Globally, fish and shell fish account for about 16% of animal protein consumed, while consumption for the same commodity for the rest of Africa is estimated at 7.1Kg/caput/annum14. Between 1986 and 1998, these categories of food items contributed about 29-35% of the animal protein in the diets of people in the West African sub-region. In Nigeria, fish constitute 40% of protein intake16 where per capita fish consumption has been estimated at 8.8 kg/caput/annum6. So many reasons have led to the increasing importance of fish as dietary requirement. These reasons include easy digestibility, and the increased health benefits of consuming fish in place of red meat. Fish consumption has been shown to have anti-cancerous effects and minimizes the risks of cardiovascular ailments in humans13. This acknowledged characteristic of fish in the diet of humans is mainly due to its proximate composition.

Proximate composition is traditionally used as an indicator of nutritional value of food materials231921 and this has shown that the principal components of fish are water, ash, protein, lipids and minute quantities of carbohydrates9. Other constituents also include vitamins and minerals15. There may be considerable variations in fish tissue concentrations of these components within species, sex, and sexual condition209. These components have also been shown to fluctuate even amongst fishes of different ecological systems209. Fish consumed in Nigeria are sourced from marine, freshwater ecosystems and farmed fish. However, ethnocentric biases for fish from a particular ecosystem have influenced the choice of fish for food in some parts of Nigeria (personal observation). Thus, the objective of this study is to establish a data base for:

Those who are interested in the use of these species for fish to enrich food protein intake and for comparing the proximate composition of wild and farmed fish.

The fish samples used in this study were two fresh water species namely Alestes nurse and Oreochromis gallilaeus purchased in lot from artisan fishers at Oguta Lake, Imo State, Nigeria. The other two marine species namely Scomberomorus tritor and Pseudotolithus elongatus were bought from a local cold room in Umuahia, Abia State, Nigeria. All fish samples were transported cold in an ice chest and stored in frozen condition until they were analyzed. Before analysis, samples were thawed at room temperature of 25-28⁰C.

Sample Preparation

In the laboratory, fish samples were weighed to the nearest 0.1gm on a metler balance. Their total length (to the nearest 0.1mm) was also obtained from a measuring board. The fish samples were cleaned, de-scaled, eviscerated and cut length–wise along the vertebral column. The head, bones and visceral parts were discarded. To obtain fish fillets, each fish was cut along its full length. Each of the four fish species was replicated three times to yield a total of 12 samples. All samples were blended into a smooth paste in a 3.8 L kitchen-type blender (Warning Products, New Hartford, CT) which was thoroughly cleaned and dried between samples.

Chemical Analysis

Fresh blends of fish paste were analyzed for proximate composition. Moisture, crude protein, crude lipid and ash contents were determined according to AOAC2. Carbohydrate was determined by difference17. Metabolizable energy (ME) was calculated using equation (1), while Nitrogen Free Extract (NFE) was calculated using equation (2) below.

ME = 0.0226CP+0.0407EE+0.0192CF+0.0177NFE......1

NFE = %DM – (%Ash + % CF+CP+%EE)..……2

Where, MF = Metabolizable energy (MJ/kgDM)

NFE = Nitrogen Free Extract (g/kg)

CP = Crude Protein (g/kg)

EE = Ether extract (g/kg)

CF = Crude fiber (g/kg)

DM = Dry matter

Statistical Analysis

All data were presented as means ± standard error. The SPSS software (version 11.5) was used for statistical analysis. Comparisons among sampling were made by one-way analysis of variance (ANOVA), at 5% confidence level using Duncan’s multiple range tests.

The proximate composition of two freshwater fish species (A. nurse and O. gallilaeus) and two brackish water fish species (S. tritor and P. enlongatus) is presented in Table 1. Percentage moisture content varied between 70.63± 0.57% and 72.26±0.20% in freshwater fish species. In brackish water species moisture content varied between 69.52±0.10% and 72.51±0.20%. The ether extract in Alestes nurse and Oreochromis gallilaeus varied from 3.22±1.0 to 3.41±0.20% respectively and in S. tritor and P. enlongatus this parameter varied from 1.31±1.0 to 2.95% respectively. There were significant differences (P≤0.05) in the mean percentage of both moisture and ether extract in the species of fish under study.

