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Proximate Composition and Mineral Contents of Farmed and Wild Fish in Kenya

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The Kenyan government is promoting fish farming to boost fish supply and reduce rural poverty and malnutrition. This study was carried out to determine the differences between the chemical composition of wild and farmed fish particularly Nile tilapia (Oreochromis niloticus) and African Catfish (Claris gariepinus) species. Wild fish were obtained from City market Nairobi, while farmed fish were obtained from selected ponds in Sagana and Kamulu (Kenya). Proximate composition, mineral and heavy metal content of the fish were determined using standard methods of analysis. Farmed fish contained significantly higher moisture content than the wild fish. Protein content in wild fish ranged from 21.9 – 22.1 % compared to 16.0 – 19.2 % in farmed fish. Fat content in wild fish ranged from 3.0 – 3.8 % compared to 1.9 – 4.8 % in farmed fish and ash from 1.8 – 2.1 % in wild compared to 1.1 – 1.5 % in farmed fish. The concentration of minerals in wild fish ranged from 2.8 – 3.0 mg/100g of iron, 5.5 – 5.6 mg/100g zinc, and 39.9 – 43.8 mg/100g calcium compared to the lower values of 1.9 – 2.4 mg/100g of iron, 28.2 – 37.0 mg/100g calcium and 4.3 – 5.0 mg/100g zinc in farmed fish respectively. Unfortunately, the wild fish was found to contain significantly higher concentrations of heavy metals as compared to the farmed fish (P < 0.05). From the results, it was evident that wild fish were nutritionally superior to the farmed fish but, they also contained significantly higher levels of toxic heavy metals.
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Journal of Food Research; Vol. 9, No. 3; 2020
ISSN 1927-0887 E-ISSN 1927-0895
Published by Canadian Center of Science and Education
53
Proximate Composition and Mineral Contents of Farmed and Wild
Fish in Kenya
Jennifer K. Raymond1, Arnold N. Onyango1 & Christine A. Onyango1, 2
1School of Food and Nutrition Sciences, Jomo Kenyatta University of Agriculture and Technology, Kenya
2Taita Taveta University, Kenya
Correspondence: Jennifer K. Raymond, School of Food and Nutrition Sciences, Jomo Kenyatta University of
Agriculture and Technology, P.O. Box 62000, Nairobi, Kenya. Tel: 254-714-788-477. E-mail:
jennykeji@gmail.com
Received: March 17, 2020 Accepted: April 21, 2020 Online Published: May 17, 2020
doi:10.5539/jfr.v9n3p53 URL: https://doi.org/10.5539/jfr.v9n3p53
Abstract
The Kenyan government is promoting fish farming to boost fish supply and reduce rural poverty and
malnutrition. This study was carried out to determine the differences between the chemical composition of wild
and farmed fish particularly Nile tilapia (Oreochromis niloticus) and African Catfish (Claris gariepinus) species.
Wild fish were obtained from City market Nairobi, while farmed fish were obtained from selected ponds in
Sagana and Kamulu (Kenya). Proximate composition, mineral and heavy metal content of the fish were
determined using standard methods of analysis. Farmed fish contained significantly higher moisture content than
the wild fish. Protein content in wild fish ranged from 21.9 22.1 % compared to 16.0 19.2 % in farmed fish.
Fat content in wild fish ranged from 3.0 3.8 % compared to 1.9 4.8 % in farmed fish and ash from 1.8 2.1 %
in wild compared to 1.1 1.5 % in farmed fish. The concentration of minerals in wild fish ranged from 2.8 3.0
mg/100g of iron, 5.5 5.6 mg/100g zinc, and 39.9 43.8 mg/100g calcium compared to the lower values of 1.9
2.4 mg/100g of iron, 28.2 37.0 mg/100g calcium and 4.3 5.0 mg/100g zinc in farmed fish respectively.
