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Research Journal of Environmental Toxicology 10 (1): 82-87, 2016
ISSN 1819-3420 / DOI: 10.3923/rjet.2016.82.87
© 2016 Academic Journals Inc.
Determination of Heavy Metal Levels in Some Commonly Consumed
Frozen Fish in Ibadan, Southwest, Nigeria
1O.K. Kareem, 1O. Orisasona and 2A.N. Olanrewaju
1Department of Aquaculture and Fisheries Management, University of Ibadan, Ibadan, Nigeria
2Federal College of Freshwater Fisheries Technology, P.M.B 1060, Maiduguri, Nigeria
Corresponding Author: O.K. Kareem,
Department of Aquaculture and Fisheries Management, University of Ibadan, Ibadan,
Nigeria
ABSTRACT
There is a growing concern on the safety of frozen fish imported from various parts of the world
where the levels of contamination of water bodies may not be well ascertained and regulatory
measures not strictly adhered to. Concentrations of four heavy metals (Lead (Pb), Cadmium (Cd),
Copper (Cu) and Mercury (Hg)) in the muscle, gill and liver were investigated for three most
consumed frozen fish species (Umbrina canosai, Clupea harengu and Scomber scombrus) within
Ibadan, metropolis. Samples were digested as described by FAO/SIDA. The concentrations of
metals were quantified using a Varian AA240 Fast Sequential Flame Atomic Absorption
Spectrophotometer (AAS). Heavy metals like Cd, Cu and Hg were significantly higher (p<0.05) in
the liver of the three species than in the muscle and gills. There was no significant difference in the
Pb concentration for the three organs in S. scombrus, whereas, Pb was significantly higher in the
liver of U. canosai (9.73 mg kgG1) and C. harengus (4.40 mg kgG1) than in the muscle
(6.92 and 2.11 mg kgG1, respectively) and gill (8.73 and 2.52 mg kgG1, respectively). Copper
concentration was marginally higher in S. scombrus (5.06 mg kgG1) with the least value recorded
in U. canosai (3.28 mg kgG1). However, no significant variation (p>0.05) was observed in the muscle
Cd, Cu and Hg levels for the three species. Comparatively, S. scombrus has the highest
concentration of heavy metals. This study revealed that the heavy metals investigated in the major
organs and flesh was all above the limits recommended by World Health Organization.
Key words: Lead, cadmium, Scomber scombrus, Umbrina canosai, Clupea harengus
INTRODUCTION
Heavy metals are natural trace components of the aquatic environment, whose level have been
reported to be on the increase in recent times due to pollution from industrial wastes, changes in
geochemical structure, agricultural and mining activities (Singh et al., 2007; Sprocati et al., 2006).
Heavy metals unlike organic contaminants are not degraded with time, but concentration can only
increase through bio-accumulation (Aksoy, 2008). Fish are often at the top of aquatic food chain and
studies have shown that they assimilate these heavy metals through ingestion of suspended
particulates, food materials and/or by constant ion exchange process of dissolved metals across
lipophilic membranes like the gills or adsorption of dissolve metals on tissues and membrane
surfaces (Melville and Burchett, 2002). On adsorption, the pollutant is carried in the blood stream
to either a storage point (bone) or to the liver for transformation or storage (Obasohan, 2008). With
fish constituting an important link in the food chain, its contamination by toxic metals causes a
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Res. J. Environ. Toxicol., 10 (1): 82-87, 2016
direct threat, not only to the entire aquatic environment, but also to humans that utilize it as food.
From a human health perspective, mercury, arsenic, cadmium and lead have been identified as
primary contaminants of concern (Burger et al., 2007). These metals have no bio-importance in
human biochemistry and physiology, but its consumption, even at very low concentration can be
toxic (Nolan, 1983; Young, 2005). Even for metals with bio-importance such as zinc, nickel and
chromium (Abduljaleel and Shuhaimi-Othman, 2011), dietary intake have to be maintained at the
regulatory limits, as excesses will result in poisoning or toxicity (Young, 2005). This is because they
combine with body biomolecules such as metal-binding protein and enzymes to form stable biotic
compounds, thereby mutilating their structures and hindering them from performing the
designated functions within the body system (Duruibe et al., 2007).
Though the maximum permitted levels of metals in seafood have been introduced in many parts
of the world for the safe consumption of fish species (Adams and McMichael, 1999) studies and
monitoring programs examining heavy metal levels in fish are becoming more and more important,
especially in developing parts of the world, where fish provide the major source of protein
(Burger et al., 1999). In Nigeria for instance, fish is appreciated as one of the healthiest and
cheapest source of protein (Ukoha et al., 2014) contributing about 55% of the protein intake, with
per capita consumption of 7.52 kg per annum and a total consumption of 1.2 million Mt with import
making up about 2/3 of the total consumption (Eletta et al., 2003). Three prominent frozen fish in
Nigerian markets include white croaker (Umbrina canosai), mackerel (Scomber scombrus) and
herring (Clupea harengus, Linnaeus 1788) and constitute a large percentage of imported fish
consumed.
