Basrah J. Agric. Sci. 25 (special issued 2), 2012 14 Fish Size as a Key Factor in Copper Toxicity to Fish: Impact of water-born copper on some physiological indices in rainbow trout Oncorhynchus mykiss of various size groups Nader A. Salman*, Jeffery L. Ullman, Xiao Q. Lu *Department of Fisheries, College of Agriculture, university of Basrah, Iraq; Department of biological Systems Engineering, Washington State university, USA Abstract. Rainbow trout (Oncorhynchus mykiss) of various size groups representing one month larvae (0.3-0.6 g), three months fingerlings (5-7 g) and adult big fish (300-500 g) were exposed to different concentrations of CuSO 4 .5H 2 O for previously determined exposure periods. Several physiological parameters were measured to compare adaptability or toxicity cases. Those include ventilation rate, heartbeat rate, body water contents and behavior as well as survival or mortality rate. The results showed a direct positive relationship between adaptability or tolerance and fish size. Physiological responses varied with the concentration of the exposure medium and fish size. Sever effects were noticed among rainbow trout fry at the levels of 200-400 ppb Cu. Similar responses were noticed among rainbow trout fingerlings but at higher concentrations (600-800 ppb Cu). Toxicity of Adult big fish has not been registered below the level of 800 ppb. The results were discussed in term of physiological disturbances occurred in response of copper concentrations in the exposure media which led to adaptive tolerance or fatigue mortality. The present study concluded that fishery management of pollutants in freshwater bodies must have different approaches as far as fish size is concerned. Key wards: copper toxicity, fish size, ventilation, residual effect, ion regulation. Correspondent author: ndrslmn1@gmail.com Introduction Heavy metals have long been recognized as serious pollutants of the aquatic environment. The accumulation of metals in an aquatic environment has direct impact on man and aquatic ecosystem. Copper is one of the wastes commonly discharged into inland and coastal waters in many parts of the world. Cu +2 is an important xenobiotic in aquatic ecosystems as well as a non- degradable and cumulative pollutant. Increased copper concentrations in waters have resulted from the release of municipal wastewater, power plant effluents and marine antifouling paints. Industrial operations such as mining and smelting contribute to the mobilization of metals, including copper (Cu), into watersheds. The toxicity of Cu +2 to teleosts has been extensively studied, as summarized by (22). Cu is an essential micronutrient for normal metabolic functioning of all organisms, including fish (17). However, it can be highly toxic to aquatic organisms, particularly fish at concentrations above the physiological threshold levels (5). Interest in the metals, required for metabolic activities in organisms, lies in the narrow range between their essentiality and toxicity