Unfortunately, the wild fish was found to contain significantly higher concentrations of heavy metals as
compared to the farmed fish (P < 0.05). From the results, it was evident that wild fish were nutritionally superior
to the farmed fish but, they also contained significantly higher levels of toxic heavy metals.
Keywords: wild fish, farmed fish, proximate composition, fatty acids, tilapia, catfish
1. Introduction
Fish is a very important source of nutrients, including proteins, minerals, vitamins, and unsaturated essential
fatty acids (PUFAS) particularly omega-3. It plays an important role in decreasing the risk of coronary diseases,
mild hypertension and preventing certain types of arrhythmias (Lorente-Cebrián, Costa, Navas-Carretero, Zabala,
Martínez & Moreno-Aliaga, 2013). Both wild fish from natural water bodies and farmed fish are widely
consumed, and have been found to differ in their nutritional value and sensory characteristics (Fuentes,
Fernández-Segovia, Serra & Barat, 2010). Such differences have been mainly attributed to differences in feed
types and availability (Jonsson & Jonsson, 2014). Fish farming may be intensive, whereby all the required
nutrients are supplied through commercial feeds in sufficient amounts; or non-intensive, whereby only part of
the fish’s nutritional requirements are supplied through feeds, while the rest comes from organisms growing in
fish ponds. In Kenya, many of the fish farmers practice non-intensive fish farming, with variations in the types
and amounts of feeds supplied.
Pollution of the environment, especially the aquatic environment with heavy metals such as lead, copper, zinc,
mercury, chromium, and cadmium, has become a worldwide problem. These heavy metals have become normal
constituents of most of the water environment, including fish (Wu, Cobbina, Mao, Xu, Zhang & Yang, 2016).
Fish are known to concentrate heavy metals mostly in the liver or muscles (flesh), the latter being of most
concern to man because it is the main tissue consumed as food (Kaile & Nyirenda, 2016). Therefore, people
consuming large amounts of fish or aquatic foods from coastal areas associated with chemical industries are at
high risk of heavy metals poisoning (Ahmed, Baki, Islam, Kundu, Habibullah-Al-Mamun, Sarkar & Hossain,
2015). Farmed fish may also be exposed to heavy metals and other pollutants from the environment.
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This study was conducted to determine the nutrient and heavy metal contents of wild fish and farmed fish grown
under different non-intensive feeding regimes.
2. Materials and Methods
2.1 Study Area
Sampling was carried out in Sagana in Kirinyaga, Kamulu, and Nairobi (city market), all in Kenya.
2.2 Study Species
The study species were Tilapia (Oreochromis niloticus) and African Catfish (Clarias gariepinus) (Figure 1).
Figure 1. Pictures representing the study species of fish; (a) Oreochromis niloticus (Tilapia) (Source: CIRAD,
Baroiller, 2012) (b) Clarias gariepinus (African Catfish) (Source: GOA, 2016)
2.3 Sampling and Sample Collection
The wild fish and farmed fish were sampled as shown in Table 1. The sampling was done four times during the
study period, namely in January to April which represented the short rain season and April and May which
represented the long rain season. Eight wild fish each of different weights; small fish (between 0.4 1.5 Kg) and
big fish (>1.5 Kg) were randomly purchased from vendors in city market as size may also influence the
nutritional composition and heavy metal profile. This was done early in the morning when the fish had just
arrived from Lake Victoria, and the sampling was done once every week during the four months of the study. On
the other hand, farmed Tilapia and African catfish each of similar weight (1 Kg) were purchased randomly from
three farmers in Kirinyaga and Kamulu with different feeding systems (A, B, and C). Farm A feed consisted of
Omena (Rastrineobola argentea) meal, mixed with lake shrimps (Caridina nilotica) meal and maize flour; farm
B feed consisted of wheat bran mixed with cotton seed meal; and farm C feed consisted of chicken droplets and
maize flour. Two farmers representing each feeding system were chosen. Sampling was done once every week
during the four months of the sampling period. The wild and farmed fish were packed in cool boxes containing
ice at approximately 4 oC and transported to the department of Food Science and Technology laboratory at Jomo
Kenyatta University of Agriculture and Technology (JKUAT). They were then degutted, filleted and kept in
freezer at 4 oC for analysis.
