Productivity, proximate composition and toxicity of naturally occurring microalgae in different stocking density of catfish under Malaysian weather conditions

Presently global warming, increasing world population and coupled with food security are some of the issues that need to be addressed urgently. Naturally occurring microalgae found in fish culture is new possibility for cheap microalgal biomass production, potential food source and maintaining sustainable aquaculture under different weather conditions. Therefore, the objectives of this study are to determine the effect of different weather conditions and fish stocking densities on the dynamics of physico-chemical water parameter in catfish tanks, to evaluate the fish and microalgal productivity, biochemical composition and fatty acid profile of naturally occurring microalgae and catfish, to document the diversity and dynamics of those microalgae, to evaluate the toxicity of mixed microalgae in different fish stocking density and weather conditions. Clarias gariepinus with size 10±0.2 cm and 18.5±0.3 g was placed in polytank of 300 L under sheltered transparent roofing. The experimental set up comprising of control tank with 25 fishes and covered to prevent microalgal growth, and other exposed tanks with 10, 15, 20 and 25 fishes for treatment 1, 2, 3 and 4, respectively. Mean light intensity and temperature variation represent different weather conditions were logged every 30 minutes. Physico-chemical water parameter were measured using standard water quality techniques sampled every 2 days. Naturally occurring microalgae were sampled every 2 days for productivity (optical density, dry weight, total chlorophyll a & b, biomass, primary productivity, biochemical composition and fatty acid profile, diversity and species compositions) and toxicity of microalgae were determined. Catfish performances were determined for fish size (weight and length), survival rate, feed consumed record, and Feed Conversion Ratio (FCR). Correlation between fish stocking density and primary productivity, biomass productivity, diversity indices of mixed microalgae in different weather conditions was also evaluated. Mean±standard error and ranking score of all data were selected for data handling and interpretation. Weather conditions were represented by an average daily light intensity and temperature with ranged from 184.02-575.07 μmol m-2s-1 and 29.29-36.90 °C. Physico-chemical water parameters were as follows; temperature (28.63 ±0.24 to 31.33 ±0.2 °C), pH (5.64±0.31 to 8.26±0.7), Total dissolve solids (TDS) (168.33±6.86 to 433.91±12.38 mg/L), Electrical conductivity (EC) (0.34±0.01 to 0.87±0.02 mS/cm), water transparency (2.13±0.31 to 28±0.00 cm), Nitrate (NO3) (6.05±0.00 to 171.02±40.42 mg/L), Ammonium (NH4) (7.02±3.11 to 105.25±18.65 mg/L), Phosphate (PO4) (95.07±13.75 to 849.65±154.00 mg/L), Dissolved oxygen (DO) (2.62±0.05 to 6.49±0.10 mg/L), and Biological oxygen demand (BOD) (16.90±0.11 to 27.96±0.08 mg/L). Overall fish stocking density with water quality cumulative rank and score was the highest in treatment 3 (20 fish), 2(15 fish), 1 (10 fish) with 37 scores in dry weather conditions, treatment 1 (42 scores) in mix weather, and treatment 1(42 scores) in wet weather. Irrespective of the fish stocking density, the water quality in the mix weather conditions were the best, followed by wet weather conditions and the dry weather conditions with the worst water quality. Dry weight, optical density, total chlorophyll (a & b), biomass and primary productivity showed the highest value in treatment 4 (25 catfish) in dry weather conditions. There was positive correlation between different fish stocking density, primary productivity and microalgal biomass productivity. Proximate composition of naturally occurring microalgae contained 36% protein, followed by 29% total carbohydrate, 14% ash, 13% crude lipid, and 8% moisture. Fatty acid profile comprised of 58.82% saturated fatty acids with, followed by 23.53% monounsaturated fatty acid and 17.65% polyunsaturated fatty acid. Fish weight and length, Feed conversion ratio (FCR) of catfish was highest in control, followed by treatment 4. The survival rate ranged from 73.33- 80% in all treatments. A total of 29 genera and 77 species of microalgae from 5 divisions were identified. The most abundant taxa were Chlorophyta (90%). Shannon diversity index (H’), Simpson index, evenness, and species richness indices ranged from 0.59-2.82, 0.21-0.97, 0.52-1.04 and 2.01-5.36, respectively. The highest stocking density of catfish (25 catfish; T4) in dry weather conditions produced highest microalgal diversity indices. Pearson’s correlation analysis between fish stocking density and diversity indices showed strong positive values in all different weather conditions. Using probit analysis, the toxicity of naturally occurring microalgae have lethality concentration (LC50) of 458.97-866.70 ppm for all weather conditions and all fish stocking density. This study concluded that the weather conditions and fish stocking density have profound effect on physicochemical water quality, microalgae and fish productivity, proximate composition and fatty acid profile, species composition, diversity indices and toxicity of naturally occurring microalgae.

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