Growth responses and metal accumulation patterns in fish exposed to chronic metal mixture concentrations

Abstract

Acute toxicity of 19 mixtures of iron, zinc, lead, nickel and manganese were determined in terms of 96-hr LC 50 and lethal concentrationsfor five fish species viz.Catla catla, Labeo rohita, Cirrhina mrigala,Ctenopharyngodon idella and Hypophthalmichthys molitrix under laboratory conditionsat constant pH (7), total hardness (200mg/L) and water temperature (30°C).The extent of metals bio- accumulation in fish body organs viz. gills, liver, kidney, fins, bones, muscle and skin at both 96-hr LC 50 and lethal concentrations were also determined. In the 2 nd phase, growth responses of five fish species were determined, separately, under chronic exposure of 19 mixtures of metals at sub-lethal concentrations (1/3 rd of LC 50 ) for 12 weeks. The bioaccumulation of metals in the fish body organs viz. gills, liver, kidney, fins, bones, muscle and skin were also determined before and after growth trails under the stress of 19 mixtures. All the five fish species showed significantly variable tolerance limits in terms of 96-hr LC 50 and lethal concentrations against 19 mixtures of five metals. Amongst 19 mixtures / treatments, the mixture of five metals (Fe+Zn+Pb+Ni+Mn) caused significantly higher toxicity to the fish, in terms of 96-hr LC 50 and lethal concentrations of 46.23±11.54 and 73.46±14.18mgL -1 , respectively. However, Pb+Mn and Zn+Pb mixtures were significantly least toxic to the fish with the mean LC 50 and lethal concentrations of 83.48±11.34 and 134.70±26.77mg L -1 , respectively. Regarding overall sensitivity of five fish species, Hypophthalmichthys molitrixwere significantly more sensitive to metals mixtures with a mean LC 50 value of 59.63±11.01mgL -1 while Labeo rohita showed significantly least sensitivity (81.73±12.73mgL -1 ).This significantly higher sensitivity of Hypophthalmichthys molitrix towards metal mixtures was followed by that of Cirrhina mrigala, Catla catla, Ctenopharyngodon idella and Labeo rohita. However, the difference between Catla catla and Cirrhina mrigala, for their tolerance limits (LC 50 ), were statistically non-significant. The overall sensitivity of all the five fish species, determined in terms of lethal concentrations, against various metals mixtures varied significantly. Regarding overall response of fish towards 19 mixtures, mean sensitivity of fish to the mixture of five metals (Fe+Zn+Pb+Ni+Mn) was significantly high (P<0.05), followed by that of four metalsmixtures (Fe+Zn+Pb+Mn) with statistically significant difference. Hypophthalmichthys molitrix were significantly more sensitive to metals mixture with themean lethal concentration of 97.85±16.20mgL -1 , followed by that of Catla catla, Cirrhina mrigala, Ctenopharyngodon idella and Labeo rohita with the mean lethal concentrations of 101.70±16.70, 105.80±12.80, 122.40±23.77 and 128.80±19.95mgL -1 , respectively. Amongst five fish species, Labeo rohita showed significantly least sensitivity towards Zn+Pb mixture (170.00±0.60mgL -1 ) while Hypophthalmichthys molitrix showed significantly higher sensitivity to Pb+Mn with the mean lethal concentrations of 18.53±0.61mgL -1 . The overall sensitivity of five fish species varied significantly for 19 mixtures with the mean higher and lower lethal concentrations of 134.70±26.11 and 73.56±14.18mgL -1 for Zn+Pb and Fe+Zn+Pb+Ni+Mn mixtures, respectively. The exposure of four (Fe+Zn+Pb+Mn) and five (Fe+Zn+Pb+Ni+Mn) metal mixtures, at sub-lethal concentrations (1/3 rd of LC 50 ), caused significantly lesser growth to all the five fish species. Amongst 19 mixtures, Fe+Zn+Pb+Ni mixture (#17) caused significantly pronounced impacts on the growth performance of all the five fish species, followed by thatof Fe+Zn+Pb+Ni+Mn (#19) and Fe+Zn+Pb+Mn (#18) mixtures. Under chronic stress, Cirrhina mrigala and Hypophthalmichthys molitix attained significantly higher weights, followed by that of Catla catla, Labeo rohita and Ctenopharyngodon idella. However, the growth of all the five metals mixture exposed fish species was significantly lesser than that of control fish (un-stressed). Significantly variable condition factor values reflected the degree of fish well-beings that correlated directly with fish growth. The metallic ion loads (under the exposure of different mixtures) of the culture media correlated inversely with fish growth due to significant effects of metal’s stress on fish body. The growth performance of fish did not vary significantly due to change in feed intake while significantly positive change in the feed conversion efficiency had affected the fish growth significantly due to the impacts of various mixtures in reducing fish metabolism and activity. Any significant change in feed intake, due to stress, is reflected in terms of fish growth showing the impacts of various mixtures on fish growth were either additive or antagonist / synergistic. Physico-chemistry of the test media (water) used for different treatments exerted significant impacts on fish growth, feed intake, and condition factor and feed conversion efficiency of fish also. Significantly better feed intake enhanced the ammonia production and excretion by the fish resulting into non-significantly positive relationship of fish weight increments with ammonia contents of the test media. Significantly higher feed intake resulted in excessive excretion of ammonia by the fish to cause significant impact on its growth. Sodium and potassium showed significantlypositive correlation with ammonia concentrations of the test media, indicating excessive release of sodium and potassium by the fish under stress of various mixtures that resulted in significantly more excretion of ammonia by the fish. Organ-wise distribution of residual metals viz. iron, zinc, lead, nickel and manganese reveals liver as the prime site of their accumulation with significantly higher persistence, followed by kidney, gills and fins of metals mixture exposed fish. The fish muscle tissues accumulated significantly (P<0.05) lower metals than liver, kidney and gills. The exposure of metals mixture at higher concentrations resulted in escalated levels of these metals in fish body that followed the general order: zinc >iron >nickel >lead >manganese which can be regarded as an indicator of cumulative response of five fish species. Accumulation of all the metals in fish body followed the general order: liver>kidney>gills>fins>muscle>skin>bones. The level of various metals in gills of fish, exposed to different sub-lethal concentrations of mixtures, was significantly higher (P<0.05) than the levels found in all other organs except liver and kidney.