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Table 1. Sampling of the wild fish from city market Nairobi and farmed fish
Wild fish
Month
Week 1
Week 2
Week 3
Week 4
1
Big
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
Small
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2
Big
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
Small
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
3
Big
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
Small
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
4
Big
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
Small
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
2tilapia+2catfish
1
A
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
B
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
C
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
2
A
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
B
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
C
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3
A
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
B
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
C
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
4
A
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
B
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
C
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
3tilapia+3catfish
2.4 Sample Preparation and Analysis
The fillet were minced and analyzed for moisture, ash contents, protein, lipid content, mineral composition and
heavy metal content.
2.4.1 Proximate Composition of Fish
Analyses of fish samples for moisture, ash, crude protein and lipid contents were carried out in triplicates
according to the standard AOAC methods (AOAC, 2006). Moisture content was determined using oven drying
method at 105 oC to constant weight whereas ash content was determined by incineration of the dried fish
sample in a muffle furnace at 550-600 oC for 6 hours (AOAC 920.153). Crude protein was determined by
estimating the nitrogen content (% protein = N x 6.25) of the fish samples using the micro-Kjeldahl procedure
(AOAC 928.08), while total fat was determined by Soxhlet extraction using methanol-chloroform solvent (2:1
v/v) (AOAC 991.36).
2.4.2 Mineral and Heavy Metals Determinations
For minerals analysis (Mg, Fe, Mn, Cu, Zn) the ash samples were digested with 2.5 ml HNO3 and 60%
perchloric acids according to AOAC method (2006). The digested samples were used for selected minerals
analysis, using atomic absorption spectrophotometer (Model A A-6200, Shimadzu, Corp., Kyoto, Japan). Heavy
metals (Hg, Pb, Cr, Cd) were also determined by AAS. Two grams from the ash sample were placed in a
digestion tube and pre-digested using10ml of HNO3 and 1ml of HClO3 acids were added and temperature
maintained at 135 oC until the liquor was colorless. The digested liquors were then filtered through a whatman 1
filter paper and diluted to 25ml with distilled water. Suitable standard solutions were prepared and their
absorbance measured to prepare a standard curve. The standard curve was used to calculate the concentration of
minerals.
2.5 Data Analysis
To determine compositional differences between fish from different sources, data were subjected to analysis of
variance (ANOVA) using the statistical analysis system SAS as outlined by Wahwua (1999) and McHugh (2011).
3. Results and Discussion
3.1 Proximate Composition
The proximate composition results of the wild and farmed fish are presented in Table 2. The moisture contents
ranged between 70.9 - 78.1 %. The farmed fish had significantly higher moisture content as compared with the
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wild fish (P < 0.05). The farmed tilapia had moisture content ranging between 75.2 78.1% whereas the wild
tilapia had the moisture content between 71.2 73.1%. The farmed catfish on the other hand, had moisture
content ranging from 73.8 - 77.3 % whereas the wild counterparts had moisture content ranging from 70.9 -
72.4 %. Therefore from the results, it is evident that both the farmed and wild tilapia fish had slightly higher
moisture content as compared to the catfish. Feeding systems also influenced moisture content, as fish from
system B and C had significantly higher moisture content than those from feeding system A. Moreover, it was
observed that the moisture content of the wild fish decreased with increasing body weight with the small catfish
and tilapia reporting 72.4 % and 73.1 %, respectively while the large catfish and tilapia having 70.9 % and 71.2 %,
respectively. Since feeding system A had higher dietary protein, the findings of the present study concur with other
studies which reported that in fish muscle, moisture and ash contents decrease with an increase in fats and protein
contents and vice versa (Mahboob, Al-Ghanim, Al-Balawi, Al-Misned, & Ahmed, 2019; Jim, Garamumhango &
Musara, 2017) and that increased water content is due to decreasing fat and protein contents in the fish body
(Mahboob et al., 2019; Jim et al., 2017). The lower moisture content in wild fish may additionally be attributed to
more physical efforts performed by the wild fish species to capture food organisms in the natural habitat than the
farmed fish which has plenty of food supply in the ponds (Jim et al., 2017).
