，これら産卵適地の経年変化とその原因について考察を行った結果，砂州波長の伸張に伴って産卵適地の箇所数が年々減少する傾向が明らかとなった。

アユ放流前 (2月) の河川においてオイカワに発生した冷水病の原因菌をめぐる在来魚種と放流アユとの同所的動的関係を冷水病菌のPCR-RFLP型を識別手段として4月から6月まで追跡した。オイカワの冷水病菌 (BS型) は, アユ放流後の5月下旬までオイカワにのみ保菌されていた。一方, アユでは冷水病の発症, 保菌が6月下旬まで確認されたが, BS型は検出されなかった。オイカワとアユの冷水病菌の間には宿主をめぐって一定の隔離があることと, 室内実験からも冷水病菌の宿主に対する感受性に違いがあることが明らかになった。

Observations on the life history of land-locked Ayu Plecoglossus altivelis were made in Lake Ikeda, in Kagoshima Pref., from 1981 to 1985.
In this study, two types of Ayu were recognized in its social behavior of immature adult fish in the Lake. One is territorial type and the other is non-territorial type. Some differences were found in the shape of dorsal fin and the color pattern of body between two social types. The territorial fish have longer posterior dorsal fin rays and darker fin membranes than those of non-territorial ones. The former have three clear yellow oval marks on their gill covers and anterior body sides, however, the latter have only one mark and its coloration is pale or faint. These variations are not related with body size and sexualky but with the social type. It is suggested that large and clear yellow marks are signal for recognition of their status among owners of territory and for showing the domination of owner to non-territorial fish.
Thus, these differences between two social types of Ayu show a possibility of being applied to the evaluation of fishing ground of this species.

The embryonic development of Plecoglossus altivelis altivelis was described on the basis of eggs inseminated artificially on October 24, 1984 in Lake Ikeda. The eggs were spherical, 0.87-0.92mm in diameter. The incubationp eriod was about 230 hours at a water temperature of 14.5-23.2•Ž (av. 20.1•Ž). The standard length newly hatched larvae had a small yolk sac (av. 5.3mm) and 61-62 myotomes. Within several days of hatching, the larvae attained 6.0-7.9mm in standard length and had completely consumed their yolk substances.
Morphological changes of the larvae accompanying their growth were observed on 1.263 specimens collected by the larval net (80cm in diameter, 0.353 in meshes), the surf zone net (1m•~5m, 1mm) and the cast net (13mm). The larval fin fold gradually disappeared, and the rudimental dorsal, the anal, the ventral and the adipose fins were visible at 11.4, 12.8, 24.3, 44.5mm in standard length, respectively. The conical teeth appeared at 24.3mm and the comb-like teeth at 55mm. Larvae smaller than 25mm inhabited the offshore waters of the lake, while those larger than 27mm appeared in the surf zone of the lake shore. The vertebral centrums of the larvae ossified at a size of 27.3mm. It is suggested that the considerable morphological changes of the ayu occurred at sizes of 25-30mm and 50-55mm in standard length. The former corresponds to the end of the larval stage and the latter to the end of the juvenile stage, respectively.

Some observations on the life history of the landlocked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda (4km in diameter, 233m in depth) in Kagoshima Pref. from 1981 to 1986. A total of about 1,263 specimens were collected with a larva net (80cm in diameter, GG50), a surf zone net (1•~5m, 1mm mesh) and a cast net (24 meshes). The larva net was towed for 10 minutes on the surface and at a mid-layer at 2-3 knots in the daytime and after sunset.
Newly hatched larvae were 4.9-5.6mm in standard length, scattered in the offshore waters of the lake. They were distributed amongst 3-24m depth layers, mainly in the 7m layer, in the daytime and appeared at the surface soon after sunset. Although individuals smaller than 25mm inhabited the offling of the lake, ones larger than about 30mm occurred at the surf zone of the shore. The larvae of the ayu began to feed on zooplankton such as Protozoa and Trochelminthes before absorption of the yolk substance, and to eat mainly active zooplankton belonging to Arthropoda as they grew. Juveniles smaller than 45mm fed on zooplankton and other animals, but individuals larger than 55mm switched their diet to algal food such as adherent diatoms and blue-green algae.
It is suggested that the standard length at the prelarval, postlarval, juvenile and young stages of this fish in Lake Ikeda are 5-8, 8-25, 27-50 and over 55mm, respectively.

