The Toxicity Of Common Heavy Metals To Several Ornamental Fishes And Its Implication

As the environment problem gets more serious, water pollution also increases. Heavy metals, which is one of the main pollutants, not only destroys aquatic species'living environment, especially fishes, but also threaten the safety of drinking water of human beings. In this study, the toxic effects of five common heavy metals, which were Cu²⁺,Hg²⁺,Cd²⁺,Cr6+,Zn²⁺, on several ornamental fishes ,such as koi, zebra fish (blue, red) (Danio rerio), red swordfish (Xiphophorus hellerii), guppy (Poecilia reticulata), tiger barb (Puntius tetrazona)were researched. The application of small fresh water ornamental fishes on water quality monitoring and early-warning was discussed too. It was hoped that this study would provide some available information for the protection of living resources, as well as laying the theory foundation for the development of water quality monitoring and early-warning system. The research consists of several parts below:1. The accumulation of Zn²⁺ and Cd²⁺ in Koi tissues from waterIn the experiment Koi was chosen as the target, the accumulation of Zn²⁺ and Cd²⁺ in its tissues from water was studied. The experiment was divided into six groups, including five treatment groups and one control group, each group has three parallels. Lentic water method was implied in the experiment, with artificial aeration providing adequate oxygen. Koi were exposed to different levels of zinc and cadmium solutions for 72 hours. After that they were dissected and taken samples. The samples were dealed with muffle at 500℃high-water temperature. Then we used the graphite furnace atomic absorption spectrophotometer machine to detect of the absorbance value of the samples under different gradient concentrations, analyzed and compared the content of zinc and cadmium in different tissues through a standard curve.The results showed that the contents of zinc in the gills and muscles increased following the raising of zinc levels between 0 to 20mg/L. However, the zinc contents in gills decreased when the levels raised beyond 20mg/L. Though it still leaded to a little increase in muscles, the amount of increment declined apparently. The contents of cadmium in gills, liver and muscles increased as the experiment levels went up between 0 to 8mg/L. Nevertheless, the contents went down when the level kept raising beyond 8mg/L. When in the same concentration, the sequence of Zn²⁺ content in Koi tissues was gill> muscle, and Cd²⁺ was liver> gill> muscle. That is to say, the ability of Zn²⁺ accumulation was gill> muscle, and the Cd²⁺ was liver> gill> muscle.2. The sensitivity of several small fishes to heavy metalsIn order to choose the available fish for water monitoring, we used small fresh- water fishes as subjects in the acute toxic experiment of four heavy metals. 24h-LC50 were calculated after the experiments. Through the comparison of 24h-LC50, the sensitivity of different fishes to heavy metals was studied. The smaller 24h-LC50 it got, the more sensitive it was.It turned out that 24h-LC50 of different heavy metals on zebra fish (blue, red) (Danio rerio), red swordfish (Xiphophorus hellerii), guppy (Poecilia reticulata), tiger barb (Puntius tetrazona) were as below: Cu²⁺:0.017,0.020,0.102,0.028,0.009mg/L;Hg²⁺:0.085,0.088,0.173,0.075,0.092mg/L;Cd²⁺:0.375,0.378,0.408,0.146,0.983mg/L;Cr6+: 34.458,34.709,22.792,10.196,23.884mg/L. In purified water, the toxicity of heavy metals to several small fish was as follows: Cu²⁺﹥Hg²⁺﹥Cd²⁺﹥Cr6+. Tiger barb (Puntius tetrazona) was the most sensitive to Cu²⁺;Guppy (Poecilia reticulata) was the most sensitive to Hg²⁺, Cd²⁺, Cr6+; zebra fish (blue, red) (Danio rerio)was sensitive to Cu²⁺, Hg²⁺ ,Cd²⁺. According to the results, deploying small freshwater ornamental fish for monitoring and early warning heavy metals is feasible and should be further studied.3. The toxicity of heavy metals to zebra fish (Danio rerio) under different conditions In order to standardize the condition of early warning system, zebra fish (Danio rerio) was used as the experiment species. Experiments were carried out to find out the influences of water sources, water temperature and the lights on the toxicity of four heavy metals.It is figured out that, in purified water, the 24h-LC50 of Cu²⁺,Hg²⁺,Cd²⁺, Cr6+ to zebra fish (Danio rerio)was 0.017,0.085,0.375,34.458mg/L ,respectively. And the consequence of toxicity was :Cu²⁺>Hg²⁺>Cd²⁺>Cr6+. In raw water of Huangpu River, the 24h-LC50 was 1.273,0.310,18.002,52.375mg/L, and the toxicity: Hg²⁺>Cu²⁺> Cd²⁺>Cr6+. In raw water condition, when the water temperature was 20,22,24,26,28,the 24h-LC50 of Cu²⁺,Hg²⁺,Cd²⁺,Cr6+ to zebra fish (Danio rerio)was Cu²⁺:1.389,1.510,1.401,1.176,0.987 mg/L;Hg²⁺:0.313,0.335,0.298,0.222,0.318 mg/L;Cd²⁺:10.666,11.243,11.175,11.392,11.827 mg/L;Cr6+: 65.208,60.312,50.468,53.944,54.130 mg/L. Between 20 to 28℃, Hg²⁺, Cd²⁺, Cr6+ had no significance difference in 24h-LC50. Water temperature had no significant effects on the acute toxicity of these three heavy metals. When the water temperature was 20 and 28℃,the 24h-LC50 of Cu²⁺had no significance difference with that when it was 22, 24 and 26℃.But the results of 20 and 28℃had significance difference. The 24h-LC50 in 20℃is significantly larger than that in 28℃, which means that the toxicity of Cu²⁺ to zebra fish (Danio rerio) in 20℃is higher than that in 28℃.In raw water ,when the light intensities were 0 to 300Lux, 300 to 600Lux, 600 to 900Lux, the 24h-LC50 of four heavy metals was Cu²⁺: 1.357, 1.363, 1.392mg/L; Hg²⁺: 0.339, 0.347, 0.353mg/L; Cd²⁺: 15.186, 17.070, 15.785mh/L; Cr6+: 53.144, 55.318, 57.190mg/L. When the light intensity was between 0 to 900 lux, the 24h-LC50 had no significance difference, that is light intensity got no significant effects on the toxicty of Cu²⁺,Hg²⁺,Cd²⁺, Cr6+.