The ash content of fish ranged between 1.2 - 2.1 % with the wild fish reporting significantly higher contents as
compared to the farmed fish (P < 0.05). The farmed tilapia and catfish had ash content ranging from 1.2 1.4 %
and 1.3 - 1.5 % respectively, whereas the wild counterparts had ash contents ranging from 1.8 1.9 % and 1.9 -
2.1 %, respectively. For the wild fish, big tilapia and catfish had significantly higher ash contents of 1.9 % and
2.1 % as compared to the small fish with values of 1.8 % and 1.9 %, respectively. However, there was no
significant difference between the ash content of the fish from the three feeding systems. These results concur
with the findings by Bhouri and colleagues (2010) which reported an increase in ash content in wild fish as
compared to the farmed fish. The observed range of ash content in this study indicated that the species is a good
source of minerals since ash is a measure of the mineral content of food item (Oladipo & Bankole, 2013).
Protein content ranged from 16.0 22.1 %. From the results, it was evident that wild fish had significantly
higher protein content (20.2 22.1%) than farmed fish (16.0 19.2 %). Farmed catfish had protein content
ranging from 16.3 - 19.2 % as compared to 21.0 - 22.1 % for the wild type. The farmed tilapia on the other hand,
had protein content ranging from 16.0 18.9%, while the wild counterparts had the content ranging from 20.2
22.0%. For the different feeding systems, feeding system A reported significantly higher contents of protein as
compared to the other two feeding systems B and C (P < 0.05). However, there was no significant difference
between feeding system B and C (P > 0.05). The high protein content in feeding system A may be attributed to
the diet which comprised of omena and lake shrimps which are richer in proteins. For the wild fish, the big
tilapia and catfish reported significantly higher protein content of 21.9 % and 22.1 % respectively as compared to
the small tilapia and catfish with protein values of 20.2 % and 20.8 %, respectively.
The fat content of the fish ranged from 1.9 4.9 %. The content of farmed tilapia and catfish ranged from 1.9
4.8% and 2.0 4.9 %, respectively, while for the wild tilapia and catfish ranged from 3.0 3.4% and 3.1 - 3.8 %,
respectively. There was no significant difference in the fat content of fish from feeding system A and wild fish.
On the other hand, feeding systems B had higher fat contents than farmed fish from feeding system A whereas
feeding systems C had lower fat contents than A. The higher fat content in feeding system B was expected as it
contained wheat bran and cotton seed which are rich in fats. For the wild fish, the big fish had slightly higher fat
content than small fish.
The results of this study concurs with the findings of a study by Adebayo, Fapohunda, & Ajibade, (2016) which
reported that catfish contain slightly higher ash, fat and protein contents than the tilapia. In addition, it confirms
that proximate composition of farmed fish is directly influenced by their diet. According to Fuentes et al. (2010)
and Rani, Kumar, Rao, & Shameem, (2016), the variation in proximate composition of fish is as a result of
differences in nutrition, living area, fish size, catching season, seasonal and sexual variations as well as other
environmental conditions. Furthermore, the results indicate that the wild fish is richer in most of the nutrients
than the farmed fish from the three non-intensive feeding systems used by farmers in Kenya. The results
corroborate the findings of other studies which compared the composition of wild and farmed fish (Job, Antai,
Inyang-Etoh, Otogo, & Ezekiel, 2015). According to these studies, the proximate composition differed based on
the type of food and habitat of the fish.