Observations on the life history of the land locked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda, Kagoshima Prefecture, 1981 to 1986.
The gonads of males and females developed rapidly from August and the GSI of both sexes indicated the highest values in October. The diameter of ovulated oocytes in the body cavity ranged from 0.74 to 0.99mm and the relation between the number and standard length of fish was represented as E=0.00188χL3.418.
The spawning season of the ayu in the lake was from early October to late November. The ayu spawned mainly at shore lines shallower than about 30cm in depth and the substrate of reds consisted of fine gravel, ranging mostly from 3-10mm in diameter. The sex composition of the ayu crowding the spawning site was not the same throughout the spawning season. Males predominated in the early period of the season, and females in the latter.
The male fish matured simultaneously and most of them crowded at spawning grounds. On the other hand, the fully matured female appeared at the spawning site one by one, while the immature ones dispersed around the site in low density. It is suggested that this mode of spawning in the ayu may be one of the important factors for sustaining its population land locked in Lake Ikeda.

The number of ayu, Plecoglossus altivelis, larvae drifting seaward was investigated at the Shou River from 1992 to 1996. The main period of seaward drifting of ayu larvae took place from early October to early December and larvae were mainly collected from 18:00 to 22:00 during the period. The number of ayu larvae drifting seaward during the period was estimated to be 3 to 29 hundred million in a year, and it was supposed that larvae drived from landlocked form, that is Lake Biwa, occupied 5-15% of the total except in 1994. It is considered that the number of ayu larvae was influenced by the water temperature and the water flow of a river before and after the period of beginning of spawning.
キ一ワ一ド：アユ，庄川，降下仔魚，降下量，推定法

1992〜1996年に庄川においてアユ降下仔魚の尾数の日周変動，体長および孵化(産卵)水域を調査した。降下尾数の日周変動は，20:00または22:00にピークを持つ単峰型で，ピーク時には1日の46.5〜56.2%を占めた。仔魚の体長は3.7〜7.6mmにあり，平均は5.8mmであった。9月下旬および10月上旬の初期の仔魚の体長は，それ以降の中後期の仔魚に比べると有意に小さく，これらは湖産系に由来すると推測された。庄川でのアユの孵化水域は，河口から5.5〜11kmの範囲に限られると推定された。

1994〜1996年に庄川のアユ降下仔魚の出現状況を，産卵場と河口域において，2時間間隔の24時間採集を行って調べた。産卵場における降下仔魚数のピークは20:00または22:00で, 河口域のピークは1:00, 3:00または7:00であった。両地点のピーク時刻の差は5〜9時間で，その時間の差は, 主に河川流量(流速)の差により生じていると考えられた。産卵場と河口域のピーク時の平均体長の差は7%未満で，1996年では有為な差はなかった。両地点の仔魚の体長分布は7.0mm未満の階層で大部分(85.3〜97.6%)が重なり合った。これらのことから，庄川では産卵場で夜に孵化した仔魚の大部分は，翌朝までには日齢0日で河口域に到達するものと推定された。

富山湾奥部の砂浜海岸の砕波帯では,アユ仔魚は10〜1月(盛期は11月)に出現し,その平均標準体長は10月に12.1±1.8mm,11月に18.3±3.0mm,12月に22.0±4.9mmおよび1月に23.3±2.7mmであった。砕波帯の沖側に隣接する水深4m以浅の浅海域では平均標準体長36.1±3.8mmの大型仔魚が1〜2月に採集された。水中観察では11〜3月にかけて砕波帯およびそれに隣接する浅海域において仔魚の群れが確認された。富山湾では10〜12月まで砕波帯を中心に生息していたアユ仔魚は,仔魚の成長や水温の低下などに伴い,2月頃までにはその沖側に隣接する浅海域へ主な生息場を移すものと考えられた。

