Sustainable fish feeds: optimization of levels of inorganic fertilizers for mass production of Oocystis sp. for climate smart aquaculture

Abstract

Use of microalgae as source of food in aquaculture production is gaining recognition due to their rapid growth rate that promises high biomass generation within a short time. The challenge faced is getting good and inexpensive nutrients source to be used in mass production of the required microalgae. This study investigated the effect of different nutrient combination in in uencing the growth rate of the of the green algae Oocystis sp. which has been indenti ed as possible protein source for the raising of Orechromis niloticus ngerlings for sh farming. Modi ed Bolds 3N Medium and commercial agricultural fertilisers (urea, NPK and DAP) media were compared to establish the appropriate combination that would result into high biomass generation but at the lowest cost possible. The Modi ed Bold 3N Medium acted as the control, at a cost of 11.28 KSh per litre, the other media were derived from urea, NPK and DAP (varying the ratio of each) at a cost of treatment 1 (0.14 KSh per litre), treatment 2 (0.18 KSh per litre) and treatment 3 (0.22 KSh per litre). The algae was cultured for ve weeks with samples taken daily for biomass analyses using chloropyhll-a concentration as the surrogate for Oocystis sp. biomass for 30 days, from each treatment was determined. The growth rate, doubling time, and divisions per day were then estimated based on this chlorophyll-a concentration. The results showed that the mean concentrations of chlorophyll-a in treatment 1 was highest (7.715 ± 0.667 µg/ml) while treatment 3 (6.441 ± 0.555 µg/ml) had the least. There was no signi cant differences in the mean concentrations of chlorophyll-a in the four treatments (Kruskal-Wallis H test: P > 0.05). The chlorophyll-a concentration varied signi cantly in each treatment with time (Kruskal-Wallis H test: P < 0.001). There was no signi cant difference in the growth rate (Kruskal-Wallis H test: P > 0.05), divisions per day (Kruskal-Wallis H test: P > 0.05), and doubling time (Kruskal-Wallis H test: P > 0.05) from the different treatments. The results of this study showed that inorganic fertilizers can be used as cost-effective media in the mass scale culture of Oocystis sp.