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Table 2. Proximate composition
Feeding system
Sample size
Percentage proximate composition
Moisture
Ash
Fat
Protein
Farm
A
24
73.8b±0.62
1.5c±0.09
3.5b±0.09
19.2c±0.65
40
75.2b±1.12
1.4c±0.21
3.4b±0.39
18.9c±1.44
B
24
76.1a±1.01
1.4c±0.02
4.9a±0.17
16.9d±1.29
40
76.3a±1.72
1.3c±0.06
4.8a±0.25
16.6d±0.92
C
24
77.3a±1.19
1.3c±0.12
2.0c±0.58
16.3d±2.03
40
78.1a±0.92
1.2c±0.16
1.9c±0.17
16.0d±1.76
Wild
Big fish
16
70.9c±1.74
2.1a±0.19
3.8b±0.33
22.1a±0.96
16
71.2c±1.06
1.9a±0.17
3.4b±0.37
21.9a±1.57
Small fish
16
72.4c±1.14
1.9b±0.30
3.1b±0.26
21.0b±1.07
16
73.1c±0.38
1.8b±0.21
3.0b±0.15
20.2b±1.48
* Values are given as means ± Std dev. Means with different superscript letters within a column are significantly
different (P < 0.05).
3.2 Mineral Composition
The results for the mineral composition of the two fish are shown in Table 3. All the fish samples examined
contained appreciable concentrations of calcium, zinc, magnesium, phosphorus and iron suggesting that these
fish could be used as good sources of minerals. The data revealed wide variations in the mineral contents of the
wild and farmed fish. The concentrations ranged from 1.9 - 3.0 mg/100g for iron, 4.3 - 5.6 mg/100g for zinc,
21.2 - 30.9 mg/100g for phosphorus, 28.2 - 46.8 mg/100g for calcium, 106.1 - 137.2 mg/100g for magnesium
and 0.4 - 0.7 mg/100g for copper.
Wild fish were found to contain significantly higher concentrations of all the minerals than the farmed fish (P <
0.05). However, there was no significant difference between the mineral contents of the big and small fish from
the wild habitats. On the other hand, there were no significant differences in Mg, P, and Zn in fish from feeding
systems A, B and C. Although the mineral content of big catfish was slightly higher than that of big tilapia, the
opposite was true for the small catfish and tilapia. In addition, there was no significant difference between the
tilapia and catfish.
Magnesium content ranged from 129.2 - 137.2 mg/100g for wild fish and 106.1 - 126.0 mg/100g for the farmed
fish. The copper content of the wild fish was significantly higher with values ranging from 0.6 0.7 mg/100g as
compared to the farmed fish with ranges of 0.4 0.5 mg/100g. For the farmed fish, feeding system A gave higher
contents of calcium as compared to other feeding systems, while tilapia from system B had lower iron levels than
the rest. Tilapia fish in the farms A and C contained slightly higher mineral content compared to catfish. This is
contrary to farm B where tilapia reported slightly lower levels of minerals than the catfish. The reason for such
differences in mineral uptake by catfish and tilapia in different feeding systems is not clear.
Generally, similar ranges of mineral contents reported in this study have been reported by FAO (2010) and
USDA (2010) permissible limits for fish. The findings of this study are comparable with the finding of Adebayo
et al. (2016) which reported calcium (6 825 mg/100g), zinc (1 12 mg/100g) and phosphorous (10 82
mg/100g). In contrast the magnesium content from this study was higher than the contents 4 12 mg/100g
reported by Adebayo et al. (2016). On the other hand, iron content from the study was lower as compared to the
3 102 mg/100g observed by Adebayo et al. (2016). The variations recorded in the concentration of mineral in
fish examined could be as a result of the rate in which they are available in the water body. Alasalvar, Taylor,
Zubcov, Shahidi, & Alexis, (2002) and El-Zaeem, El-Tawil, & Amer, (2012) reported that mineral concentration
of fish is affected by parameters such as feed type, level of dietary intake and growth. According to
PfenningKurth, Meißner, Standke, Hoppe, Zieschang, & Gutzeit, (2011), the wild fish feed on a wide diversity of
microscopic organisms and macrophytes which may be lacking in controlled farmed systems. Therefore the feed
composition of the farmed fish may also be major factor influencing their mineral content as reported by
El-zaeem et al. (2012).