河川に遡上したアユ稚魚の体長と水温の関係を1994〜2003年に富山湾に流入する庄川と神通川および神通川河口付近の海域で調べた。河川で採集されたアユ稚魚の体長分布の下限と河川水温並びに海域で採集されたアユ稚魚の体長分布の上限と海水温との間には, 有意な負の直線関係が認められた。アユの遡上開始は川と海の水温が約10℃に達した頃で, 海の水温が17℃を越えると海域では稚魚は認められなくなった。庄川の遡上期間は3月31日〜6月15日で, 積算日数は27日〜69日の範囲にあると算出された。初期遡上群の体サイズが最も大きい理由は, 河川水温に耐えられるような大型に成長した個体から遡上が可能になるためと推定された。富山湾に流入する河川において, 海産系の人工種苗の放流に際しては, 河川水温が10℃に達した時に体長9cm以上の大型個体から放流を開始し, 河川水温が14℃以上に上昇すれば体長6cm程度の小型魚でも放流が可能であると考えられた。

Survival rate and size of early larvae of ayu reared under several salinity conditions were examined in temperatures of 13-16 ℃ and of 20-24 ℃. Ayu larvae were reared by four salinities (8,16，24 and 32 PSU) in the aquaria (60cm long, 30cm wide, 36cm high) for 15 days. Survival rate of larvae reared in brackish water (8,16 and 24 PSU) was higher than that of 3ai-vae reared in seawater (32 PSU) in both temperatures. The size of larvae reared in brackish water was larger than that or larvae reared in seawater. These results suggest that survival rate and growth of ayu larvae in brackish water is better than that of larvae in seawater, and the extension of brackish water area around the near coast leads to an increase in juveniles migrating to the river.
Key words: Ayu larvae，Seawater, Brackish water, Survival rate and size

Vertical distributions of early larvae of ayu Plecoglossus altivelis reared in brackish water in a pond during daytime were observed for 18 days from hatching in 2005. Water temperature ranged from 22.9 to 25.4℃ at the surface, and from 23.1 to 25.6℃ at the bottom. Salinity ranged from 8 to 9 PSU from the bottom to the surface. Ayu larvae distributed from middle layer to surface layer, but did not gather at the bottom. It is considered that ayu larvae do not sink to the bottom by utilizing decrease of swimming ability and increase of their own specific gravity under equal brackish water density during the daytime.
Key words: Equal brackish water density, Larvae of ayu, Specific gravity, Vertical distribution

In the amphidromous form of ayu Plecoglossus altivelis Temminck et Schlegel, generally the prelarvae hatched in rivers in autumn are drifted down into the sea, where they live throughout the larval and early juvenile stages until spring. In the Shimanto estuary, however, an appreciable number of postlarval ayu occurred not only during March to May, but also during October to February. Furthermore, there was little difference in the size and age between the larvae occurring in the estuary and sea. In the estuary, they reside in the bank waters, during 10-75 day ages and 10-30mm SL. Thereafter, they seem to ascend the river without going out to the sea. The growth rate estimated from the otolith daily rings was higher in the estuary than in the sea beach.

In the amphidromous form of ayu Plecoglossus altiveus, larvae hatched in rivers in autumn are drifted down into the sea, where they live tnroughout the larval and juyenile stages until spring, kittle had Deen known about the aetails of early life history of ayu in the sea. It was recently reported that they inhabit mainly surf zone of sandy beach. On the other hand, an appreciable number of larval and jevenile ayu occurred from autumn to next spring in the bhimanto estuary. The growth rate of ayu was higher in the estuary than in the adjecent sea. These phenomena suggested that the estuary is important for ayu not only as a migration route but alsoas a nursery ground. I supposed the route of inshore migration oi larval and juvenile ayu in the sea from the distribution pattern of them in the Shimanto estuary and other ecological studies on ayu in the sea. Owing to the developments and utilizations of rivers and the sea all over Japan in recent years, natural stocks of ayu were decreased. The influence oi human works on ayu was examined, and some countermeasures for them were considered. In the Yahagi River, the various plans for the maintenance of ayu during-the spawning season are chiefly required at present.

