NITROGEN BALANCE IN THE SILVER AND BIGHEAD CARPS Ⅰ. ESTIMATION OF SEVERAL PARAMETERS IN RELATION TO THE NITROGEN BALANCE DURING THE GROWING SEASON (SUMMER) OF FISH

In terms of nitrogen, several parameters such as the metabolic rate, the assimilation effici-ency and the ingestion rate of the silver and bighead carps were determined during the summer months.On the basis of the rate of ammonia excretion during starvation periods, the resting metabolic rates of fish at 30℃ may be expressed as the following equations:Silver carp: MR=0.054W0.884(mg-N/h)Bighead carp: MR=0.073W0.782(mg-N/h)Where, MR and W denote the resting metabolic rate and the body weight(g) respectively.The active metabolic rate of both fishes increased exponentially with the increase of swimming speed. At the swimming speed(lengths/sec.) around 4, the least swimming cost(in terms of nitrogen) for both fishes were recorded.During the summer months, about 90% of food items of the silver carp in the Dong Hu Lake are phytoplankton (Microcystis), whereas the bighead carp selectively takes up cladoceran (about 30%) in addition to phytoplankton. On the basis of the assimilation efficiency for Microcystis and Daphnia of both fishes, the assimilation efficiency in terms of nitrogen for natural food organisms in the lake was estimated as 44% for silver carp and 57% for bighead.The nitrogen balance of the silver and bighead carps can be formulated as follows:Silver carp: 0.44CN=1.81W0.61+24/0.75 0.054-0.021(1-e0.216s)W0.884Bighead carp: 0.57CN=1.98W0.61+24/0.750.073-0.033(1-e0.179s)W0.782Where, CN: ingestion rate in terms of nitrogen, s; swimming speed(lengths/sec,), W: body weight(g). Calculated from the above equations, the ingestion rates(wet weight of foods/day) of the silver and bighead carps (20g in wet weight each) were 65—80% and 47—57% of their body weights respectively. These results agreed well with the rates estimated by a different method (Morishita, 1968) based on the daily changes of the gut content and rate of the food passage in fish intestines.