The minerals analyzed in this study are known to be important in human health. Therefore the high concentration
of minerals in the wild fish is advantageous since they are known to intervene in therapeutic aspects. Calcium is
required as a component of the human diet, and it is essential for the full activity of many enzymes, such as nitric
oxide synthase, protein phosphatases, and adenylate kinase. It is also necessary to maintain an optimal bone
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development (Beto, 2015). Besides, calcium is also good for growth and maintenance of bones, teeth and muscles
(Pravina, Sayaji, & Avinash, 2013). Normal extra cellular calcium concentrations are necessary for blood
coagulation and for the integrity, intracellular cement substances (Mohanty et al., 2019).
Magnesium is an essential mineral for cell function as it acts as a co-factor of pyruvate dehydrogenase, an
enzyme which transforms pyruvate into acetyl-CoA used in the citric acid cycle to carry out cellular respiration
to release energy. The recommended daily allowance (RDA) for Mg is 2000 mg per day for a healthy adult
(Lenntech, 2013). Phosphorus is also an important mineral as it has been reported to form the structure of teeth,
bones and cell membranes (Butusov & Jernelöv, 2013). It also acts as a cofactor for many enzymes and activates
the vitamin B complex.
Other elements such as zinc and iron varied in concentration among all the fish studied. These elements are
equally important in trace amounts as observed, but they tend to become harmful when their concentrations in
the tissues exceed the metabolic demands (Adebayo et al., 2016). Zinc is an essential element in human diet as it
plays an important role in maintenance of normal glucose tolerance and in the release of insulin from beta cells
of islets of Langerhans (Piero, Njagi, Kibiti, Ngeranwa, Njagi, & Miriti, 2012; Praveeena, PaSula, & Sameera,
2013). Therefore the availability of zinc in fish could mean that the fishes can play valuable roles in the
management of diabetes, which result from insulin malfunction. In addition, it is involved in most metabolic
pathways in animals and humans (FAO, 2010).
Iron on the other hand, is important for metabolic reactions and the regulation of cell growth and differentiation.
It is an essential trace element for haemoglobin formation, normal functioning of the central nervous system and
in the oxidation of carbohydrates, protein and fats. Iron is important for children, women of reproductive age and
pregnant women since they are most vulnerable to micronutrient deficiency and anemia (WHO, 2015). Iron
deficiency occurs when the demand for iron is high, particularly in growth, high menstrual loss and pregnancy
and the intake is quantitatively inadequate for or contains elements that render them unavailable for absorption
(Kumaran et al., 2012). Besides, iron acts as a cofactor in catalase, an enzyme that catalyzes the conversion of
hydrogen peroxide to water and oxygen (Soetan, Olaiya, & Oyewole, 2010). Iron and zinc are also antioxidant
micronutrients and their presence could boost the immune system (Prashanth, Kattapagari, Chitturi, Baddam, &
Prasad, 2015). Since deficiencies in calcium, iron and zinc are common in the developing world, and are the
leading cause of many ailments (Mohanty, Sankar, Ganguly, Mahanty, Anandan, Chakraborty, & Asha, 2016) fish
eating is encouraged as this may contribute to alleviation of this problem in the vulnerable groups.