White croaker (Umbrina canosai) is a species of croaker occurring in the Eastern Pacific. It has
been taken from Magdalena Bay, Baja California to Vancouver Island, British Columbia, but are
not abundant north of San Francisco (Walsh et al., 1995). Meanwhile, the Atlantic mackerel
(Scomber scombrus) is a coastal species found only in the north Atlantic (Uriarte et al., 2001).
However, herring (Clupea harengus) is found in shallow, temperate waters of the North Pacific and
the North Atlantic oceans, including the Baltic Sea, as well as off the west coast of South America
(FAO., 2012).
Heavy metal occurrence have been reported in Mackerel and Sardine available in some
Nigerian markets (Beetseh and Abraham, 2013; Ukoha et al., 2014) with values for cadmium
ranging from 0.016-0.105 mg LG1 and lead values ranging from 0.013-0.105 mg LG1. Though these
values fall within the recommended tolerable limits, there is need to expand the species examined
and update the status at various intervals and locations. This study investigates the concentration
of some heavy metals in the gills, flesh and liver of some commercially consumed frozen fish in
Ibadan metropolis (Largest city in West Africa) to ascertain their safety for human consumption.
MATERIALS AND METHODS
Study site: The study site, Ibadan is located on longitude 350’ East of the Greenwich meridian and
latitude 723’ North of the equator with a population of 1,991,367 (Areola, 1994).
Reagents/apparatus: All reagents used were of analytical grade. Deionized water was used for
solutions preparation and dilutions. All glass wares were soaked in 10% HNO3 for 2 h and later
rinsed with distilled de-ionized water prior to use for metal analysis. Ultrapure standard solutions
of 1000 mg LG1 of the metals (Merck, Darmstadt, Germany) were used for calibration. Nitric acid
(65%) and hydrogen peroxide (30%) of ultrapure quality for digestion were from same source.
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Res. J. Environ. Toxicol., 10 (1): 82-87, 2016
Sample collection and preparation: Two major fish storage facilities were selected within the
metropolis using purposive sampling and three most prominent fish species in the market namely,
white croacker (Umbrina canosai), mackerel (Scomber scombrus) and Herring (Clupea harengus
Linnaeus, 1788) were collected bi-monthly for 12 months (Each species from each facility were in
triplicate groups per sampling period). Samples were placed in prewashed polyethylene bags and
taken to the laboratory in icebox.
In the laboratory, the fish were thawed at room temperature, weight and the total length
measured, before dissecting for analysis. Fish samples were dissected to separate the muscle, gills
and liver as recommended by UNEP/FAO/IOC/IAEA (1984). The separated organs were neatly
placed in labeled acid washed petri dishes and dried to constant weight at 80°C for 2 days
(Abubakar et al., 2015). Teflon mortar was used for pulverization and homogenization of tissues
before digestion.
Digestion of samples: The labeled samples were digested (in triplicate) according to methods
described by FAO/SIDA (1983). The 0.5 g of each sample was weighed into a dry digestion tube.
Five milliliter of 2:1 Nitric per chloric acid (H2O2/HNO3) was added. The mixtures were swirled
gently and allowed to digest on a hot plate in a fume chamber for 2 h at 80°C until the brown fumes
disappears (Igwemmar et al., 2013). The digests were allowed to cool and filtered into 25 mL
volumetric flasks with Whatman No. 1 filter paper and made up to mark with de-ionized water.
Sample blanks were carried out throughout the digestion processes. The metallic content of the
digested samples and the blanks were quantified using a Varian AA240 Fast Sequential Flame
Atomic Absorption Spectrophotometer (AAS) and vapour generation accessory (Varian VGA 77)
with closed end cell for mercury. The quality of analytical procedure was confirmed by analyzing
standard reference materials of mussel. Concentration of Pb, Cd, Cu and Hg were expressed as
mg kgG1.
Statistical analysis: Data resulting was subjected to analysis of variance (ANOVA) to determine
the differences in the heavy metal concentration of the three fish species and organs. Duncan
Multiple Range Test was used to separate differences between means. Statistical significance level
was set at p<0.05 using SPSS version 17.
RESULTS AND DISCUSSION
The mean total length and weight of sampled fish are shown in Table 1. Umbrina canosai had
the highest average weight of 848.75±5.50 g while S. scombrus and C. harengus had 420±6.20 and
339.17±5.25 g, respectively. Mean total length ranged from 34.08 in C. harengus to 41.17 cm in
