ABSTRACT
Some biological aspects of the gold fish Carassius auratus [L. 1758] were studied in the Euphrates river near Al-Mussaib Power Station in the middle of Iraq, during the period of September 2002 to December 2003 .A total of 301 specimens were collected, with total length ranged between 6.2 to 25.5 cm and total weight between 10 to 175 gm. The b value obtained from length-weight relationship was volued 2.89. The age of fishes ranged from 1 to 4 years. From Von Bertalanffy growth pattern, Linfinity was 27.9 cm and K was 0.36. The highest mean of highet values of natural mortality rate coincides with the gonad weights at cold and warm months. The present study recorded increased values in the total length on the account of the weight of male as compared with female. Thus, the gold fish reached an optimum length in short time, although, there is increasing in the natural mortality rate of fishes increased and the gonads, were developed two months earlier compared with the same fish species in natural environment. This can be due to the thermal effluents effect of Al-Mussaib power station