Ecological studies on spawning of ayu and downstream migration of their larvae in the Yahagi River irom September to December 1996.
l. Spawning grounds of ayu
1) Seven spawning grounds oi ayu were found in the Yahagi River (10-50 km from the river mouth).
2) Spawnings of ayu were observed from late September to mid November. Main spawning grounds were shifted downstream during this period.
3) Few eggs of ayu clung to gravel of spawning ground containing much fine sand and mud, where the rate of died egg was high.
4) In the Yahagi River, some treatments, such as washing away fine sund and mud, are required to maintain the spawning grounds of ayu.
2. Ecology of spawner
1) Spawners of ayu were collected at two stations(Aoi-ohashi and Toyota-ohashi) in the Yahagi River from September to November 1996. The samples were measured standard length, body weight, and gonad weight.
2) The sizes of spawners ranged from 7.8 to 15.6cm SL with a mode at 10-12cm SL at Aoi-ohashi. At Toyota-ohashi, they ranged from 9.6-20.3cm SL with a mode at 14-16cm SL.
3) The number of spawners decreased in early November at Aoi-ohashi, and in mid-October at Toyota-ohashi.
4) Mature females first appeared in late September both at Aoi-ohashi and Toyota-ohashi. The peak of gonadosomatic index of females was observed in mid-October at both stations.
5) The spawning period was estimated from late September to mid- November with a peak in early or mid-October at Aoi-ohashi. On the other hand, that was estimated from late September to late October with a peak in early or mid-October at Toyota-ohashi.
6) The conservations for the spawner were quickly required in the Yahagi River, for example, closed season and sanctuary.
3. Ayu larvae during downstream migration
1) Ayu larvae during downstream migration were collected at four stations (Toyota-ohashi, Mej-yosui, Miyai-hashi and Yonezu-hashi) in the Yahagi River from October to November 1996.
2) Larval ayu occurred from early October to early December at Toyota-ohashi and Meiji-yousui which are located at the middle reaches of the river. They occurred from late October to middle December at Miyai-hashi and Yonezu-hashi which are located at lower reaches of the river.
3) Larval ayu occurred abundantly in October at Toyota-ohashi and Meiji-yosui. At Miyai-hashi and Yonezu-hashi, however, few larvae occurred in this month.
4) Most of the individuals which were collected at Meiji-yosui absorbed the yolk, but yolk-sac larvae just after hatching were collected abundantly at Miyai-hashi which is located locate below Meiji-yosui.
5) These phenomena indicate that larvae hatched above Meiji-yosui died during downstream migration without reaching to the sea.
6) We think that increasing the volume of flowing water is an effective in reducing the mortality rate during their downstream migration.

Ecological studies on ayu in the Yahagi River were made from April to December 1997.
1. Upstream migration of Juvenile ayu
1) Juveniles of ayu during upstream migration were collected using a cast net at three stations in the Yahagi River (Furukawa-branch, Otogawa-meeting and Meiji-yousui) during April-June 1997.
2) Main size of ayu during upstream migration at Furukawa-branch (near the river mouth) was 5-8 cm SL. In the other stations, main size of ayu were larger than Hurukawa-branch, indicating that juvenile ayu ascends the river with growth.
3) The age in days of ayu starting their upstream migration was 140-190 days.
4) The growth rate of the fish collected in the upper area in the river was higher than that of fish collected in the lower area.
5) Although larvae hatched from early October to early November 1996 in the spawning grounds above the Meiji-yosui, the main hatch date of ayu during upstream migration was between early November and early December 1996. This leads the conclusion that almost of the larvae hatched above the Meiji-yosui died before reaching to Mikawa Bay.
6) Juvenile ayu took not only epilithic algae but also Arthropods.
2. Ayu living in the river
1)Adult of ayu living in the river were collected by Korogasi at three stations in the Yahagi River (Aoi-ohhashi, Toyota-ohhashi and Hirose) from June 1997 to September 1997.
2) The size of ayu were smaller at Aoi-ohhashi than at Toyota-ohhashi and Hirose.
3) At Hirose, ayu under 18 cm got thin in summer.
4) The rates of landlocked forms of ayu in the samples were under 10%, 40-70% and 40-70% at Aoi-ohhashi, Toyota-ohhashi and Hirose, respectively. These rates decreased with month at all stations.
3. Spawning grounds
1)Five spawning grounds of ayu were found in the Yahagi River (between 30-50 km from the river mouth) in autumn 1997.
2 ) The spawning period of ayu in the Yahagi River was during late September-late November.
4. Downstream migration of larva
1) Downstream migration of larvae were collected using conical plankton nets at four stations in the Yahagi River (Toyota-ohhashi, Meiji-yosui, Miyai-hashi and Yonezu-hashi) from Octber to December 1997.
2) Few larvae collected at Toyota-ohhashi and Meiji-yosui, but numerous larvae collected at Miyai-hashi and Yonezu-hashi. This indicate that the main spawning grounds were formed below Meiji-yosui.
3) The size of larvae was ranged from 5.0 to 7.5 mm with a mode 6-7 mm. The sizes of larvae were larger at Toyota-ohhashi and Meiji-yosui than at Miyai-hashi and Yonezu-hashi.
4) Most of the individuals collected at Meiji-yosui had been completed the yolk absorption. On the other hand, most of the individuals collected at Miyai-hashi and Yonezu-hashi were newly hatched larvae.
5) These facts lead the conclusion that almost of the larvae hatched above Mej-yosui died before reaching to Miyai-hashi.

