International Journal of Marine Science, 2017, Vol.7, No.9, 76-87
83
3 Remarks
The heavy metal pollution of marine environment may be evaluated by analysing seawater, sediments and biota.
Because of metal levels in marine organisms are generally notably higher than those in other items of marine
ecosystem (Bryan, 1976; Phillips and Rainbow, 1994). Phillips (1980) represented that for an organism to be a
good indicator of metal contamination, there should be a simple correlation between metal values in the
surroundings and in the organism. Fish are broadly chosen as sentinels of pollution in marine coastal area (Bat,
2014; Ahmed and Bat, 2016a; 2016b; Ahmed et al., 2016) and may accumulate metals over background levels and
so, verify their likely as biomonitors of heavy metal pollution (Storelli and Marcotrigiano, 2005). Most living
being need minor quantities of many essential metals such as iron, manganese, copper and zinc for their vital
processes (Bryan, 1976). Besides, these ones have toxic when they passed over definite limits (Rainbow, 1985). In
the present study, the concentrations of Cu, Zn, Fe and Mn have been measured in skipjack tuna
(
K.
pelamis
)
collected from Karachi fish harbour of Arabian Sea. Skipjack tuna is a pelagic fish and is the top of food chain.
They feed on mainly fish, crustaceans, cephalopods and mollusc (Kojadinovic et al., 2007; Jakimska et al., 2011).
It is also notified that metabolic rates (Korsmeyer and Dewar, 2001) and digestion and growth rates (Storelli et al.,
2005) of the pelagic fish are three and two to five folds higher than those of many fish species of same length,
respectively. Because of their high rates of metabolism and xenobiotic uptake (Kojadinovic et al., 2007; Jakimska
et al., 2011), it may results in a rise in the concentrations of heavy metal taken up and hence accumulated.
Bustamente et al. (2003) pointed out that difference in metal levels coupled with food and nutrition temperaments
of benthic and pelagic fish species and found that benthic fish generally accumulated higher metal levels than
those in pelagic fish. This finding is disagreeing with the results of Topping (1973) who put forwarded that
primarily plankton nourishment fish comprise much higher metal levels than those benthic nourishment fish from
Scottish waters. Jakimska et al. (2011) found that carnivorous species accumulated higher heavy metals than
herbivores and omnivores. These results are similar to Bat et al. (2012a) which show high heavy metal levels in
sprat which is pelagic and zooplanktonivorous fish and has high metabolic rate. This makes skipjack tuna ideal
bioindicator for assessing heavy metal pollution in marine ecosystems (Kojadinovic et al., 2007). Heavy metal
levels varied among the tissues and organs with seasonally. This may be attributed to factors such as water
temperature and food. The data in Table 2, Table 3, Table 4 and Table 5 on the dispersion of the metal levels in
the organs and tissues of the skipjack tuna represent that there is a significant concentration of these metals in
liver than those in edible parts. These findings were similar to those reported by other studies in Karachi coasts
(Yousuf et al., 2013; Ahmed et al., 2014). This is also agreed with the study of Bat et al. (2012b) that heavy metal
levels of bottom fish
Psetta maxima
in liver were greater than those in edible tissues and gonad for both male and
female, recommending that liver is an organ in which metals are accumulated. The results for these metals also
demonstrated that there was disparity in metal levels seasonally. Fe is the maximum available in large quantities
of the metals investigated. In skipjack tuna, gill was the second target tissue for Mn and Cu. Dallinger et al. (1987)
emphasizes that one of the major route of heavy metal assimilation in fish are water, through the gills. It is
interesting to note that Zn in muscle tissue is higher than those in in gills and gonads. Cu in muscle tissues is also
greater than those in gonad. From the current study, gonads generally accumulated low heavy metals except Fe.
Maximum Cu and Zn levels in dorsal muscles were 7±2 μg/g dry wt. Statutory thresholds are not existing for
requisite elements in European countries. Nevertheless, in the edible tissues of skipjack tuna the mean metal levels
were beneath the maximal allowance values for people consumption set by compared the international regulations
(Table 8). The Cu and Zn levels in kidney, gills and gonads were always lower than the permissible maxima, but
liver Cu and Zn levels exceeded the allowable levels. It should be noted that the concentrations of metals were
given as dry wt. in the present study. Even so it is recommended that the liver of skipjack tuna should be ejected
and decently cleaned before consumption.
In addition, the Provisional Tolerable Weekly Intake (PTWI) level is a calculate of the quantity of a metal that
may be eaten by human over a vita without perceptible hazard. PTWI is builded according to the Joint Food and
Agricultural Organization for the United Nations (FAO) / World Health Organization (WHO) Expert Committee
on Food Additives (JECFA) (Anonymous, 2010a). PTWI levels were accessed in the current investigation to give
