Extraction and Characterization of Oils of the Different Genera of Fishes From the Freshwater of Lower Niger River

Abstract

Investigations were carried out on the oils and fishmeal of fishes collected from the Onitsha regions of the lower Niger river for possible nutritional and industrial applications. A total of 27 species belonging to 25 different genera were collected. From the results obtained, the highest moisture content was recorded for Electricus membranicus with a value of 88.03% while the least value of 59.93% was recorded for Hypeopisus bebe. Extraction of oil and percentage oil yield analysis showed that high percentage oil yields for the fresh and dry fishes of 8.19 and 22.80 were obtained for Hyperopisus bebe and Schilbe mystus respectively while low values for both the fresh and dry samples of 0.21 and 1.05 respectively were obtained for Tilapia zilli. The difference in means for the percentage oil content between the fresh and dry samples was statistically significant (P<0.05). Based on the percentage oil yield of the fresh samples, they were classified into lean, low, medium and high fat fishes. The lean fishes constituted 33.33%, low fat fishes 40. 70%, medium fat fishes 22.22% while high fat fish constituted 3.70% of the total number of fishes sampled. Good yields of fishmeal for the fresh and dry samples of 27.91% and 94.10% were obtained for Papyrocranus afer and Chrysichthys nigrodigitatus while the lower yields of 10.51% and 56.95% were obtained for Electricus membranicus and Hyperopisus bebe respectively. The difference in means for the percentage yield of fishmeal for the fresh and dry samples was statistically significant (P<0.05). The highest values for the cholesterol content in the oils, fresh and dry fishes were 113.00 mg /g of oil, 587.73 mg /100g of fresh fish and 1880.01mg /100g of dry fish recorded for Heterotis niloticus, Brycinus brevis and Brycinus brevis while the least values were 9.50mg /g of oil, 9.41mg/100g of fresh fish and 48.20mg/100g of dry fish recorded for Labeo senegalensis, Tilapia zilli and Tilapia zilli respectively. The difference in means for cholesterol contents in the fresh and dry fishes was statistically significant (P<0.05). The fatty acid analysis showed that the predominant fatty acids in the oils of the fishes of the lower Niger river were linoleic, palmitic, oleic, linolenic and palmitoleic acids. Stearic, docosahexaenoic, eicosapentaenoic, myrisic and lauric acids were present in moderate amounts while myristoleic, arachidonic, capric and caprylic acids were present in very low levels. The nutritionally important omega-3 fatty acids EPA and DHA were each present in 14 samples but predominantly in Protopterus senegalus, Heterobranchus bidorsalis and Mormyrus macrophthalamus at levels of 6.90, 6.60 and 0.97 g /100g fish oil for EPA while DHA was predominantly found in Synodontis clarias, Tilapia zilli and Chrysichthys nigrodigitatus at levels of 5.45, 2.91 and 2.71 g/100g of fish oil respectively. High values for n-6 and n-3 fatty acids for the fish oils were 59.24% and 42.36% obtained from Brycinus brevis and Polypterus senegalus senegalus while the least values were 10.73% and 4.51% obtained from Polypterus senegalus senegalus and Chrysicththys nigrodigitatus respectively. A high value for percentage C-20 + C -22 n -3 fatty acids for the oils was 6.90, recorded for Polypterus senegalus senegalus while 0.37 and 1.21 were recorded for Synodontis clarias for the fresh and dry fishes respectively. However, low values of 0.07 and 0.01 were obtained for Protopterus annectus and Clarias ebiriensis for the oils and dry fishes respectively. Hydrocynus forskalii, Distichodus rostratus, Citharinus citharus, Parachanna obscura, Tilapia zilli, Dagetichtys lakdoensis, Mormyrops anguilloides and Lates niloticus all had a low value of 0.01 for the fresh fishes. The n-3/n-6 is an expression of the levels of n-3 fatty acids against the levels of n-6 fatty acids. A higher value of 3.95 was obtained for Polypterus senegalus senegalus while a lower value of 0.12 was obtained for Brycinus brevis for the oils, fresh and dry fishes. The expression C-20 + C – 22 n – 3/n -6 is more indicative of the value of fish oils and a good value of 0.64 was obtained for Polypterus senegalus while the least value of 0.002 was recorded for Clarias ebiriensis for the oils, fresh and dry fishes respectively. The n-3/n-6 and the C-20 + C-22n-3/n-6 values obtained for the fishes of the Onitsha regions of the lower Niger river fall within the range of values obtained for the temperate fishes although some temperate fishes have very high values. The study therefore indicates that the oils of some of the fishes of the lower Niger river could be good for the control of cardiovascular diseases and production of nutraceuticals or pharmaceuticals while the fishmeal could serve as raw material for the production of animal feeds.