Ecophysiological Effects Of The Waterborne Zinc (Zn²⁺) On Two Common Fishes In The Yangtze River

In order to determine ecotoxicological effects of waterborne Zn²⁺,laboratory tests were conducted on the juvenile of southern catfish?Silurus meridionalis??18.92±0.09 g?and the grass fish?Ctenopharyngodon idellus??19.68±0.17 g?during acute and chronic exposures to the solutions of zinc sulfate heptahydrate?ZnSO4.7H2O?at constant water temperature?27.5±0.5??and hardness?25-30 mg CaCO₃/L?.Two series of experiments were conducted in the present study: 1.In the acute exposure experiment,the southern catfish were exposed to nominal Zn²⁺ concentrations of 0?control?,7.0,8.0,9.0,10.0 and 11.0 mg Zn/L for southern catfish and 0?control?,2.0,3.5,5.0,6.5 and 8.0 mg Zn/L for grass fish,respectively.The symptoms of acute poisoning fish were observed.The mortality rate in the exposure process was recorded in order to determin the median lethal concentration in 96 hours?96 h LC50?of waterbone Zn²⁺ for southern catfish and grass fish,respectively.2.In the chronic exposure experiment,the concentrations of waterbone Zn²⁺ were 0?control?,149.60?low concentration?,299.30?high concentration??g/L for southern catfish and 0?control?,and 83.41?low concentration?,166.82?high concentration??g/L for grass fish,which were equivalent to approximately 0,1/60,and 1/30 of 96 h LC50,respectively.After 8 weeks of chronic exposure,the parameters of growth performance,resting metabolism,biochemical composition and Zn bioaccumulation in tissues were tested to investigate the effects of the waterborne Zn²⁺ on southern catfish and grass fish.The main results in the present study were shown as follows:1.The relationship between the concentrations of waterbone Zn²⁺?the independent variable,x?and fish mortality?the dependent variable,y?,were described by y = 23.34x-159.54?southern catfish?and y = 13.845x-19.289?grass carp?,respectively.Using the linear interpolation method,we got the 96 h LC50 for southern catfish and grass fish were 8.98 mg/L and 5.00 mg/L,respectively.2.After 8 weeks of chronic exposure,the final weight,specific weight growth rate,condition factor and hepatosomatic index of southern catfish decreased with the increasing Zn²⁺ concentration,and those in the high exposure group were significantly lower than those in the control group?P<0.05?.Meanwhile,the kidney index and the resting metabolic rates increased with the increasing Zn²⁺ concentration.The kidney index in the low concentration group was significantly higher than that in the control group?P<0.05?,and the resting metabolic rates in the high concentration group was significantly higher than that in the other two groups?P<0.05?.3.The final weight and the specific weight growth rate of grass fish decreased with the increasing Zn²⁺ concentration,and the differences were significant among the three groups?P<0.05?.But there was no significant difference for condition factor,hepatosomatic index,the kidney index and resting metabolic rates among the three groups.4.The contents of crude protein,ash,dry mass and energy density of southern catfish decreased with the increasing Zn²⁺ concentration,but the tendency of the fat content was inverse.The contents of protein in Zn concentration groups were significantly lower than that in the control group?P<0.05?,and the energy density in the high exposure group was significantly lower than that in the control group?P<0.05?.There was no significant difference for contents of fat,ash and dry mass of the catfish among three groups.5.The contents of crude protein and ash of grass fish increased with the increasing Zn²⁺ concentration,but the tendency of the fat content,dry mass and energy density were all increased at first and then decreased.The content of ash in the high exposure group was significantly higher than those in the other two groups?P<0.05?,The dry mass in two exposure groups and the energy density in the low concentration group were significantly higher than those in the control group?P<0.05?.There was no significant difference for the contents of protein and fat of grass fish among three groups.6.The contents of Zn in organs?liver,gill,kidney,intestines,brain and muscle?and the whole body of southern catfish all increased with the increasing Zn²⁺ concentration.The contents of Zn in liver,gill,intestine,and whole body in the high concentration group were significantly higher than those in the control group?P<0.05?.The differences for the Zn contents in either muscle or brain was not significant among the three groups.The bioaccumulation of Zn in the organs of the test fishes exposed to Zn²⁺ could be ordered from higher to lower as following: intestines > gill > kidney > liver > whole fish > brain > muscle.The contents of Zn in intestines,gill and kidney were significantly higher than those in other organs?P<0.05?,and the content of Zn in muscle was significantly lower than that in the other organs.7.The contents of Zn in organs?hepatopancreas,gill,kidney,intestines,brain and muscle?and the whole body of grass fish all increased with the increasing Zn²⁺concentration.The contents of Zn in hepatopancreas,gill,intestine,muscle and whole body in the high concentration group were significantly higher than those in the control group?P<0.05?.There was no significant difference for the contents of Zn in brain among the three groups.The bioaccumulation of Zn in the organs of the test fishes exposed to Zn²⁺ could be ordered from higher to lower as following: hepatopancreas > gill > kidney > intestines > whole fish > brain > muscle and hepatopancreas > intestines > kidney > gill > whole fish > brain > muscle.The contents of Zn in hepatopancreas was significantly higher than those in other organs?P<0.05?.There was no significant difference for the contents of Zn in intestine and gill in two Zn exposure groups,but those were significantly higher than those in other organs except hepatopancreas?P<0.05?.The content of Zn in muscle was significantly lower than that in the other organs.The conclusions suggested in this study were as follows:1.Compared to grass fish which has no mechanism of muco-blocking against skin permeability,the skin mucus of the southern catfish could hinder the absorption rate of Zn²⁺ through the penetration.It should be the reason that cause higher Zn accumulation in grass carp compared with the southern catfish.Meanwhile,cough behavior of southern catfish could remove the foreign bodies of gills,to keep the gill clean and help maintain respiratory functions of catfish under the stress of Zn²⁺ exposure.Therefore,the mechanism of muco-blocking against skin permeability and cough behavior might be the important reasons for southern catfish to have more tolerance to Zn toxicity than that of grass carp.2.The pattern of energy allocation of the southern catfish and grass fish were changed when exposed to waterborne Zn²⁺.Two species of fish both need to pay additional energy to supply energy consumption in the process of resistance to environmental stress.The rate of protein or fat and energy deposited in fish reduced,thus the growth declined.Protein was preferentially used by southern catfish,while fat was preferentially used by grass fish,to provide extra energy for the detoxification when exposed to waterborne Zn²⁺.3.The waterbone Zn²⁺ uptake was mainly through the gill of fish,then distributed to other tissues via the circulation of the blood.Muscle is not the active tissue for Zn bioaccumulation,and the content in the muscle is not obviously influenced by environmental concentrations of heavy metals This indicated that it is not appropriate to rely on the metal content of muscle in the fish as an indicator of environmental pollution.in the field investigation.4.Zn contents of all tissues in grass carp were all higher than those in southern catfish in this study,which is different from the universal phenomenon that Zn contents in herbivores fish was generally lower than that in carnivorous fish in field conditions.It is due to only waterbone Zn²⁺ exposure pathway existed in laboratory tests.There was no food exposure pathways and the difference of blocking of skin permeability,which should be the main biological mechanism to result in the difference of Zn content among two species of fish in present experiment.The accumulation of Zn in fish tissues is determined by various biological mechanisms,rather than merely depended on the Zn concentration of the environment.