The nitrogen balance in plaice, sole and perch.

In both ecology and fish-cultivation, a knowledge of the efficiency with which food is converted into growth is of considerable importance. Conversion to growth, however, is but one aspect of the balance of nutrients in general, and it is therefore appropriate to consider the whole of this balance, and to do so with particular reference to selected constituents of living matter, the turnover and cycling of which may be traced from prey to predator. Nitrogen—of which the major portion is contained in proteins—and energy are the most commonly used constituents in such work. The primary nitrogen ' balance' is between the input to the animal, i.e. that which is absorbed into the bloodstream from the food, and the output, in which are included the nitrogen retained as growth or stored in the body and that which is metabolized. The balance is summarized by the following verbal equation, which forms a useful starting point to more detailed discussion: