However, the
microbial biomass undergoes a constant process of degradation and if
the whole system is not in a steady state, ammonium may be released
to the solution with time. The tagged protein harvested by the fish is
partially digested, and the rest is excreted as fecal material. In
addition, metabolic processes in the fish lead to an excretion of
ammonium (Hepher, 1988), including some of the digested tagged
nitrogen. The measured enrichment of 15N in the fish tissues is the
difference between the gross uptake and the excretion processes, and
thus represents the “net uptake”, an important entity, but only part of
the picture. It seems that the ratio of gross uptake to net uptake may
differ under different conditions such as fish age, fish variety,
environmental conditions, properties of substrate etc.