蚊幼虫天敵魚グッピーの汚水に対する耐性に関する観察

抄録

It has been reported in our previous studies that a fresh water fish of the family Poeciliidae, the guppy or Poecilia reticulata (Peters), has established in a number of areas in Asia after being introduced from South America as a pet fish, and served as an efficient predator of mosquito larvae in sewage waters. The fish was found also to be highly adapted for breeding in polluted waters and effective in cleaning organic matters and microoganisms. In this connection, studies were made on the tolerance of the guppies to the septic pollution of water and for identification of the cause of the toxicity. When 5g of the powder of laboratory animal food is added to 10 liters of water and kept at 23℃, the dissolved oxygen in water becomes undetectable within several hours; the guppies, however, released in such water can survive many days apparently by taking oxygen from the water surface. The absence of dissolved oxygen is therefore not the direct cause of the death of the guppies, though it may be fatal to other fish species. When larger amounts of the septic substance is added to water, such as over 10g of the animal food into 10 liters of water, the water becomes toxic to the guppies after about 4 days, and the toxicity lasts for about a week. However, the guppies exposed to the water before and after such a toxic period stays apparently healthy, though the dissolved oxygen may be undetectable. Interestingly enough, the guppies which survive keep on eating the fouling substances and the microorganisms, and eventually the water recovers its oxygen content after most of the organic matter is consumed by the guppies. The toxicity of septic waters to the guppies was found to be highly correlated with the concentration of H_2S in water. The water becomes fatal to the fishes from about the fourth day when the H_2S concentration increases to over 1ppm, and remains so for about a week until the concentration drops again to below 2ppm. The pH level during this period is usually below 6,but it gradually changes to near neutral in connection with the increase of NH_3 in the water. The septic water also becomes non-toxic to the fishes after providing aeration for a few hours. Under experimental conditions, H_2S dissolved in distilled water exhibits toxicity to the fishes at concentrations higher than 1ppm, while the toxicity of ammonia in water is seen at concentrations above 11ppm. It was also observed that the toxicity of H_2S and NH_3 is remarkably reduced by mixing the two solutions at adecuate ratios. The solution of (NH_4)_2S is safe to the fishes even at 63ppm, and becomes toxic only at concentrations 125ppm and higher. It is presumed that the accumulation of H_2S produced by microorganisms under anaerobic condition is the principal cause of the toxicity, and subsequent increase of ammonia in water acts as a neutralizing agent. It was further observed that the toxicity to the fishes could be easily minimized by providing aeration into the septic waters in connection with the recovery of dissolved oxygen. The D.O. concentration of the septic water rapidly increases after commencement of the aeration, but in most cases the concentration starts to drop after several hours, probably the result of the reproduction of aerobic microorganisms which consume oxygen for oxidizing the fouling substances. The secondary increase of D.O. concentration occurs some time later, after most of the septic substances are saturated with oxygen.