The phosphorus mass-balances (Fig. 2) showed no differences in
retained TP (51.5–53.9%, i.e. 7.4–7.8 g TP kg feed−1) or solid TP
waste (37.0–41.3%, i.e. 5.4–6.0 g TP kg feed−1), while the output of
dissolved waste TP decreased significantly (P=0.018) with PPC supplementation
from 10.5% in the diet A to 3.0% in diet D corresponding
to a decrease from 1.5 to 0.4 g TP kg feed−1. The residual TP varied
from 8 to 15% with no obvious trends related to the dietary treatment
groups, meaning that less TP was recovered than consumed by the
fish. As for N, residual TP reflected general measurement uncertainties,
time variation in sampling, and the fact that different analytical
methods had to be applied for measuring retained and solid
versus dissolved waste TP.
3.3. Growth and feed conversion ratio (experiment 2)
The fish generally accepted all diets, and the sum of feed waste
was less than 1% of the administered feed per diet. The fish grew
from an average initial weight of 65.7±13.3 g to an average final
weight for all dietary treatment groups of 182.0±48.1 g during the
57 feeding days of the second experiment. There were no significant
differences in overall SGR, TGC or FCR between the dietary treatment
groups during the 57 feeding days (Table 3). Twelve fish died during
the study, and the deaths were not associated with any specific dietary
treatment group.
4. Discussion
Organic production of herbivorous and omnivorous aquaculture
species such as tilapia and catfish appears to be relatively straight forward
as organic feedstuff may largely cover their nutritional needs
and therefore readily replace conventional feedstuff (e.g., Craig and
Mclean, 2005; Li et al., 2006). Formulating organic diets for carnivorous
species is a much larger challenge due to their high protein/
essential amino acid requirement and the ban against adding
synthetic amino acids in organic feed (EU, 2007). The present study