We first checked the N balances to estimate total nutrient
discharge and used that to estimate the discharge of other
components. The FCR of 1.86 was used to estimate total
waste. We calculated the N output on the basis of a dry
matter content of 90% for feed and 30% for fish, and an N
content of 22% for feed and 16% for fish. This resulted in a
total N excretion of about 33 kg/t fish. We did not
distinguish between N from respiration and N from
excretion, and for N, we used the total N in the water
column because the amounts of N from N2, N2O, H3NO3.
H2NO2, NH4OH and NH3 in the water, sludge and
sediments are not stable but vary according to the pH and
the availability of oxygen. Consequently, if any N evaporated
as NH3, N2 or N2O, its impact was not included in the
assessment.