Based on the biological measurement data of small yellow croakers collected in the sea area in the north of 29°N and the east of 127°E in 2004,the trend of annual change of power b was analyzed by Bootstrap method at four different temporal scales including day,week,half-month，and month.The bootstrap analysis showed that the values of power b during the spawning period were significantly smaller than those of the other periods in the same year (P<0.05).When the average b value from the samples was used for the determination of the periods of spawning and non-spawning,only the results based on the time scales of week or longer were reliable.However,when the b value of a single sample was used for the determination,the results based on the time scales of only half month or longer were reliable.The trend of annual change of power b,with low values during April~May and a peak during November~December,became more obvious with the increase of temporal scales.