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

Abstract:

Picture a small-scale fish farmer in Vietnam's Mekong Delta trying to raise common carp fingerlings on a shoestring budget as fish-meal prices climbed 46% between 2019 and 2023. Azolla pinnata, a free-floating aquatic fern, can be produced locally in farm ponds at roughly VND 12,000 per kg dry matter, against VND 75,000 per kg for commercial fish meal. This research tested whether azolla could partially replace fish meal in common carp fingerling diets without compromising growth or health. Five replacement levels (0%, 15%, 30%, 45%, 60%) were tested in a 90-day feeding trial with Cyprinus carpio fingerlings (initial weight 4.8±0.3 g) across 15 cemented tanks at the university aquaculture station. Weight gain peaked at 30% replacement (62.8 g), a 7.5% improvement over the fish-meal control (58.4 g). Feed conversion ratio (FCR) followed the same pattern, improving from 1.82 at 0% to 1.68 at 30%, before deteriorating to 2.12 at 60%. Survival remained above 93% up to 45% replacement and dropped to 88.4% at 60%. Specific growth rate, protein efficiency ratio, and whole-body composition confirmed the 30% replacement as the optimum. At this level, feed cost fell by VND 5,600 per kg feed with no production penalty. The findings recommend 30% azolla replacement as a practical cost-reduction strategy for small-scale carp farmers in the Mekong Delta.