Nitrogen requirement for maintain of Pagrosomus major

The nitrogen balance method and nitrogen-free diet were used in present study to determine nitrogen requirement for maintenance(Nm)and nitrogen requirement for maintenance per unit metabolism body weight(Nm')of red sea bream(Pagrosomus major).Fishes with body weights of 50,100,150,200 and 250 g were fed by the diets containing a gradient protein level from 38% to 46%.The results from nitrogen balance experiment show that the amount of nitrogen deposition varied from 0.15 to 0.30 mg·g-1·d-1,accounting for total ingested nitrogen from 12.1% to 20.3%.The amount of faecal nitrogen excretion varied from 0.20 to 0.32 mg·g-1·d-1,accounting for total ingested nitrogen from 16.4% to 21.6%.The endogenous nitrogen excretion,a main part of total ingested nitrogen consumption,varied from 0.80 to 0.96 mg·g-1·d-1,accounting for total ingested nitrogen from 62.8% to 67.7%.A positive correlation was observed between Nm and body weight,while a negative correlation was observed between Nm of unit body weight and growth duration.There is no significant difference(P0.05)among Nm'obtained from different growth stages.The average of Nm' was 0.498 mg·g-1·d-1.The results from nitrogen-free diet experiment show a negative correlation between nitrogen requirement for maintenance and feed intake of nitrogen-free diet;With the decrease of feed intake of fish,the nitrogen requirement for maintenance was increased;The average of Nm' was 0.510 mg·g-1·d-1 with fasting of fishes.This value is close to 0.508 mg·g-1 obtained from fishes with 150 g body weight in nitrogen balance experiment.The nitrogen balance method is recommended to be a right method for determining nitrogen requirement for maintenance in consideration of fish stress and result stability.This study also supplied a calculated result about the protein content levels in diets,which are necessary for maintaining fish body protein at different growth stages.The calculation is based on the amount of nitrogen necessary for maintaining body protein per kg fish weight.