Cadmium, Copper, Lead and Zinc Tissue Levels in Bonga Shad(Ethmalosa fimbriata) and Tilapia (Tilapia guineensis)Caught from Imo River, Nigeria

MATERIALS AND METHODS

Bonga shad (Ethmalosa fimbriata) and Tilapia (Tilapia guineensis) were caught using gill nets from three locations representing the upper, middle and lower reaches of the river. The first station (OBG) is located on 4^o29' N,7^o8' E, upstream of Imo River (Rivers State) where a number of small-scale industrial activities are sited. The second station (IAS) is located in Ikot Abasi (Akwa Ibom State), where the Aluminium Smelting plant is sited. Moreover, domestic wastewaters are channeled into the water body within this area. The third station (DRN) representing the downstream region of the river was chosen far from any industrial activities. The fishes collected at each station were labeled with an identification number, the date and place of collection, before they were taken to the laboratory for analysis in ice-packed coolers, until being placed into a freezer at 4°C.

Pretreatment of Samples Collected
Samples of fishes from each location were taken to the laboratory on the same day and later dissected to remove the liver and epixial muscle on the dorsal surface of each species of fish. The livers and pieces of edible muscle tissues (8.0 g) were dried at 105°C until they reached a constant weight. Each dried sample was ground, using porcelain mortar and pestle and 0.5 g of the powdered form was separately digested using perchloric and nitric acids in a silica beaker and later evaporated to dryness on a hot plate. The white residue of each digest was dissolved in 10 mL of 20% nitric acid. The solutions were filtered into acid-washed volumetric flasks and diluted to 50 mL prior to elemental analysis. Replicates, three blanks and standards were treated similarly. Concentrations of cadmium, copper, lead and zinc were then determined using an inductively coupled plasma spectrometer (Optima 3000-Perkin Elmer).

RESULTS AND DISCUSSION

The investigated metal levels in liver and muscle tissues were measured as mg kg^-1 on a dry weight basis. Importantly, considerations will be made on the tissue elemental contents and the bioconcentrated ratio in muscle with respect to liver. However, more emphasis will be on the heavy metal concentration in the muscle because of its dietary significance. The mean concentrations recorded for copper (Cu), cadmium (Cd), lead (Pb) and zinc (Zn) in muscle and liver tissues of E. fimbriata- Bonga shad and Tilapia guineensis-Tilapia are presented in Fig. 1 and 2, respectively. As indicated in Fig. 1, zinc levels were relatively higher in both the muscle and liver tissues of E. fimbriata than other elements considered. Zinc concentrations in muscle tissues were 7.78±5.40, 11.48±0.20 and 14.00±3.12 mg kg^-1 dry weight at IAS, DRN and OBG stations, respectively. On the other hand, Zn levels in the liver tissues of E. fimbriata indicated higher values with DRN, OBG and IAS stations recording concentration of 39.26±1.00, 33.00±2.41 and 26.41±3.53 mg kg^-1dry weight, respectively.

Fig. 1:	Tissue elemental concentration in Ethmalosa fimbriata caught from Imo river ecosystem

Fig. 2:	Tissue elemental concentrations in Tilapia guineensis caught from Imo river ecosystem

The average concentrations of copper obtained at OBG, IAS and DRN stations for liver and muscle tissues of the fish species were 6.03±1.00, 4.29±1.16, 2.62±0.10 and 0.47±0.24, 0.21±0.40, 1.93 mg kg^-1 dry weight, respectively. The variation of Pb and Cd levels in E. fimbriata followed the trend as Zn and Cu, which recorded higher concentrations in the liver than muscle tissue. The concentrations of Pb (0.56±0.28, 0.19±0.36, 0.09±0.24 mg kg^-1 dry wt.) and Cd (0.09±0.01, 0.1±0.01, 0.02±0.01 mg kg^-1 dry wt.) found in the muscle tissues at OBG, IAS and DRN stations, respectively were however lower than values obtained from the liver tissues, which were Pb: 1.06±0.02, 0.55±1.20, 1.32±0.10 mg kg^-1 and Cd: 0.28±0.03, 0.40±0.01, 0.03±0.01 mg kg^-1 dry wt. In general, the trend in elemental bioaccumulation in E. fimbriata in both muscle and liver tissues was Zn>Cu>Pb>Cd.

