The age and size at which Chinese carp attain maturity varies greatly with climatic and environmental conditions. Males usually mature 1 year earlier than females (Table 9). Among climatic factors, temperature has a pronounced effect on the maturation rate, as maturity occurs earlier in subtropical and tropical regions. In China grass carp and silver carp mature after being exposed to approximately 15,000 degree-days (°D), which is calculated in degrees Celsius as the sum of average daily water temperature for the days when water temperature exceeds 15°C. 126 According to Li and Wang, 127 silver carp need about 20,000°D and bighead carp about 26,000°D. The number of degree-days may differ, however, as temperature acts with other factors, especially feeding conditions, which are very important. An abundant food supply usually accelerates the maturation process because of greater growth rates, but in some cases overfeeding on certain foods, particularly prepared feeds under cultural conditions, may actually retard maturation. 9 Also, Chen et al. 128 suggested that exclusive feeding of grass carp on hydrilla could cause extensive fat accumulation, which may result in the retardation of gonad development.