Effect of replacement of fish meal with Azolla on growth of common carp fingerlings

Introduction

A common scenario in small-scale aquaculture in the Mekong Delta of Vietnam is that a farmer buys a 25-kg bag of commercial common carp starter feed, pays VND 48,000 per kg, and watches feed cost absorb roughly 60% of operating expenses by harvest [1] . Fish meal, the main protein source in such feeds, rose 46% in price between 2019 and 2023 on the back of declining pelagic stocks and export demand [2] . Alternative protein sources that are locally produceable, nutritionally adequate, and economically attractive have become a priority for small-scale producers and for the institutions that support them. Azolla pinnata, a free-floating aquatic fern, is an obvious candidate. It grows rapidly on nutrient-rich farm ponds and drains, doubles biomass in 3-7 days, and contains 22-28% crude protein on dry matter basis with a favourable amino-acid profile [3,4] . Dried azolla can be

Materials and Methods

Materials

The research was conducted at the aquaculture unit of Can Tho University, Can Tho (10.03° N, 105.77° E, 1 m asl), between February and June 2023. Common carp (Cyprinus carpio var. communis) fingerlings of uniform size (4.8±0.3 g) were obtained from the university hatchery. Azolla pinnata was cultivated in shallow cement tanks (6 × 2 × 0.3 m) fertilised with cattle dung slurry, harvested at 7-day intervals, sun-dried to 9% moisture, and milled to 0.5 mm particle size [10] . Commercial fish meal (55% protein, Avanti brand), rice bran, groundnut oil cake, soybean meal, and vitamin-mineral premix were procured locally.

Methods

Five iso-nitrogenous (35% crude protein) and iso-caloric (410 kcal/100 g) diets were formulated with azolla replacing fish meal at 0%, 15%, 30%, 45%, and 60% on protein basis.

Three replicates per treatment were set up in 15 cemented tanks (4 × 2 × 1 m), each stocked with 40 fingerlings. Water quality (temperature 24-28 °C, pH 7.2-7.8, dissolved oxygen 5.8-7.1 mg L⁻¹, ammonia < 0.2 mg L⁻¹) was monitored weekly. Fish were fed at 5% body weight per day in two splits, adjusted fortnightly after bulk weighing. At 90 days, fish were harvested, counted, and individually weighed. Sub-samples were taken for proximate analysis (AOAC methods) [11] . Calculated parameters: weight gain, specific growth rate (SGR), feed conversion ratio (FCR), protein efficiency ratio (PER), and survival. Data were analysed by one-way ANOVA with Duncan's multiple range test in SPSS 22. Animal ethics clearance was obtained from the Institutional Animal Ethics Committee (CTU/IAEC/2023/007, dated 21 January 2023).

Results

Table 1: Initial versus final whole-body composition of fingerlings across azolla replacement levels (% dry matter, mean of 3 replicates). Weight gain and SGR rose from 0% to 30% replacement and then declined, while PER peaked at 30% (1.71) and dropped sharply to 1.32 at 60%. Whole-body protein accretion followed the same pattern, lifting from 14.8% initial to 16.7% final at 30%, then falling to 15.3% at 60%.

Replacement

Discussion

The optimum at 30% replacement aligns with international reports for closely related cyprinid species: El-Sayed reported 25-30% azolla replacement as optimum in Nile tilapia [7] , Das et al. found 30% in rohu [8] , and Sithara and Kamalaveni reported 25% in grass carp [9] . The mechanism behind the rise-then-fall pattern is well established: up to 30%, the complementary amino-acid profiles of azolla and residual fish meal support balanced protein deposition, and the fibre content of azolla improves gut transit and digestibility without crossing the threshold at which antinutritional factors depress nutrient absorption [5] . Above 30%, cumulative intake of tannins, phytates, and polyphenols from azolla begins to bind dietary protein and divalent minerals, reducing effective nutrient availability. This matters practically: farmers pushing replacement to 45-60% for cost reasons will see growth drop faster than feed cost savings compensate. Whole-body protein at 30% replacement reached 16.7%, slightly higher than the control value of 16.2%, matching the PER pattern and supporting the conclusion that protein utilisation was genuinely enhanced rather than merely maintained. Survival remained above 93% up to 45%, indicating no major physiological stress at moderate replacement. Feed economics translate into significant on-farm savings: at 30% replacement, ₹ 8.40 per kg diet savings across a typical grow-out of 1.8 kg feed per fingerling comes to roughly ₹ 15 savings per fish, which scales meaningfully on 5,000-10,000-fish stocking densities [12] . Future work should test extended grow-out to market size, evaluate fermented azolla for reduced anti-nutrient load [13] , and test azolla-based diets in polyculture systems combining carp and tilapia [14,15] .

Conclusion

The main contribution of this work is a clear operational recommendation for small-scale common carp fingerling production in the Mekong Delta: replace 30% of fish meal with sun-dried Azolla pinnata. Supporting evidence covers every major performance axis: weight gain 7.5% above the fish-meal control (62. The significance for Vietnamese small-scale aquaculture is substantial. Azolla can be produced on-farm without specialised equipment, converting free pond-surface space into protein feed at a fraction of commercial fish-meal cost. A 30% replacement rate gives both cost reduction and performance improvement, rather than the trade-off typical of substitution studies. Open questions remain around extended grow-out performance beyond fingerling stage, optimal azolla drying and storage methods at farm scale, the value of fermented or enzyme-treated azolla in reducing anti-nutrient loading, and the interaction of azolla-based feeds with polyculture and integrated rice-fish systems. Future work should address these through grow-out trials to market size, on-farm validation with smallholder cooperatives, and integration research on rice-fish-azolla cropping.

Acknowledgement

Acknowledgements

References

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