Aquaculture is gaining considerable importance all over the world as a mean of improving world fish production which is currently on decline due to dwindling output from capture fishery (FAO, 2009). Expansion in aquaculture is strictly related to improvement in nutrition, and up gradation of fish husbandry practices- a challenge for future development in aquaculture. Mass scale fish production heavily depends on the amplification of proper feeding protocols to satisfy nutritional requirements of the cultured species.

Table 1 Growth comparison of different fish species fed on different feed ingredients

Amylase secretion level remained uniform when fishes were fed on rice polish except Hypophthalmichthys molitrix where level was significantly lower than its counterparts. Though highest value was observed in Cirrhinus mrigala but level of significance was not as prominent as observed in former species. Much lower amylase concentrations were observed in Catla catla, Labeo rohita and Hypophthalmichthys molitrix when fed on soybean meal and cotton seed meal (Table 2). Cirrhinus mrigala again topped other species in amylase secretions when fed on cotton seed meal.

Table 2 Concentrations of amylase in different fish species when fed on different feed ingredients (IU/min/mL)

showed the highest lipase activity when fed on soybean meal while lipase activity was the highest in Cirrhinus mrigala when fed on rice polish. Lipase secretion levels were similar and quite lower than its counterparts in Hypophthalmichthys molitrix in all the ingredients fed (Table 3). Catla catla secreted lowest concentrations when fed on cotton seed meal while the second highest when fed on rice polish. Labeo rohita did equally well in all the ingredients but levels were quite higher than Hypohthalmichthys molitrix but lower than Catla catla and Cirrhinus mrigala in some combinations (Table 3).

Table 3 Concentrations of lipase in different fish species when fed on different feed ingredients (IU·min-1·mL-1)

Table 4 Concentrations of protease in different fish species when fed on different feed ingredients (IU·min-1·mL-1)

1.5 Water Quality Parameters
Due to daily exchange of water all the physico-chemical parameters remained within the acceptable range (Table 5).

Table 5 Ranges of water quality parameters observed during the course of experiment

2 Disscussion
Growth and survival data are powerful tools for understanding the effects of quality of feed on growth and linked metabolic mechanisms in fish (Wang et al., 2005). Our findings favorably fit in and support this statement because we observed the highest and uniform growth in all the fish species when fed on rice polish. Change in species and type of feed significantly affected the fish growth which was very much evidentwhen Catla catla was fed on soybean meal (Table 1) or when Cirrhinus mrigala was fed on cotton seed meal. Growth comparison of Catla catla showed significantly higher growth when fed on soybean meal followed by rice polish and cotton seed meal. Hypophthalmichthys molitrix grew significantly less than its counterparts (Table 1). Findings of Yasmin (1987) support and confirm our findings who reported better performance of Cirrhina mrigala than Lobeo rohita and Catla catla in artificial feed supplemented pond. Our observations are quite in line with previous investigations because Cirrhinus mrigala elicited superior growth performance on all the feed ingredients than its counterparts. Findings of Ashraf et al.(2008) further verify our investigations who observed different growth responses when Cirrhinus mrigala fingerlings were fed on different feed ingredients. Hypothalmicthis molitrix generally showed poor growth in all the ingredients nevertheless it was selectively comparable with Cirrhinus mrigala when it was fed on soybean meal or with Catla catla when it was fed on cotton seed meal. 

The nature and composition of the diet administered strongly triggers the development and secretion of digestive enzymes which affects the metabolic capabilities of fish. These enzymes had been an effective tool for identification of particular components of animal’s diet (Kanou et al. 2000). When sea bream specimens were fed on protein rich and non-proteic diets, it reflected type of diet in the secretion of gut enzymes. Protein rich diets resulted in higher amounts of proteases while amylase and lipase levels were greater in fish fed low protein diets. Fish may adapt their metabolic functions to the dietary substrates, through a regulation in enzyme secretion, in order to improve the utilization of particular feed ingredients (Caruso et al., 2009). Results were quite interesting in current studies where all the enzymes studied viz. amylases, lipases and proteases differed in different species and when fed on different ingredients (Table 2, Table 3, and Table 4). These findings strongly suggest that secretions of intestinal enzymes are dependent both on fish species and type of diet. This emphasizes that there are dietary as well as anatomical adaptations in fish. Fernandez et al.(2001) pointed out that digestive adaptations in different species exhibit closer correlation with their diet rather than on their taxonomic category. Our studies support former part of their findings but contradict the later part further confirming that taxonomical differences do have bearing on digestive adaptations in fish. This view was also confirmed by the results of Kuzmina (1996a) who indicated that changes in digestive enzyme activity could be affected by feeding behavior and biochemical composition of feed. Chan et al. (2004) and German et al. (2004) investigated the digestive enzyme activities in four closely related prickle back fishes, including two herbivorous and two carnivorous species. Their results showed that the activities of digestive enzymes correlated more strongly with phylogeny rather than with the fish’s natural diets. Influence of the genetic strains on the activities of brush border enzymes was demonstrated in the crosses of Oreochromis mossambicus and O. aureus (Hakim et al., 2006) and in the silver perch Bidyanus bidyanus (Hakim et al., 2006). Furthermore, the activities of digestive enzymes were also influenced by many other factors such as the ages of the fishes (Kuzmina, 1996), temperature and season (Kuzmina et al., 1996b) and the composition of their diets (Zambonino Infante and Cahu, 2001). Significantly higher amylases were observed in Cirrhinus mrigala when fed on rice polish and soybean meal, higher lipase levels were evident in Catla catla and similarly variable proteases were present in different species when fed on different feed ingredients. Chaudhuri et al.(2012) measured enzymes (i.e. α-amylase, invertase, cellulose, alkaline protease and pepsin) from liver, stomach and intestine of ten carnivorous species and did not find any positive relationship between prey preferences and digestive enzymes. These studies further strength ours and suggest that enzyme patterns are more affected by phylogeny rather than adaptability. The findings of Essa et al.(2010) further support our view point who observed that Tilapia fed on diets containing different probiotics have appeared to improve the digestion of protein, starch and fat that could be due to higher level of enzyme activities, which may explain the better growth and feed utilization. Notwithstanding all these findings, the relationship between digestive enzyme activities and feeding habits in fishes is still not very clear.

Zambonino InfanteJ.L., and CahuC.L., 2001, Ontogeny of the gastrointestinal tract of marine fish larvae, Comparative Biochemistry and Physiology, 130(4): 477-487