3.3. Digestive enzyme activities in the liver and the intestine of grass carp
The activities of trypsin, amylase and lipase in the liver and the
intestine of grass carp fed with duckweed and chironomid larvae
were presented in Figs. 4–6. Trypsin activity in the liver of fish fed
with chironomid larvae was significantly decreased (P b 0.05) at
30 days compared to 0 day (Fig. 4). Trypsin activity in the liver of fish
fed with duckweed was significantly higher than those fed with chironomid
larvae at 30 days (P b 0.05) (Fig. 4). Amylase activity in the liver
and the intestine of fish in both groups were significantly decreased
(P b 0.05) at 60 days compared to 0 day (Fig. 5). At 30 days, amylase
activity in the intestine of fish fed with duckweed was significantly
lower than those fed with chironomid larvae (P b 0.05) (Fig. 5). Lipase
activity in the liver and the intestine of fish with chironomid larvae was
significantly increased (P b 0.05) at 30 days compared to 0 day (Fig. 6).
Fish fed with duckweed exhibited significantly lower lipase activity in
the liver than those fed with chironomid larvae at 30 days (P b 0.05)
(Fig. 6).
4. Discussion
The digestive system plays an essential role in vertebrate physiology
as the site of nutrient digestion and absorption (Johnson, 1994). Previous
studies demonstrated that the time of exposure of ingested food
to proteolytic enzymes rises with increasing gut length in herbivorous
fish, therefore many herbivorous fish have long coiled digestive tract
(Hofer and Schiemer, 1981). In this study, we showed that during the
food transition stage from zooplankton or benthos to aquatic
macrophytes, the relative length and weight of gut were significantly
increased at 30 days. When the fish finished the transition stage, with
the increased body length and weight at 60 days, the gut relative length
returned to levels at 0 day. Fish gut was long enough for digestion of
plant food.
Moreover, differential feeding patterns of grass carp fed with plant or
animal diets were closely correlated with different growth rates. Fish fed
with duckweed had significantly higher growth than those fed with chironomid
larvae at both 30 and 60 days. Plant food stimulated not only
body growth, but also gut growth. Although total length, body length,