Feed intake, nutrient utilization, growth and survival
Milk-fish larvae studied in this experiment showed good palatability for all the diets as evidenced from excellent acceptability and there were no issues among encountered related to the intake of the micro-extruded and marumerised larval diets. The feed intake during the course of the experiment showed a decreas trend as the dietary corn oil inclusion level was enhanced, the differences being non-significant and highest feed intake observed in F3C1 dietary group was significantly different from other four experimental dietary groups.
After 42 days, there were significant differences in the survival rates and growth performances among the experimental groups (Table 2).
Larvae fed with F3C1 diet showed the best final body weight (FBW), which was significantly different from those fed the other four experimental diet. Furthermore, growth parameters such as weight gain (WG), average daily gain (ADG), specific growth rate (SGR) and weight gain percentage (WG%) of larvae showed (
P<0.05) significant differen up to 2% corn oil replaced diets, followed by fish fed with F1C3 and the lowest growth observed in C4 diet (Table 2). Similarly, the higher FER compared to larvae fed the other four experimental diets but there was no statistically significant difference among dietary treatment groups. Fish fed the C4 diet had a lower survival rate (52%) than fish on other dietary treatments. Based on comparative performance analysis among the dietary groups, the present study revealed that up to half the quantity of fish oil in milkfish larval diets can be replaced by corn oil (at the tested inclusion levels,
i.e., 2% fish oil and 2% corn oil F2C2 group), without affecting their growth parameters and wellbeing. Our earlier study showed that the optimal dietary soy lecithin inclusion level for larval diet of milkfish was 5.75 g kg
-1 whereas 3.5 g kg
-1 of dietary soy lecithin inclusion meets the minimal requirement
(Sivaramakrishnan et al., 2021).
Whole body larvae fatty acid composition
The fatty acid composition of milk fish early fry at the end of the feeding experiment is presented in Table 3.
In terms of fatty acid profile, the n-3 polyunsaturated fatty acids concentration in F2C2 diet accounted for 13.95% of total fatty acids (Table 3), which is equivalent to 1.71-2.0% of the diet. This suggests that a dietary content of 1.71-2.0% of n-3 PUFA could meet the essential fatty acid requirement of milkfish and effectively support growth and survival during their early life stages. Similarly, other marine fishes such as red sea bream
(Fujii et al., 1976), turbot
(Gatesoupe et al., 1977) and gilthead sea bream
(Kalogeropoulos et al., 1992) and grouper
(Lin and Shiau, 2007) showed maximal growth at 0.5, 0.8 and 1% of n-3 PUFA, respectively.
The presence of eicosatrienoic acid (20:1n-9) in tissues is an indicator of deficiency of essential fatty acid in striped bass and palmetto bass
(Webster et al., 1994). Similarly, oleic acid (18:1 n-9) occurrence was also reported to be the indictor for the deficiency of essential fatty acids in red sea bream
(Fujii et al., 1976) and gilthead sea bream
(Rodriguez et al., 1994). In this study, 20:1n-9 was linearly increasing with increasing the dietary corn level and highest concentration was observed in C4 group. As milkfish is a brackishwater fish with generally scanty D5-desaturase and it is likely that 20:1 n-9 can be produced. These findings were in agreement with our previous study in milkfish larvae where linoleic acid (C18:2 n-6) and oleic acid (C18:1 n-9) content increased with higher inclusion of soy lecithin in the experimental diets. In contrary presence of higher EPA and DHA level was associated with lower growth; whereas, the increasing proportion of phospholipid addition to neutral lipid was found to be superior for growth, feed utilisation and survival in the milkfish larvae
(Sivaramakrishnan et al., 2021). A similar result also reported in European sea bass larvae
(Cahu et al., 2003). The fatty acid bioconversion ability has been documented in the previous studies in milkfish as reported by
Benitez and Gorricita (1985) the presence of significant amounts of PUFA in the liver in spite of their deprivation in the natural food. If the fish were on nutrient plane that was essentially fatty acid-deficient, it is more likely that 20:2 n-9 and/or 18:2 n-9 would be produced
(Lin and Shiau, 2007).
According to,
Wu et al., (2002) and
Lin and Shiau (2007) a high tissue 20:1 n-9 concentration in tissues of grouper to be a indicator deficiency of essential fatty acid and in the present study, elevated 20:1 n-9 concentrations in milkfish larvae fed the F1C3 and C4 diets (Table 3) also indicate a sign of essential fatty acid deficiency. This may explain the poor growth of the two groups. The omega-3 fatty acids such as alpha linolenic acid, EPA and DHA were critical fatty acids which is more important than omega-6 fatty acid like. linoleic acid for milkfish larvae
(Borlongan, 1992; Borlongan and Benitez, 1992; Sivaramakrishnan et al., 2021). The whole-body nutrient composition of milkfish larvae fed with diet containing various lipid sources in the diets must have influenced by the composition of the dietary fatty acids
(Borlongan and Benitez, 1992; Kumar et al., 2014; (Balito-Liboon et al., 2018; Sivaramakrishnan et al., 2021).
In the current work, DHA (docosahexaenoic acid, 22:6 n-3) and EPA (eicosapentaenoic acid, 20:5 n-3) concentrations in the F4 diet were 11.87 and 4.76% of the total lipid concentration respectively (Table 3), which was equivalent to 1.47 and 0.59% of diet. The n3/n6 ratio in this diet was calculated to be about 0.95. This ratio seemed to be an equal against earlier recommendation of n3/n6 1 to for enhanced growth of marine fishes
(Lin and Shiau 2007;
Wu et al., 2002). It is interesting to observe growth of milk fish larvae was actually suppressed when fed diet having n3/n6 ratio is 0.57. Thus, the F4 diet (n3/n6: 0.95) of the current study meets the requirement and is to be used as the requirement.