This paper deals with the variation of water quality between upand downstream ponds in relation to their food trout production. The major part of the present investigations was conducted in the trout farms at Otomi Village, Yamagata Pref. in 1955 and 1956. Although they have produced rainbow trout at a rate of 150 tons per cubic meter per second of water flow annually, their production is considered to be nearly the maximum limit, because the more the ponds were located down stream, the more a time lag in production appeared and the lower the efficiency of production per unit of pond area became. The results of water analyses showed that while the water used flows down through the ponds, there are considerable decrease in dissolved oxygen, accompanied with the reduction of pH values, and increases in free carbon dioxide, ammonium- and nitrite nitrogen, phosphates, suspended volatile matters and alkalinity. Under the conditions where the dissolved oxygen content was reduced to 3cc/l the behavior of "surfacing" of trout was often observed, their feeding activity during the daytime being restricted. Further, under such environment, especially in downstream ponds, the blood corpuscle resistance of trout is observed to be decreased. In the case where the ample oxygen was present, however, even when 0.5mg of ammonium nitrogen per litre was present together, there was no difference in the blood resistance of trout between up- and downstream ponds. As a result, it can be said that the major factor involved in water qualities in relatlon to the trout production is dissolved oxygen and its content should be maintained more than 3cc/l. The rainbow trout fingerlings in their growing stages consume 230cc of oxygen, excreting 17mg of ammonium nitrogen and 2.7meq of some substances which increase alkalinity in water, per kg of body weight an hour. Therefore, in anticipation of a possible relationship between the oxygen consumption and the rates of excretion, it was presumed that these relations might be available for the evaluation of environment of trout ponds. Thus, in the trout farms where the ponds were arranged in series and where trout were heavily raised, the accumulation of ammonium nitrogen of water was observed at a rate of 0.1mg per 1cc decrease of dissolved oxygen. This value is approximate to 0.075mg which is determined in an experiment. On occasions in which the rate of accumulation of nitrogen per unit desease in oxygen might be higher than 0.1mg, some considerations should be given to the water changing rate in ponds; their sanitation and the inflow of water containing nitrogenous substances.

この研究では食用マスの生産を限定する養魚用水の水質変動に関して検討を行なった.主な調査を行なった1955/56年の山形県大富村の養魚池では,上下流に互って川水を反復利用し毎秒1㎥の水量当り年間150tonの食用マスを生産していたが,当時の生産量は,下流池ほど出荷ピークの遅れと池面積当りの生産率の低下が認められたことから,生産限界に近い値と推定された. 養魚用水の水質を分析した結果,下流池では,pH,溶存酸素量の低下があり,遊離炭酸,アンモニウム塩,亜硝酸塩,燐酸塩,揮散物,及びアルカリ度の増加が認められた.溶存酸素量が3cc/lに減少すると養殖魚の慢性的鼻あげがみられ投餌が制限される.下流池ではしばしばこの現象が観察された.このような環境の悪化は魚の赤血球抵抗力を減少せしめるが,酸素量が多い場合には0.5mg/lのアンモニウム窒素が共存しても上下流池の魚の抵抗力には差が認められなかった.食用マス生産の水質要因として重要なのは溶存酸素であり,この許容量は3cc/l以上と推定した. 成育期の投餌されているニジマスはkg体重,1時間当り酸素230ccを消費し,アンモニウム窒素17mg,アルカリ度増加成分2.7meqを排泄する.これらの排泄量は呼吸活動と一定の関係にあると考えられ,この関係を用いて養魚池の環境評価を行なうことができる.連続して池が配列され集約的に食用魚を生産している養魚場では,池水の溶存酸素1ccの減少につき0.1mgのアンモニウム窒素の増加がみられた.一般に養鱒が成立する限りでは,池水のアンモニウム窒素量が0.5mg/lをこえることはまれであろう.単位酸素の減少に対するアンモニウム窒素の蓄積率が0.1mg以上の場合には,魚以外の増加要因につき検討する必要があると考えられる.

長崎大学水産学部研究報告, v.17, pp.68-82; 1964