on the average nutrient contents of outlet water and
estimated sludge composition in the pond at the end of
the production cycle for nine other ponds. To fit the nutrient
balance, we assumed water refreshment at a rate of
2,500 m3/t fish, which is equivalent to replacing 0.4 m
water column daily. Though such an assumption seems
questionable, the results reported by Yakupitiyage et al.
(2006) suggest that over 50% of the P and N from
superfluous feed and faeces in semi-intensive tilapia ponds
leave the pond through various processes (e.g. leaching,
infiltration, immobilisation and mineralisation). These
nutrients will be used by the vegetation and not contribute
to any impact. Thus, leaving sludge in the ponds during the
production cycle will decrease nutrient discharge and
related impact categories such as eutrophication, since total
suspended solids are reduced by sedimentation and mineralisation
at the bottom. Consequently, leaving the sludge in
place will reduce the amount of energy used for pumping
and for the related impact categories, providing energy is
not needed to aerate the pond for the air-breathing
Pangasius.