Feed use, growth and feed utilization
During the first 2 weeks of period 1, the appetite of all groups
was quite low and some mortality occurred. The average survival
rate was 95.3% (SD = 1.8%). Within the two fish groups, there
was no significant difference in the feed use due to dietary treatments.
The average final weights were 2.71 and 4.79 kg for size
groups I and II, respectively. For each size group, there were
significant differences in the final weights among the dietary
treatments (Table 3). The fish fed diet 3 in group II were initially
smaller than the other ones, which may have interfered with some
of the results on this treatment. The TGC was significantly higher
in group I than in group II during the whole experiment. The
lower TGC in period 1 than in period 2 may be related to the low
appetite mentioned above. The TGC calculated for the whole
experiment was significantly different due to dietary treatments
within both fish groups (Fig. 1), while the TGC for each period
differed (Table 3). For the smallest size group, the highest TGC
for the whole experiment was found on diet 3 (DP/DE ratio
of 18.8 g MJ–1), which was significantly higher than that on diet 1
(DP/DE ratio of 14.1 g MJ–1), and in period 2 there was a
significantly higher TGC on diet 3 than on the other diets (Table 3).
For both size groups, the TGC was lowest in fish fed diet 1.
FCR differed significantly due to dietary treatments within the
two fish groups (Fig. 1). In group I, the feed conversion ratio was
significantly higher in fish fed diet 1 compared with fish fed the
other diets. The lowest FCR in group II was obtained on diet 2
(DP/DE ratio of 16.4 g MJ–1).
The average nitrogen retention was higher in group I (41%)
than in group II (33%), and the energy retention differed in the
same manner with 52% and 42% for the two groups, respectively.
The nitrogen retention increased with decreasing DP/DE
ratios from 21.9 to 16.4 g MJ–1 where the positive effect levelled
off (Fig. 2). The energy retention was lowest in fish fed diet 1
(DP/DE ratio of 14.1 g MJ–1) within both fish groups and was not
significantly different among diets 2 to 4 (Fig. 2).
For the diets resulting in lowest FCR for the fish groups, it can
be calculated that group I required about 21 MJ DE for producing
1 kg growth, while group II required 23–24 MJ DE per kg
growth.