島根県西部の主要河川である，高津川と江の川では天然アユの溯上量を増大させるために漁業協同組合が中心となって祿々な取り組みを行っている.しかし，近年の夏季から秋季にかけての少雨傾向と，堰堤による砂利供給量の不足により’下流部のアユ産卵場の河川環境は年々悪化してきている.そこで，高津川および江の川におけるアユの主要産卵場の機能回復を「造成」によって図ること，さらにそこでの産卵状態を検証することを目的として調査を行った.

島根県西部の高津川では，漁協が中心となってアユ産卵親魚の保護を目的として産卵期の禁漁期間を大幅に延長するとともに産卵場造成にも取り組んでいる．しかし，具体的な資源管理目標値が定められていないことが管理を進める上で問題となっている．そこで，高津川のアユ資源を管理するための具体的な数値目標を設定するための基礎資料として，高津川のアユ漁場の適正収容量を検討した．

島根県西部の主要河川である，高津川と江の川では，近年の夏季から秋季の少雨傾向と，堰堤による洪水調整ならびに砂利供給量の不足により，下流部のアユ産卵場の環境は年々悪化している．そこで，2008年に引き続いて両河川におけるアユ産卵場の機能回復を「造成」によって行い，さらにそこでの産卵状態を検証した．高津川では３箇所7,170m2，江の川で１箇所3,660m2を造成し，うち産卵面積の割合は高津川で65％，江の川で50％であった．また，造成した産卵場のうち高津川の長田の瀬以外は10cm 以上の埋没深があり「効果有り」と判断できた．

島根県西部の主要河川である，高津川と江の川では，近年の夏季から秋季の小雨傾向と，堰堤による砂利供給量の不足により，下流部のアユ産卵場の環境は年々悪化している．そこで，両河川におけるアユ産卵場の機能回復を「造成」によって行い，さらにそこでの産卵状態を検証した．高津川では3ヶ所5560m2を造成し，産卵面積の割合は57%であった．造成した産卵場のうちバイパス上の瀬以外は10cm以上の埋没深があり「効果あり」と判断できた．一方，江の川は親魚数が少ないことが予想され，自然産卵場の面積で十分収容できると判断し，造成は行わなかった．

江の川のアユ資源回復に向けた取り組みを進めるうえでの増殖目標となるアユ漁場の適正生息数を検討した．アユの再生産が確実な浜原ダムより下流域を踏査し，5つの河床型（早瀬，平瀬，淵，トロA，トロB）に区分した．それぞれの水面面積と，江の川の環境を考慮して決定した収容密度から適正生息数を検討した．その結果，標準的な適正生息数は239万尾（167t）と試算された．また，平均密度は0.66尾/m2であり，天然遡上主体の河川での平均的な密度1尾/m2と比較すると，やや少なめの密度と判断された．

島根県西部の主要河川である江の川では，近年の夏季から秋季の小雨傾向と，河川構造物による砂利供給量の不足により，下流部のアユ産卵場の環境は年々悪化している．そこで，本河川の谷住郷の瀬のアユ産卵場としての機能回復を「造成」によって図り，さらにそこでの産卵状態を検証した．造成面積は2,270m2 であり，産卵面積の割合は83% であった．また，産着卵の埋没深は10cm 以上であり，造成の効果があったと判断できた．

