Plant feed sources are cheaper than fish
meal but are also higher in fibre and anti-
nutritive factors such as phytin, gossypol,
trypsin inhibitors, lectins, etc. Therefore,
feed enzymes, now more than in the past,
have a significant role to play in increasing
the utilization of plant protein sources by
aquaculture species.
Experience from other animal species
would suggest the young animals grown
under aquaculture conditions should
benefit most from the inclusion of feed
enzymes. However, research with small
fish, as with immature land animals, has
produced variable responses to enzymes.
Kolkovski et al. (1997) found the feed
intake and growth of seabass larvae
(Dicentrarchus labrax) were unaffected by
a pancreatin enzyme supplement. Yet,
larvae of gilthead seabream (Pagrus
aurata) fed a pancreatin supplemented
micro-diet had 30% higher assimilation
rates than larvae offered the control diet
(Kolkovski et al., 1993). Despite the
improved assimilation rate of the enzyme-
supplemented larvae, their intake was
only half that of larvae given a live feed
regime. This suggests that the charac-
teristics which attract fish to feed may be
as important as improvements in diet
digestibility produced by enzyme supple-
mentation.
The variable response to enzyme
supplementation is also found with older
fish. Neither the feed intake nor feed
conversion efficiency of Atlantic salmon
parr (Salmo salar) were affected by supple-
mentation with an -amylase (Carter et al.,
1992). Similarly, Renitz (1983) found that
proteolytic enzymes did not improve
weight gain or feed conversion efficiency
(FCE) of rainbow trout given a standard
USA Fish and Wildlife Service starter diet.
Yet, carp given diets supplemented with a
multienzyme pre-mix at 5 or 10 g kg^1 had
12.3 and 27.5% faster growth rates than
control-fed fish (Ye et al., 1995). Gorskova
and Yu-Dvinen (1984) working with Coho
salmon and Carter et al. (1994) working
with Atlantic salmon have also measured
improved animal performance with
enzyme supplementation.
For fish, as for other monogastric
species, phytate P is an anti-nutritive
factor. The level of phytase activity
required to improve P digestibility varies
depending on the fish species and diet. For
example, Li and Robinson (1997) showed
that 250 U kg^1 of phytase was required to
replace inorganic P in a practical channel
catfish diet. Increasing the phytase level to
500 or 750 U kg^1 produced no improve-
ment in intake, weight gain, FCR, bone ash
or P compared with fish given the 250 U
phytase kg^1 -supplemented treatment.
Lunari et al. (1998) showed that 1000 U
kg^1 of phytase increased P digestibility
from 58.6 to 68.1% for rainbow trout.
Most research with feed enzymes in
aquaculture species has added the enzyme
supplement to the whole diet. In contrast,
Cain and Garling (1995) gave rainbow trout
(Onchoryhnchus mykiss) diets containing
phytase-treated or untreated soybean meal.
Phytase treatment either produced equal or
better trout growth rates and FCE than
those given the control diet. The efficiency
of phytase action can depend on the weight
of the fish. Phytase supplementation
reduced effluent P by 65% with 2 g fish
compared with 88% for fish weighing 17 g.
These results show that inclusion of
phytase can reduce the amount of
inorganic P required in fish diets by
increasing the availability of P in plant
feed ingredients. In addition, the P concen-
tration of fish effluent can be reduced
by phytase supplementation, which may
have significant implications for water
quality.
Two points now emerge:
● There is an increasing trend for fish
meal to be replaced in aquaculture diets
by other protein sources including those
from plants. This will mean that the
importance of feed enzyme supplemen-
tation will continue to increase in the
future.
● The response of fish to proteolytic and
carbohydrase enzymes has been variable.
Only phytase has proven consistently to
improve digestibility in a range of
situations.Feed Enzymes 415