U. canosai. Lead, mercury, copper and cadmium were detected in all the fish samples collected.
Lead was significantly lower (p<0.05) in the muscle of C. harengus with a value of 2.11 mg kgG1,
followed by 6.92 and 9.45 mg kgG1 in U. canosai and S. scombrus, respectively. In gills, Pb
concentration were significantly higher (p<0.05) in U. canosai (8.73 mg kgG1) and S. scombrus
(10.38 mg kgG1) whereas, concentration in the liver ranged from 4.4 mg kgG1 in C. harengus to
Table 1: Mean total length and Mean wet weight of experimental samples
Fish species Mean total length (cm) (±SD) Mean wet-weight (g)
Umbrina canosai 41.17±5.24 848.75±218.95
Scomber scombrus 36.88±1.92 420.00±65.64
Clupea harengus 34.08±2.37 339.17±52.34
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Res. J. Environ. Toxicol., 10 (1): 82-87, 2016
Table 2: Mean concentration of heavy metals in fish
Species Organ Pb Cd Cu Hg
Umbrina canosai Muscle 6.92±0.59a0.60±0.15a3.28±0.80a0.46±0.00a
Gill 8.73±0.32ab 0.59±0.21a2.89±0.61a0.07±0.01a
Liver 9.73±0.74b1.62±0.32b9.20±0.64b0.85±0.10b
Clupea harengus Muscle 2.11±0.28a0.62±0.19a3.63±0.80a0.02±0.01a
Gill 2.52±0.28a0.78±0.17a3.93±0.60a0.06±0.01a
Liver 4.40±0.48b1.66±0.34b7.62±0.24b0.92±0.07b
Scomber scombrus Muscle 9.45±1.38a0.61±0.27a5.06±0.36a0.05±0.01a
Gill 10.38±0.91a0.91±0.12a5.77±0.98a0.09±0.00a
Liver 13.18±2.03a2.18±0.71b9.74±0.37b1.31±0.32b
Maximum acceptable limits (FAO/WHO) 0.40 0.10 0.40 0.5-1.0
Values are Means±SEM, Values in each column with the same superscript are not significantly different at p<0.05
13.18 mg kgG1 in S. scombrus. Lead is described as a classical chronic or cumulative poison, which
may result in neurological, hematological, behavioral, renal, cardiovascular and reproductive effects
at levels above the tolerable limit (FAO/WHO., 2011). Lead concentrations were highest in all
organs of S. scombrus (Table 2) compared to other species and this may be attributed to the feeding
habit of this specie as well as the level of habitat contamination. Romeo et al. (1999) stated that the
concentrations of metals in gills reflect their concentration in water where the fish lives, whereas
the concentration in the liver represents storage of metals in water. High concentration of Pb in
the liver is in agreement with various authors working on other species (Storelli et al., 2012;
Bashir et al., 2013). This is because the liver is the primary organ responsible for the detoxification,
transportation and storage of toxic substances (Uzairu et al., 2009).
Cadmium concentration in fish muscles ranged from 0.60 in S. scombrus to 0.63 mg kgG1 in
U. canosai as shown in Table 2. Cadmium concentrations were highest in fish liver, with the
highest value recorded in S. scombrus (2.18 mg kgG1) and least in U. canosai (1.62 mg kgG1). The
results obtained in this study were lower than values reported by Abubakar et al. (2015) for
S. scombrus. Compared with the other organs, the muscle had the lowest levels of cadmium. This
is because of the inactive nature of this tissue which is responsible for its in ability to accumulate
heavy metals to a large extent (Eneji et al., 2011).
Cooper is an essential element which enhances enzymatic activity of the body
(Igwemmar et al., 2013). The concentration of copper in the gills of species were statistically similar
(p<0.05), ranging from 2.89 mg kgG1 in U. canosai through 3.93 in C. harengus to 5.77 mg kgG1 in
S. scombrus. Values followed the same trend for muscles. However, copper concentration was
significantly higher in the liver of U. canosai and S. scombrus than C. harengus. The values
recorded in this present study are higher than that reported for S. scombrus and U. canosai
(Igwemmar et al., 2013), but they fall far below the recommended safety limit of 30 mg kgG1.
Mercury is a natural occurring metallic element which can be present in foodstuffs by natural
causes. Fish ingest contaminated mercuric food which passes through the gastrointestinal tract and
gets distributed, accumulated or detoxified by the liver (Mieiro et al., 2011). The concentration of
mercury in muscles ranged from 0.02 in C. harengus to 0.05 mg kgG1 in S. scombrus. This metal
is absorbed by the gills and carried by the blood stream to the liver where it is detoxified and
excreted as bile before it gets to the flesh, leading to the less toxicity of heavy metals in the flesh
(Romeo et al., 1999). Mercury level in the liver range from 0.85 in U. canosai to 1.31 mg kgG1 in
S. scombrus.
Except in the muscle of U. canosai where Hg concentration was higher, S. scombrus recorded
the highest concentration of all heavy metals in the muscles, gills and livers examined in this
study. The pattern of metal content in S. scombrus and U. canosai is Pb>Cu>Cd>Hg, while it is
Cu>Pb>Cd>Hg for C. harengus.
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CONCLUSION
This study showed that the levels of heavy metals in gill, liver and muscle of the three species
(S. scombrus, U. canosai and C. harengus) are higher than the acceptable limit recommended by
FAO and WHO except mercury level in S. scombrus liver. This accumulation of heavy metals
especially in fish muscle is of great health concern to human. It is therefore, become pertinent for
government to monitor frozen fish importation more effectively and create public awareness on the
implication of heavy metal accumulation in food on human health.
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