江の川のアユ産卵場環境は近年の夏季から秋季の小雨傾向と河川構造物による上流からの砂利供給不足により年々悪化している．本河川の谷住郷・長良の瀬のアユ産卵場としての機能回復を造成によって図り，産卵状況を自然産卵場と合わせて検証した．造成面積はそれぞれ1,890m2と1,940m2であり，産卵面積の割合は22%と55％であった．また産着卵の埋没深は一部を除き10cm程度であり，造成効果があったと判断できた．一方，自然産卵場は，セジリの瀬では産着卵が確認されたが，イチノセ，ハネノセでは確認されなかった．

The samples from 6 natural populations of amphidromous race collected in Kochi Prefecture, 2 from landlocked populations, and 2 artificially propagated populations of Ayu, Plecoglossus altivelis were used for examining isozyme markers in order to clarify genic variability and differentiation of populations. Among 20 loci examined, 14 loci were polymorphic in total samples. The genic variability was not low in comparsion with the other fish species, as shown in the number of alleles per locus ranging from 1.25 to 1.60, the proportion of polymorphic loci per population from 0.250 to 0.450, and heterozygosity from 0.034 to 0.090. The level of genic variability of landlocked form was lower than the amphidromous form. The allele frequency was very similar within a race as shown by the genetic distance (D=0.0001-0.0008 in amphidromous form, D=0.0003 in landlocked form). On the other hand, the genetic distance between amphidromous and landlocked forms was relatively large (D=0.0035). Distinct change was observed in the allele frequencies and genic variability in the hatchery population, possible effects of founder population and selection being suggested.

島根県西部の主要河川である高津川では，近年の夏季から秋季の小雨傾向と，河川構造物による砂利供給量の不足により，下流部のアユ産卵場の環境は年々悪化している．そこで，「造成」によってアユ産卵場としての機能回復を図り，さらにそこでの産卵状態を検証した．高津川では虫追の瀬および長田の瀬で造成を行い，造成面積は合わせて3,866m2 であり，産卵場面積の割合は114% となった．また，産着卵の埋没深は虫追の瀬では10cm に達しなかったが，長田の瀬では10cm 以上で造成の効果があったと判断できた．

新潟県南部に位置する海川で、アユ産卵場における着卵数と流速および床石サイズの閨係を調査した。
流速17.5cm/sと31.8cm/sの調査箇所では、20cm型枠内に200粒前後の着卵が確認されたが、流速や水深の増加に伴い着卵数が減少する傾向が認められた。
着卵が見られた床石の最小サイズは、0.1X0.1cm、最大サイズは9.0X4.6cmであった。確認された着卵の多くは（63.5%)、長径の2.0cm以下の床石に付着し、長径1.1〜1.5cmの石に付着した卵の割合が最も高かった（30.5%)。
海川では流速が穏やで、水深の浅い場所に多くのアユ卵が確認され、小型の床石に多くのアユ卵が付着していた。
海川の繁殖時におけるアユの体長は、他の河川のそれより小さい傾向にある。アユは繁殖時の体サイズが小さい個体ほどより小型の砂礫がある場所を産卵場として選択することから、体サイズの小さい親魚アユが生息する河川では、産卵場の造成場所の床石のサイズや流速に留意する必要があると考えられた。

To examine regional specific diversity in development and growth of Ayu (Plecoglossus altivelis) larvae, we collected and compared collections from the Kalong estuary in Vietnam, and the Shimanto and Muko estuary, and the Niigata coast in Japan. Among the four areas, most of the morphometrics through ontogeny were similar except that the snout tended to be shorter and the anus hardly migrated in Kalong larvae. The snout length increased gradually with growth in the Vietnamese larvae, while this value increased significantly until ca. 10 mm BL, subsequently being constant up to 30 mm body length (BL) in the Japanese larvae. The water temperature when the larvae were collected was higher in the Kalong than in most of the Japan sites. Growth-rates estimated from otolith increments were from highest to lowest, Niigata (mean = 0.54 mm/day), Kalong (0.47), Shimanto (0.38) and Muko (0.34). The higher growth-rates were obtained not in Niigata of highest latitudinal region, but in Kalong of lowest latitudinal region. This indicates that Ayu could experience their early developmental stages from the cool temperate to tropical regions, implying the potential biodiversity of this fish species in the world.