All the metals considered were detected in the target organ (liver) of Tilapia guineensis albeit at different levels and their respective mean levels at the three stations in the liver and muscle tissues were: Cd (0.26±0.24 and 0.02±0.01 mg kg^-1); Zn (45.91±18.17 and 10.96±2.07 mg kg^-1); Pb (0.47±0.39 and 0.59±0.19 mg kg^-1) and Cu (12.30±8.93 and 1.10±0.84 mg kg^-1). The concentrations of all these metals in liver tissues of Tilapia species were comparatively higher than levels obtained from the muscle tissues, with Zn displaying the highest tissue load (Fig. 2). This observation presupposes the low metal absorption potential in the muscle and moreover confirms earlier reports that target organs such as liver, gonads and gills of aquatic organisms have inherent potential to bioaccumulate heavy metals at enhanced levels compared to the muscle tissue (Abdel-Moniem et al., 1994; Arellano et al., 1999; Huang, 2003; Yilmaz, 2005). In general, muscle tissues of aquatic biota are not considered as excellent bioaccumulator of heavy metals (Kargin and Erdem, 1991; Shoham-Frider et al., 2002; Yilmaz, 2005).

However, the liver is the preferred organ for metal accumulation as could be deduced from the present study (Karadede and Unlu, 1998; Shoham-Frider et al., 2002). In both the Tilapia and Bonga shad, relatively high concentrations of Zn and Cu were found in the liver. Similar reports of preferential bioaccumulation of Zn and Cu in liver to muscle tissues of biota are available (Amundsen et al., 1997; Roditi-Elasar, 1999; Al-Yousuf et al., 2000; Yilmaz, 2003). Several researchers have documented the anomalous concentrations of essential elements especially Zn and Cu than other heavy metals in tissues of fishes (Farkas et al., 2000; Wong et al., 2001; Chou et al., 2003). Moreover, in aquatic organisms, the liver plays a prime role in the sequestration of adsorbed metal through detoxification by cysteine-rich metal binding proteins (metallothioneins), synthesized in response to changes in heavy metal balance (Langston et al., 1999).

The degree to which a metal is concentrated by a species, expressed in this study as the muscle to liver ratio (MLR) of metal concentrations was relatively higher for Pb (0.34) than Cd (0.26), Zn (0.29) and Cu (0.28) in E. fimbriata. Thus, the order of metal MLR in E. fimbriata is Pb>Zn>Cu>Cd. However, a different order was found in T. guineensis species: Cd>Pb>Zn>Cu. The MLR was higher for Cd (0.42) than Pb (0.38), Zn (0.20) and Cu (0.09). This presupposes that the rate of homeostasis of essential elements was relatively higher in E. fimbriata than T. guineensis (Chen and Chen, 2001). This is expected and the observation is supported by the higher concentration of the non-essential element, Cd in E. fimbriata which could have induced the metallothioneins to displace the essential metals (Zn, Cu) from pre-existing MTs (Riba et al., 2003). Cadmium levels in T. guineensis were lower in both muscle and liver tissues (Fig. 2).

Tissue elemental load in aquatic organisms is variable and species dependent, as well as being a function of the biota’s environment (Eja et al., 2003; Canli and Atli, 2003). However, the affiance of metal uptake from an impacted ecosystem through direct or indirect ingestion or absorption through biomembranes in food and water could differ in relation to metabolism, physiology, ecological requirements, pollution gradients of pelagic column, sediment and food as well as the prevailing temperature and salinity (Romeo et al.,1999). T. guineensis are benthopelagic species with sedentary habits. Therefore, the relatively higher tissue elemental concentrations observed when compared to E. fimbriata may be attributed to its feeding habits and the contamination gradients of the pelagic column of Imo River with respect to its sediment compartment. Benthopelagic fishes have been reported as having high levels of heavy metals derived primarily from residual toxic levels in water and sediment (Huang, 2003). More so, taking the species factor into consideration, the heavy metal difference were significant (p>0.05). In general, this study revealed that the muscle tissues of E. fimbriata indicated higher preference for the bioaccumulation of non-essential elements (Pb, Cd) than Zn and Cu. This was however in contrast to the muscle metal bioaccumulation preference indicated by T. guineensis for essential elements.

CONCLUSIONS

Since there are many environmental factors which have been claimed by various researchers to influence the bioaccumulation of heavy metals to toxic levels in organisms, it would be unreasonable to attribute the levels of these metals in E. fimbriata and T. guineensis to any specific factor(s). It is perhaps more rational to posit that these elemental tissue load might have been largely influenced by their feeding habits as well as the contamination gradients of the Imo River surface water and sediment. From the results, the concentrations of heavy metals in liver and muscle tissues of both species investigated did not exceed the acceptable levels proposed for human consumption (USEPA, 1995). However, the levels of Cd and Pb in the species call for serious concern because they could bioaccumulate and biopersist in their tissues over time. This study therefore highlights this potential danger (Cd- and Pb-poisoning), which could occur owing to long-term consumption of these species. Thus, precautions need to be taken in order to forestall future heavy metal pollution in the river as well as in the biota. This research is however a precursor to the holistic approach needed to fully assess the elemental contents of sediments, water and several other biota of the Imo River ecosystem.