Fish are ideal indicators of heavy metal contamination in aquatic ecosystems because they occupy different trophic levels . Fish species bioaccumulate trace elements in different tissues, predominantly from their diet [136,137]. Many fish adapt to a wide variety of food sources and often switch from one food source to another as environmental and food supply conditions change . Due to this capacity of trophic adaptability, trace element concentration in water and tissue trace element content do not have a consistent relationship. For example, fish from lakes with trace element levels below detection in water can carry trace element burdens that present a human health hazard. Yet, elevated concentration in water has also been measured in lakes where fish have a relatively low burden .
Trophic level differences in different fish species with respect to elemental content have been reported by a number of workers [139-142], although much of the work is with reference to mercury. In general, carnivorous species have higher levels than herbivorous, omnivorous, and planktivorous species  and larger carnivores have higher levels than smaller carnivores. However, bottom-dwelling fish can sometimes have higher levels than carnivorous fish, particularly when they are ingesting sediments . For example, Campbell  found that bottom-feeding redear sunfish (Lepomis microlophus) had higher levels than bass and bluegill sunfish, which are predominantly water-column feeders. Villar et al.  found a difference in the metal content in the tissues of the two fish species, Prochilodus lineatus (detritivore) and Pterodoras granulosus (omnivore). P. lineatus had a high concentration of metals in the tissues because it primarily fed on the metal-rich detritus. Thus, it is essential to understand the feeding location as well as the trophic level to understand contaminant levels.
Burger et al.  estimated trace element levels (As, Cd, Cr, Cu, Pb, Mn, Hg, and Sr) in 11 species of fish occupying different trophic levels with varied dietary habits. They found that species-specific differences existed in all the species studied with respect to all the trace elements. Bowfin and channel fish (both piscivores) had the highest level of all the elements except Mn and Sr. They also found that trophic relationships alone were not able to account for the elemental concentration in different fish species and suggested that metal levels in fish may also reflect age; older and larger fish have higher levels of trace elements. Such a correlation has been seen for mercury .
Stewart et al.  demonstrated the role of food web structure and physiology of trace element accumulation in the prey species in the differential bioaccumulation of trace elements by fish species; this resulted in some species of fish having very high concentrations of Se while others did not in the San Francisco Bay. Two dominant food webs present in the estuary region of the bay were based upon bivalves and crustacean zooplanktons. The dominant bivalve, Potamocorbula amurensis, had a tenfold slower rate of loss of Se from the body compared to that of the crustacean zooplanktons. This resulted in higher tissue Se concentration in the bivalves, which was then reflected in the higher tissue levels of Se in the predatory species of the bivalves. The tissue concentration far exceeded the threshold levels at which Se acted as teratogen and carcinogen, whereas concentration of Se in the predators of the zooplankton was less than the threshold value.
In this case, concentration of Se in water and sediments was not high (<1|g/lt), but in some of the top consumers like white sturgeon, the tissue levels were as high as >10 |g/g. This case is an example in which basic physiological and ecological processes can drive wide differences in exposure and effects among different species. These processes are rarely considered in traditional risk assessment studies of contaminant impacts. Besser et al.  also found that metal bioavail-ability to higher order consumers such as trout can be substantially modified by processing metals in the stream food web. Substantial variation in diets of higher consumers such as fish with respect to trace metal content exist due to differential accumulation of metals among invertebrate taxa and differences in taxonomic composition among different locations [149,150].
The differences in the bioaccumulation and trophic transfer potential of different trace elements may be related to their bioavailability in aquatic environments, chemical characteristics of the element, and food web processing of the element. Chen and Folt  found that, although As and Pb bioaccumulate in aquatic biota, the concentrations of both these elements biodiminish with increasing trophic levels. The elemental content in fish species was 10 to 20 times lower than what was present in the zooplanktons. Higher levels of As were present in planktivorous fish compared to the omnivores and piscivores. Kay  and Handy  concluded that Cd also did not biomagnify in the aquatic food web; however, methyl mercury had the capability to bioaccumulate and biomagnify in aquatic biota [135,153]. Besser et al.  found that Cd had a higher bioac-cumlation factor compared to Pb, suggesting that Cd was highly available in the stream food web.
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