Table 1: Nutritional composition of two freshwater and two brackish water fish species.

Data are means of triplicate determinations. ± is Standard deviation, Means with different sub-scripts within rows are significantly different (P≤0.05), NS= Not Significant.

Carbohydrate content in S. tritor was 3.51±0.10% and was significantly higher (P≤0.05) than values of 2.49±0.10% for P. elongatus and 2.47±0.40% for O. gallilaeus. Ash content in A. nurse and O. gallilaeus was 2.20±0.2% and 2.47±0.40% respectively while in S. tritor and P. elongatus ash content was 2.38±0.3% and 2.5±0.2% and was significantly different (P≤0.05).

Crude protein levels in freshwater fish species ranged from 20.26±2.00% in A. nurse to 21.52±0.10% in O. gallilaeus while in brackish water species this parameter ranged from 21.52±0.10% in S. tritor to 22.86±1.00% in P. elongatus. Crude fiber content in A. nurse and O. gallilaeus were 0.93±0.10% and 0.86±0.10% respectively while in S. tritor and P. elongatus it was 0.76±0.01% and 0.80±0.01% respectively.

The energy values for the individual fish species under study is presented in Table 2. The percentage Nitrogen Free Extract (NFE) was 0.67±0.20 in A. nurse and 2.47±0.40 in O. gallilaeus. The percentage value for this parameter was 2.75±0.01 in S. tritor and 1.14±0.0 in P. elongatus. The total energy content in the fish species under study also followed the same trend as NFE. There was no significant difference (P≤0.05) in the metabolizable energy content of the samples. Although S. tritor had the highest value of 0.669 mg/kg DM it was not significantly different. However, the total energy content of the fish species varied significantly (P≤0.05).

Table 2: Energy values of two freshwater and two brackish water fish species.

Data are means of triplicate determinations. ± is Standard deviation, Means with different sub-scripts within rows are significantly different (P≤0.05), NS= Not Significant.

The results for moisture content and ether extract obtained in this study corroborated with the literature reports of similar studies182221. The variation in moisture level amongst various fish species as observed in this study agrees with earlier report. Clucas3 reported that monthly variation in moisture and fat content varied 57.07-74.0% and 7-24% respectively in the same species within a year. Lipid levels in the studied species of fish were below 5% and as such these fish species can be said to be lean fish11. The result of this study is in compliance with the general observation that relatively lower lipid levels are expected in brackish water fishes which is primarily attributable to periods of starvation during migration201811.

Carbohydrates content for the studied fish species are relatively higher than 0.5% expected for typical striated fish muscles9. Carbohydrate levels in brackish water species were significantly superior (P≤0.05) than their counter parts from fresh water. It stands to reason that the levels of fat are relatively low in all the fishes the relatively high carbohydrates in brackish water species are requirements for prolonged periods of starvation during migration.

Ash contents in the different species of fish are significantly different (P≤0.05) between fish species of the same and fish species of different habitats. This observation is in tandem with earlier research reports1021. Dale et.al45 reported an inverse relationship between protein and ash levels in Tilapia meal but this was not confirmed in this study. The values for protein recorded for the various species in this study were in agreement with some earlier reports127. There was no significant difference (P≤0.05) in protein levels within fish species and within species of different habitats. The percentage total energy in both fishes from fresh and brackish waters values indicate significant differences (P≤0.05) but such differences are not habitat induced.

This study was primarily concerned with the chemical composition of fillets of two species of fish each from freshwater and brackish water habitats with a view to determining if there are any habitat imposed differences in their composition. The nutritional composition of all the fish species obtained from both fresh and brackish water environments were comparable to literature values for the components studied. The observed differences in nutritional composition amongst fish species can be categorized into:

Parameters for which there are significant differences amongst species and for which such differences are traceable to differences in habitat e. g. moisture, ether extract, and carbohydrate content.

These results did not confer any superiority in quality of fish from any of the habitats from which they were obtained. It also indicated that the species under study are low-fat, high-protein fish species that can be recommended in diets to enrich food protein intake. Fresh water fishes being higher in oil content could contribute to healthy nutrition and can be explored for fish oils and formulation of fat–based vitamins.