Nutritional Value and Risk Assessment of Toxic Heavy Metals of Selected Fish Species from Freshwater Ecosystems in Ethiopia
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Date
2019-09
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Addis Ababa University
Abstract
One of the traditional sources of animal protein of the developing world is through livestock
rearing. Unfortunately, the livestock production is under increasing pressure from the combined
effects of human population growth, shortage of grazing land and expantion desertification.
Therefore, it is important to look for fish, which is the cheap source of animal protein and
an important source of high quality protein as it contains large amounts of essential amino acids.
In addition, fish contains crude lipids, which supply the body with energy as well as omega-3 fatty
acids- the healthy fats that playa role in regulating blood cholesterol and supporting proper brain
functioning. Moreover, fish is rich in iron, zinc, magnesium, and copper. Despite this fact,
biochemical composition and toxic metal accumulation of different fish species from various
freshwater ecosystems had gained little emphasis in Ethiopia. Therefore, the present study was
conducted to evaluate the nutritional quality, health benefits and toxic heavy metal risk assessment
of selected fish species collected from four freshwater ecos ystems in Ethiopia.
In this study, a total of 80 (40 females and 40 males) fish samples of two freshwater fish species,
namely African catfish and Nile tilapia (C gariepinus and 0. niloticus) were collected from four
freshwater bodies of Alwero River, Abay River, Lake Hawassa and Lake Ziway found in Ethiopia.
The fish samples were transported to the Center for Food Science and Nutrition, Addis Ababa
University for further analysis. Proximate analysis of the fish samples was done by Association of
Official Analytical Chemists (AOAC) method. Minerals in fish samples were determined by
Atomic Absorption Spectrophotometry (AAS). Heavy metals were determined by inductively
coupled plasma optical emission spectrometry (ICP-OES) with following a microwave digestion
procedure.
The analysis of the proximate composition of both fish species (male and female) from all sampled
sites for maximum and minimum dry matter content for male and female were respectively 7.96%
and 11.76% (p < 0.05); 7.06% and 12.00% (p<0.05). Dry matter fish muscles content showed a
variation among fish species and sites offish collection in lakes and rivers. Even though sex has
no significant (p>0.05) effect on dry matter content, female fish had higher (7.06 - 12.00% dry
weight (DW) as compared to male fish (7.96 - 11.76%). The dry matter contents in the gill tissues
of 0. nilolicus and C gariepinus male fishes were in the range of 6.62 to 10.66% and for female
fishes were in the range of6.80 to 9.11 % in DW and sex has no significant (p>0.05) effect on dry
matter content of gill tissues from all sampled sites.The results on the proximate composition of 0. niioticlIs fish muscle from all sampled sites for the
maximum and minimum protein, fat, ash and carbohydrate contents were 15.22% and 12.44%
(p < 0.05); 2.76% and 2.15% (p > 0.05); 106% and 0.65% (p > 0.05); 5.47% and 1.55% (p< 0.05).
For C. gariepinlls maximum and minimum protein, fat, ash and carbohydrate contents were
20.23% and 10.75% (p< 0.05); 4.62% and 2.11% (p < 0.05); 1.55% and 109% (p > 0.05); 2.53%
and 0.28% (p< 0.05) , respectively.
The analysis of the proximate composition revealed that fat and ash content of the fish species
ranged from 2.69 to 4.60 and 0.65 to 152 g/IOOg wet weight (WW), respectively. The lowest
moisture was detected in C. gariepinlls from Lake Ziway (71.11 %) with high fat content (4.62 %).
The protein content ranged from 12.44 - 15.22% and 10.75 - 20.23% for 0. niloticlIs and C.
gariepinus from all fresh water bodies, respectively. The difference in fat content was the major
cause for the change in caloric values for C. gariepinus fish tissue ranged from a 6199 -12206
kcalllOOg (260.36-512.65 kJ) followed by 0. niloticus ranged from 69.75-82.16 kcal 1l00g
(292.95-345.07 kJ) from all fresh water bodies. Sexes were not significant (P > 0.05) influence in
terms of the four proximate compositions (protein, fat, ash and caloric values) measured in muscle
tissues of 0. niloticlIs and C. gariepinus collected from all sample sites.
Five essential mineral elements Fe, Ni, Cu, Mn, and Zn and seven toxic heavy metals (Hg, Sn, Pb,
As, B, Cd and Cr) were analyzed for each of the fish species in muscle and gill tissues from four
different freshwater bodies. Iron, zinc, copper, nickel and manganese contents were in the range
of33.54 to 122.29 mg/kg, 20.21 to 47.43 mg/kg, 0.75 to 10.87 mg/kg, 0.16 to 1.16 mg/kg and 2.48
to 69.82 mg/kg DW, respectively. The result showed that iron and z inc were the most abundant
minerals detected in gill and muscle tissues.
Toxic heavy metal concentration were assessed in gill and muscle tissues of C. gariepinlls and 0.
niloticlis collect from Alwero River, Abay River Lake Hawassa and Lake Ziway. In gill tissue, the
highest accumulated heavy metals were Cd (0.58 mg/kg DW) and Pb (4.56 mg/kg DW) and were
observed in 0. niloticlIs species in all water bodies. The highest concentrations of Sn (222.89
mg/kg DW) and Hg (14.52 mg/kg DW) were observed in gills and muscle of 0. niioticlIs from
Lake Ziway and Alwero River, respectively, while the highest concentrations ofCr (4.95 mg/kg
DW) and As (30.92 mg/kg DW) were observed in gills and muscle of C. gariepinlls from Lake
Ziway, respectively. A surprisingly too high As content was 30.92 mg/kg DW in muscle tissue ofC. gariepinus from Abay River and Hg content was 14.52 mglkg DW in muscle tissue of 0.
niloticlIs from Lake Ziway, may be due environmental contaminants from municipal, agricultural
and industrial sources may enter the food chain, accumulate in fish muscles. Those two fish species
from Abay River and Lake Ziway are limiting its consumption for infant and pregnant women.
However, the estimated daily intakes of all metals were below the acceptable daily intake (ADI)
and oral reference dose (RID) set by international guideline. The target hazardous quotient (THQ)
and hazard index (HI) values were «1) for all examined metals in both species in all sampled sites.
There are no risk for consumptions of 0. niloticlIs and C. gariepinlls muscle meat for consumers.
Therefore muscles of studied both fish species should be safe for utilization in human diet from
those regions in Ethiopia.