The reproductive characters such as hatching time, egg diameter and total length of hatched larvae were observed to find genetic differences between two local races of ayu, Plecoglossus altivelis altivelis. The two forms were reared consistently in the same tank and under the same environmental conditions from the young to adult stages in order to standardize environmental effects in the inter- and intra-racial cross breeding tests.
Significant differences between the two local races were observed in such characters as days to 50% hatching and total length of hatched larvae. The larvae of the landlocked form hatched out significantly earlier than those of the amphidromous form. The egg diameter and total length of the landlocked form hatched larvae were significantly shorter than those of the amphidromous form. Paternal effects were found when comparing these reproductive characters between intra-racial and inter-racial crosses, which suggests that these differences between the two local races might be determined by genetic factors.
キーワード：遺伝的差異，生殖形質，湖産アユ，海産アユ

The growth increments in otolith were validated to establish the age determination method of the ayu Plecogdossus altivelis an amphidromous salmonoidei fish with 1 year life span.
Newly hatched larvae have ca. 5 embryonic increments in a circular sagitta of ca. 15μmradius. No mark peculiar to the time of hatch was detected in sagitta. Sequential samplings of known age fish from a rearing pond and those released in a river demonstrated that the days after hatching and the number of sagittal increments outside the 15μm radius presented close linear regressions with the slopes of 1.007 (1-150 days), 0.950 (120-180 days) and 0.871 (215-355 days), suggesting a daily deposition of the increment in sagittae. This was supported by the tetracycline marking of otolith in both larvae and juveniles.
Ages of fish over 200 days old tended to be slightly underestimated, because the earlier in-crements arround the focus became invisible due to thickening and brownish coloration of otolith.accompanied by fish growth, and further because the occurrence of conspicuous opaque bands (checks) corresponding to the tentative cessation of increment formation increased in older fish. The error, however, did not exceed 5 days (2%) till 300 days old, and the age determination method was applicable to the ecological study of this species for almost the entire life cycle.

Ayu larvae, amphidromous salmononidei fish with a 1 year life span, were aged with otolith daily ring in order to reveal the ecological aspects of their downstream migration from the spawning ground.
The age of larvae ranged from 0 to 12d, body length from 4.6 to 8.8mm and sagittal otolith diameter from 20.5 to 48.0μm. Mean age of larvae collected in the Nagara, Kiso and Tone River was 4.5, 1.8 and 4.0d, respectively, and mean body length was 7.2, 6.8 and 6.9mm. Age, body length and otolith diameter had tendency to increase in the lower reaches of the rivers, whereas yolk volume decreased towards rivermouth.
Age, otolith diameter and yolk volume did not differ significantly between two samples collected up and down of a dam near the rivermouth of the Tone River, although body length was larger in the downstream, 7.4mm, than in the upperstream, 6.9mm. The fish density was about 4 times higher at the upperstream of the dam than at the downstream, suggesting that the drifting larvae were pooled in the dam.

矢作川中流域の瀬で捕獲されたアユPlecoglossus altivelis 45個体の胃内容物と後腸内容物を解剖して比較した.アユは大型糸状藻のカワシオダサCldophora glomerataを摂餌し，胃内容物として発見されるが，それは後腸でもほとんど消化されていない.従って，初夏に大発生するカワシオダサはアユの成長を阻害している可能性が高い.一方，7月から9月上句の夏季には瀬で優占している藍藻の一種Homoeothrix janthinaを主な餌料としていた.

乱獲や環境破壊は，アユ，マダイ，ヒラメなどの水産資源を減少させた。種苗放流の目的は，人為的に激減させた対象種を捕填して漁獲量を増大すること，および再生産過程を介して資源水準を回復させることにある。アユの種苗放流は，生態系を復元する試みである一方で，生態的撹乱を助長するリスクの一部が顕在化している。これらリスクの多くは技術的に回避可能な課題であると考えられる。アユサブチームは，アユの種苗放流が生物の多様性（種，他種，生態系）に与える影響を評価する手法を確立し，負の影響を軽減するとともに天然アユの資源回復を目指した放流技術の高度化に閨する研究を実施している。

