Effect of Feed Supplementation with Seaweed and Use of Novel Membrane Bioreactor Wastewater Treatment Method on the Quality of Farmed Nile Tilapia

Abstract

The Kenyan aquaculture sector has a great potential of expansion and with a growing population, it can play an important role towards achievement of food and nutrition security and promoting sustainable aquaculture production in Kenya. There has been an increase in the demand for fish and fish products given the fact that fish is a rich source of protein and essential fatty acids. With the growth in aquaculture, there has been an increase in the need for quality feeds to produce farmed fish of high nutritional value and also access to adequate water resource due to competition from other water uses. In order to achieve improved output from fish farming, a seaweed-based fish feed was formulated and municipal wastewater was treated using the self-cleaning membrane bioreactor (MBR) and used in rearing the Nile tilapia (Oreochromis niloticus). The brown seaweed Sargassum portieranum was identified, based on literature review, and used in supplementation of tilapia fish feeds to improve the fatty acid profile of the fish. For the feed supplementation, three experimental diets were prepared at 0% (control), 5% and 10% (dry weight basis) inclusion levels of the seaweed. One hundred and eighty Nile tilapia were randomly distributed into three groups in triplicate and fed the experimental diets for 12 weeks. In a separate experiment, to assess the effect of water quality on fish growth and muscle quality, three water treatments were used; a) MBR treated water, b) maturation pond water and c) tap water (control). Weight and length gain were measured after every two weeks throughout the experimental period. The biochemical composition was determined using standard methods of the Association of Official Analytical Chemists (AOAC) while all microbial safety analysis was done according to standardized ISO methods. The concentration of minerals and heavy metals in the water and fish muscles were analyzed using atomic absorption spectroscopy (AAS) while the fatty acid profile was determined using Gas chromatography–mass spectrometry (GC-MS). In addition, the physicochemical properties (pH, dissolved oxygen, conductivity, biological oxygen demand, chemical oxygen demand, nitrates and ammonia) of the water were determined. Analysis of variance (ANOVA) was used to establish significant difference in the data using R, version 4.0.2 Software. Based on the review on the nutritional properties of seaweeds, the brown seaweed species was identified to contain the highest content of total lipids compared to the red and green seaweed species. After feeding the tilapia fish for 12 weeks, the final body weight differed significantly (P < 0.05) between the fish that was fed on seaweed diet supplemented at 10% (66.12 ± 2.24 g) and un-supplemented diet (59.19 ± 1.03 g). The weight gain and length gain in the fish that was supplemented with seaweed at 5% and 10%, ranged between 28.08 to 35.00 g and 3.13-3.87 cm respectively. The highest total lipid content was in the fish diet that was supplemented at 10% (0.93%). The fatty acids palmitic acid (20.33-21.91%), linoleic acid (24.65-37.34%) and docosahexaenoic acid (19.51-26.16%) were the predominant saturated, omega-6 and omega-3 fatty acids respectively in the fish muscle. Although the biological oxygen demand, chemical oxygen demand, pH and nitrates values of the maturation pond water were significantly higher than those of the MBR water, both water qualities met the recommended standards for water for use in aquaculture as set by Food and Agriculture Organization (FAO). However, the heavy metal content (Cu, Pb and Cr) in the maturation pond and MBR water exceeded the permissible levels for discharge in the environment as per the National Environment Management Authority (NEMA) standards (fifth schedule for effluent discharge in the environment). Besides, the survival rate of the fish was significantly affected (P = 0.001) by the water quality; with the control at 95.56%, MBR, 86.67% and maturation pond, 76.67%. Likewise, the crude protein, fibre and lipid varied significantly in the three treatments. The heavy metal content in the fish reared in the maturation pond and MBR were above the safe levels for human consumption. The findings of this study also demonstrated that apart from the water used in rearing fish, feeds can also be a source of contaminants in fish. In conclusion, the present study revealed that the brown seaweed has the potential to improve the fatty acid profile of fish as an alternative supplemental lipid source. At low inclusion levels, the brown seaweed is beneficial to fish growth and overall nutritional quality of final product. In addition, the present study provided evidence that MBR wastewater is a promising technology in treating wastewater for alternative uses. It effectively treats to meet most of the standards for water used in aquaculture. It is recommended that, an addition step like reverse osmosis be added to upgrade the water quality to remove heavy metals.