Introduction

In recent years heavy metal concentrations were found to be raised in the coastal environment due to discharges from industrial wastes, agriculture and urban sewage. As a result, aquatic organisms are exposed to elevated levels of heavy metals in the coastal environment1011. Heavy metals can be accumulated by marine organisms through a variety of pathways, including adsorption and ingestion12.

Fishes have widely consumed in many parts of the world because it is rich in protein, contains low cholesterol and high percentage of poly unsaturated fatty acids, liposoluble vitamins and essential minerals especially containing omega fatty acids1. The aquatic organisms exposed to the heavy metal from the runoff water tend to accumulate in their body but fishes are more commonly affected than the other a13 quatic species4.

Fishes are found to be good bio-indicators or biosensor for heavy metal contamination to the aquatic environment and extensively used in marine pollution monitoring programs. Hence, the aim of the present study is to evaluate the level of concentrations of two edible fish species (Nemipterus japonicas and Sardinella longiceps) in the Ennore region of Chennai coast.

Materials and Methods

Study area

The study area located on the north part of Chennai, the major industrial units located in the Ennore – Manali belt alone release about 100 million litres of effluents (treated or untreated) per day with high metal concentration into the nearby water bodies including the coast7. Ennore estuary was one of highly polluted estuary due to heavy industrialization and the improperly treated effluents ultimately reached through Ennore bar mouth and finally enter into the Bay of Bengal8.

Fig. 1: Map showing the study area.

Sample collection and analysis

Fishes Nemipterus japonicas and Sardinella longiceps were collected during two season (post monsoon and pre monsoon) 2006. Fishes were washed, separately placed in an ice box and brought to the laboratory as frozen at -20˚C until analysis.

The frozen fish samples were thawed at room temperature and dissected using stainless steel scalpels. One gram of accurately weighted muscles from dorsal surface of fish, entire intestine, liver and gills from each sample were removed carefully to avoid any contaminations. The samples were washed with double distilled water, dried into the filter paper, weighed, packed in polyethylene bags and kept at -20˚C until analysis.

The samples were digested with aquaregia using Teflon bombs and after complete digestion the samples were cooled at room temperature and diluted to 25 ml with double distilled water. All the digested samples were analyzed for metals (Zn, Cu, Fe, Cr and Cd) using Flame Atomic Absorption Spectrophotometer (Perkin Elmer Analyst 700) and were expressed as (µg g-1) wet weight of tissue6.

The instrument was calibrated with chemical standard solution prepared from commercially available chemicals (Merck, Germany). Analytical blanks were run in the same way as the samples and the concentrations were determined using the standard solutions prepared in the same acid matrix. The quality of the data was checked by the analysis of standard reference material (DORM-2, National Research Council and Canada).

Results and Discussion

The concentration of heavy metals (Zn, Cu, Fe, Cr and Cd) in gills, liver, intestine and muscle of fish species N. japonicas and S.Longiceps were estimated during post monsoon and pre monsoon and summarized in Fig 2a-d.

Fig. 2: Heavy metals in different organs of Nemipterus japonicas (a and c) and Sardinella longiceps (b and d) during post monsoon and pre monsoon season 2006.

During post monsoon the high accumulation of Zn 10.75 µg/g-1 and Cr 0.85 µg/g-1 were recorded in the gills region. Cu 2.02 µg/ g-1; Fe 23.75 µg/g-1 and Cd 0.007 µg/g-1 were high in the liver and intestine region of N. japonicas (Fig 2a). Whereas S. longiceps the Zn 11.05 µg/ g-1; Cu 2.62 µg/g-1 and Cd 0.007 µg/g-1 were high in the gills portion. Fe 26 µg/g1 was high in the intestine and Cr 1.10 µg/g-1 was high in the muscle portion (Fig 2c).

Pre monsoon season Zn 24.3µg/g-1 was recorded high in the muscle and Cu 1.71 µg/g-1; Cr 0.87 µg/g-1 were high in the liver. The Fe 27.85 µg/g-1 and Cd 0.009 µg/g were high in the gills of N. japonicas (Fig 2 b). Whereas the S.longiceps the Zn 25 µg/g-1 and Mn 4.95 µg/g-1 were high in the intestine region. Cu 1.89 µg /g-1 and Cd 0.013 µg/g-1 were recorded high in the liver. Fe 29.15 µg/g-1 and Cr 1.51 µg/g-1 were registered high in muscle portion (Fig 2d).

In post monsoon and pre monsoon season the sequence of metal concentrations in N. japonicas and S.Longiceps were Fe>Zn>Cu>Cr>Cd. The results indicated that the bioaccumulation of heavy metals Zn, Cu, Fe, Cr and Cd in gills, liver, intestine and muscle of fish species (N. japonicas and S.Longiceps) were high during pre monsoon than the post monsoon season. The accumulation of heavy metals in fish species varies with the species of different aquatic environments. The high concentration of metals in the gills of N. japonicas and S. Longiceps reflects the levels of heavy metals in the water where the fish live, thus the gills in fish are often recommended as environmental indicator of water pollution than any other fish organs. High accumulation of metals in liver portion may also be due to the difference in physiological functions of muscle and liver. Thus, the liver is often recommended as a target tissue in aquatic environments13.

The variation in the level of heavy metals in fish (N. japonicas and S. Longicep) species depends upon its feeding habit, age, size, length of the fish and their habitats14. The metal concentration in muscle is important because it is the chief edible part of the fish. However the affinity for metal absorption from contaminated water and food may differ in relation to ecological needs, metabolism and the contamination gradients of water, food and sediments as well as other factors such as salinity, temperature and increasing agents9.

The present results seem to be similar with the findings of Batvari2 exhibited by the metal concentrations in fish species of Carangoidel malabaricus and Belone stronglurus from Pulicate lake north of Chennai southeast coast. Padmini and Vijaya geetha8 reported the metal accumulation in the grey mullet and Mugil cephalus fish species from the Ennore estuary southeast coast of India.

Conclusion

In the present study, accumulation of heavy metals Zn, Cu, Fe, Cr and Cd in the fish species N. japonicas and S.Longiceps were found to be high during pre monsoon than the post monsoon season. The seasonal variation of heavy metals in the fish species might be due to the physico-chemical and biotic factors of the coast, which influence the bioavailability of metals. However the concentrations of the heavy metals were below the limited values prescribed by of FAO3 and compared with other ecosystems. The concentrations of metals in edible parts of the fishes were within the permissible limits and are safe for human consumption.

Table 1: Heavy metal concentrations in certified reference material (NRCC-DORM-2 Dogfish muscle) from the National Research Council, Canada.

Table 2: Concentration (µg/g-1) of heavy metals in different organs of fish species during post monsoon and pre monsoon.