Table 3. Mineral composition of farmed and wild fish
Feeding system
Fish type
Sample
size (N)
Mineral composition of fish sample (mg/100g)
Calcium
Iron
Magnesium
Phosphorus
Zinc
Copper
Farm
A
Catfish
24
35.0b±2.0
2.4b±0.1
117.2b±2.1
25.0b±1.8
5.0b±0.2
0.48b±0.07
Tilapia
40
37.0b±1.0
2.4b±0.2
124.2b±1.3
25.3b±1.1
4.4b±0.5
0.44b±0.01
B
Catfish
24
34.1b±1.3
2.3b±0.1
126.0b±1.9
24.2b±1.8
4.8b±0.4
0.39c±0.04
Tilapia
40
31.5c±1.8
1.9c±0.1
112.1b±2.7
21.2b±0.4
4.6b±0.6
0.42b±0.02
C
Catfish
24
28.2c±2.0
2.2b±0.2
106.1b±7.2
23.2b±0.4
4.3b±0.5
0.35c±0.07
Tilapia
40
30.2c±1.2
2.2b±0.3
118.2b±2.3
24.4b±0.9
4.4b±0.3
0.44b±0.03
Wild
Big
fish
Catfish
16
43.8a±2.2
3.0a±0.6
134.2a±4.3
29.5a±1.5
5.5a±0.8
0.59a±0.02
Tilapia
16
39.9a±2.2
2.8a±0.2
129.2a±7.1
28.3a±1.1
5.4a±0.8
0.55a±0.09
Small
fish
Catfish
16
42.4a±1.1
2.8a±0.4
131.3a±3.9
30.4a±2.1
5.3a±0.9
0.62a±0.08
Tilapia
16
46.8a±1.9
3.0a±0.1
137.2a±5.1
30.9a±1.9
5.6a±0.6
0.69a±0.04
* Values are given as means ± Std dev. Means with different superscript letters within a column are significantly different (P < 0.05).
3.3 Heavy Metal Content
The results for the heavy metal composition are presented in Table 4. The concentrations ranged from 0.1 - 0.3
mg/100g for chromium, 0.3 - 0.7 mg/100g for lead, 0.2 - 0.4 mg/100g for cadmium and 11.9 - 17.6 μg/100g for
mercury. The heavy metal content of the wild fish was significantly higher with values ranging from 0.24 0.30
mg/100g chromium, 0.5 0.7 mg/100g lead, 0.3 0.4 mg/100g cadmium and 16.2 18.3 μg/100g mercury as
compared to the farmed fish with ranges of 0.13 0.23 mg/100g chromium, 0.3 0.4 mg/100g lead, 0.2 0.3
mg/100g cadmium and 11.9 15.4 μg/100g mercury. Although there was no significant difference between the
values of the small and big fish from wild habitat, the big catfish reported slightly higher heavy metal contents as
compared to the big tilapia. This was contrary to the small catfish which reported slightly lower heavy metal
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59
levels than the small tilapia fish. The high accumulation of heavy metals in wild fish as compared to farmed fish
depends on its amount in water and sediments which act as the most important reservoir or sink of metals and
other pollutants in the aquatic environment (Gupta, Rai, Pandey, & Sharma, 2009; González-Fernández,
Garrido-Pérez, Nebot-Sanz, & Sales-Márquez, 2011). Heavy metal contamination in sediment can affect the
water quality and bioaccumulation of metals in aquatic organisms, resulting in potential long-term implication on
human health and ecosystem (González-Fernández et al., 2011).
For the farmed fish, feeding system A was significantly higher in heavy metal content as compared to the other
feeding systems. On the other hand, farm B reported significantly lower heavy metal content. The tilapia fish
from farm A and C reported slightly higher levels of heavy metals particularly chromium and lead whereas the
tilapia fish from farm B reported lower levels. Heavy metals in system A may be at least partly derived from the
R. argentea and C. nilotica components of the feed.
Lead had a higher content as compared to the other heavy metals whereas mercury reported lower levels.
