3.2. Pen experiment
In pens, the reductions in final weight, growth rate and PER attributable to CoP were
much lower than in aquaria. Fish fed the diet with 130 g kg 1 CoP had similar growth and
feed utilization compared to those fed the control diet (P>0.05) (Table 4; Fig. 1). The
results in pens concur with findings of Bayne et al. (1976) with tilapia and of Bautista et al.
(1999) with Cachamay (Colossoma sp.Piaractus sp.), who found similar growth in fish
fed diets containing CoP and control diets. The amount of feed that could be fed to the fish
decreased with increasing levels of CoP in the diet (260 and 390 g kg 1). This is in
accordance with the results from aquaria trial and suggests indirectly that the palatability
of diets high in CoP (and high in fibre content) may have been a main factor limiting feed
intake and growth of fish.
Contrary to results in aquaria trial, in pens, tilapia growth was not altered by dietary
CoP levels between 0 and 130 g kg 1. The natural productivity of the pond could explain
why results in pens differed from those in aquaria. In the no feeding treatment, natural
food supported fish growth for 2 weeks and after that period, when the fish reached about
25 g, the available natural food only was enough to maintain fish weight for the remaining
experimental period. Assuming that the same amount of natural food was available to the
fish in the other treatments, the higher FCR values found in pens could result from
overfeeding. Our results have shown that the potential inclusion of CoP in tilapia diets
may be limited up to 130 g kg 1 when fish are raised in earthen ponds and natural food is
available.
Our findings may have an especial relevance in those areas where CoP is readily
available, and fish culture is practiced in extensive and/or semi-intensive conditions using
omnivorous and/or herbivorous fish species such as Oreochromis, Colossoma and different
carp species. If these fish species are reared in earthen ponds or pens instead of
concrete tanks or raceways, CoP will have a potential as a feed ingredient in fish diets as it
was found in our study. In contrast with cereal by-products, however, the high moisture
content and the presence of ANFs in CoP will to some extent limit its potential use in fish
feeds. One important consideration on this context will be the processing cost to produce
CoP meal that may be high because of the extra cost of drying. Unfortunately, up to date,
there is no reported data estimating this cost at an industrial scale, which could allow a
more precise cost comparison with other commercial feed ingredients of similar origin.