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Length, weight, structural index and sexual maturity in tambaqui Colossoma macropomum were analysed. 449 specimens were obtained from the commercial fishing fleet arriving at Manaus Harbour from the floodplain areas from May 1992 to July 1993. The objective of this study is to describe changes in structural index during one hidrological cycle and to interpret them according to the water level fluctuations and to also determine the time, reproductive period and length in sexual maturity. Sex ratio in tambaqui was 1:1 however, the length of females was significantly longer than males'. Length/weight relationship was described by the expression: Pt = 0.00058*Cp2.9039. b value was significantly different from 3 which meant an alometric growth. Lower condition index values (Kn) were found from August to October at the end of receding water and dry period. The feeding index was significantly lower during the receding water and dry period. Lower hepatosomatic index values were found in December during the dry period. Fishes in lengths longer than 550 mm which were more common from September to December during the dry period, stored larger cavitary fat reserves. Fishes smaller than the standard length of 550 mm stored less fat reserves and were more common during the rising of the water and flooding period. Spawning period of tambaqui extends from September to February in a total spawning synchronized with "repiquetes" (native term used to name a sudden recending and rising of the water level). Length of tambaqui was 60.69 cm, at sexual maturity.

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