In 2001 - 2003 population numbers of ayu were estimated by direct underwater counting with a line transect method from April to December in the Nezugaseki River. Ayu migrated upstream from the sea into the river on May to June in 2001 -2003. The maximum numbers of ayu in the river were estimated as 70,000,105,00 and 27,000 individuals, respectively, and the fish survived 11-18% on October and then spawned. In 2003 a mark-recapture test revealed that the hatchery-reared ayu released in the Nezugaseki River showed similar survival and growth to wild ayu stock.
Key words: ayu, population number, line transect, stock enhancement

The distribution and feeding habits of the larvae of ayu Plecoglossus altivelis altivelis were investigated in the coastal waters of Tosa Bay from 2000 to 2004. The larvae (chiefly yolk-sac and preflexion stages, 4.0-13.5 mm BL) were distributed horizontally 3 km inshore and vertically on the surface, being in‰uenced by coastal waters. Average feeding incidences were 40.8-47.9 % in yolk-sac and 46.6-81.5 % in preflexion stages. They fed on copepods, chiefly Oithona and Paracalanus type, which were dominant in the zooplankton fauna of the waters. In accordance with the development of the larvae, the copepods consumed gradually changed from the nauplius to copepodite stage. When the monthly frequency of hatching of ayu larvae inhabiting surf zones close to the waters investigated were compared among year groups, it was suggested that the degree of assemblage of early larvae near the coast and the density of copepods were signiˆcant for their survival until migration to the surf zones.
キーワード�Fアユ仔魚，河川水，食性，餌環境，生残

矢作川に生息するアユの流下仔魚について12年に渡る調査結果を報告した.流下仔魚の流下数は年によって変動し，約0.2〜10億尾の範囲だった.流下の盛期は概ね11月頃にあり，日周的には深夜1:00〜3:00にピークを迎えた.卵黄指数の組成には年変動および季節変動はみられず，毎年指数2および1の個体が多かった.
キーワード：矢作川，アユ，流下仔魚，長期モニタリング

海域におけるアユの初期生態を2000年10月から2001年4月の期間調査した.調査項目は，アユ仔魚の流下量の季節変化，河口周迈における仔魚の成長と孵化日組成，三河湾内全域での分布，碧南火力発電所放水ロに出現する稚アユの体長および孵化日組成とした.流下しているアユ仔魚は調査期間の10月上旬から12月中旬を通じて採集されたが，その中心は10月上旬から11月上旬の間にあった.これは河口周迈で採集されたアユ仔魚の組成が11月下旬をピークにしていたのとずれていた.三河湾の全域にわたる分布調査では，渥美半島側で知多半島側よりはるかに多くのアユが採集された.月別では，12月に最も多くのアユが採集された.その後1月と2月は非常に少ない量しか採集されなかった.これは水温の低下によってアユが波打ち際からの移動を余儀なくさせられたためとみられる.放水ロでは2月中旬から4月上旬までアユが採集された.その日齢査定により，出現期間の最初の頃は早く生まれた個体が出現し，時期が進むにつれて順次遅く生まれた個体に移りかわる傾向がみられた.
キーワード：アユ，初期生態，三河湾，矢作川

矢作川における天然アユの生態調査は，矢作川天然アユ調査会と豊田市矢作川研究所とが共働して1998年より行われている（山本・内田，2007；豊田市矢作川研究所，2008）．その中で，矢作川における天然アユの現存量を把握するための基礎データを得る目的でアユの遡上調査を毎年実施してきた．ここでは明治用水頭首工における天然アユの遡上数のデータを掲載する．

Catches of marine juvenile ayu Plecoglossus altivelis altivelis in the coastal waters of Kui Channel, Wakayama Prefecture, fluctuate widely. Environmental factors for the catches in the past years have been examined by the multiple regression analysis. The variable selection according to the Akaike Information Criterion (AIC) showed that the annual catches of the Juveniles are explained by the quantity of down-wept larvae in the last year in the Hidaka River (X₁), the catch of clupeoid Juveniles by the fishery from last October to last January (X₂), the wet weight of plankton biomass in last December (X₃), and the amount of precipitation in last October (X₄) . The following equation is proposed to obtain the forcast of catch (Y) based on these factors:
Y=2.898+0.321X₁-0.022X₂+5.970X₃+0.016X₄ (R²=6.837) .