According to WHO (2011), the maximum allowable concentration for Pb and Cd were 200, 50 and 10000 ppb,
respectively. However, such food limits are not defined to all the elements (Agah, Leermakers, Elskens, Fatemi,
& Baeyens, 2009). The element levels of fish muscles in this study were below the maximum allowable
concentration suggested by WHO (2011) and have no threat to public health. Although, fish is the main source of
mercury in human diet (Mania, Wojciechowska-Mazurek, Starska, Rebeniak, & Postupolski, 2012), mercury was
found to be the least accumulating metal during this study. The present study agrees with the results obtained by
Bosch, O'Neill, Sigge, Kerwath, & Hoffman, (2016) which found out that mercury is least accumulated in fish
and is high in wild fish as compared to farmed fish. According to Perugini, Visciano, Manera, Zaccaroni, Olivieri,
& Amorena, (2014) the concentration of heavy metal contaminants in fish is strongly influenced by fish age,
origin of the fish, its species and pH and temperature of water. It is also related to the tissue sampled, the season
of harvest especially for farmed fish, and the composition of the diet (Hussain, Muhammad, Malik, Khan, &
Farooq, 2014). The health and safety qualities of fish is an advantage to the fish farming industry owing to the
fact that, unlike fishermen, fish farmers can control for the presence of toxic contaminants and pathogens in their
fish throughout the production process. Whereas the diet of wild fish is totally beyond human control, the
development of formulated diets, which are used in aquaculture, makes it possible to directly control
contaminant levels (Craig, Helfrich, Kuhn, & Schwarz, 2017; Davidson, Barrows, Kenney, Good, Schroyer, &
Summerfelt, 2016).
Table 4. Heavy metal composition of farmed and wild fish
Feeding
Fish
Sample size
Chromium
Lead
Cadmium
Mercury
Farm
A
Catfish
24
0.20b±0.05
0.39b±0.06
0.28b±0.04
15.2b±1.7
Tilapia
40
0.23a±0.01
0.42b±0.05
0.23c±0 .02
15.4b±1.7
B
Catfish
24
0.17c±0.06
0.36b±0.01
0.24c±0.02
11.9c±1.0
Tilapia
40
0.13c±0.02
0.31c±0.04
0.21c±0.06
13.5c±1.1
C
Catfish
24
0.14c±0.05
0.34c±0.05
0.26b±0.02
12.6b±0.7
Tilapia
40
0.19b±0.09
0.37b±0.09
0.27b±0.01
14.8b±0.9
Wild
Big fish
Catfish
16
0.27a±0.07
0.58a±0.04
0.34a±0.05
17.6a±1.5
Tilapia
16
0.24a±0.07
0.52a±0.03
0.33a±0.04
18.3a±2.5
Small fish
Catfish
16
0.26a±0.03
0.61a±0.02
0.39a±0.06
16.2a±1.5
Tilapia
16
0.30a±0.08
0.66a±0.07
0.44a±0.07
17.3a±1.6
* Values are given as means ± Std dev. Means with different superscript letters within a column are significantly
different (P < 0.05). The concentration is expressed as mg/100g except for mercury which is expressed in
μg/100g
4. Conclusion
The findings from this study reaffirm that fish are nutritious and can provide important nutrients needed for
normal body function and maintenance. The attributes of nutritional value differed significantly between wild
and farmed fish. It is important to note that wild fish is rich in most nutrients as compared to fish farmed under
the three non-intensive feeding systems studied. Although the wild fish had higher heavy metals as compared to
farmed fish, the values were below toxic levels. For farmed fish, the nutritional quality is dependent on the
feeding practices. Thus, manipulating the feeding systems might deliver designer fish having preferred quality
and nutritional compositions.
http://jfr.ccsenet.org Journal of Food Research Vol. 9, No. 3; 2020
60
Acknowledgement
The authors would also like to acknowledge Fish farmers and School of Food and Nutrition Science of the Jomo
Kenyatta University of Agriculture and Technology.
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