Page 64 - The Toxicology of Fishes

44 The Toxicology of Fishes

in the intestine, so BaP absorbed from food enters circulation with its metabolites (Van Veld et al., 1988).
When bluegill sunfish during aqueous exposure to BaP were fed uncontaminated food, the rate of
elimination of BaP and metabolites was ten times greater than with no food, and the rate of conversion
of BaP to polar metabolites was also increased (Jimenez et al., 1987). This suggests that having food
in the gastrointestinal tract, in addition to increasing elimination of BaP by partitioning to feces, facilitates
the exchange of BaP from blood to the gastrointestinal mucosa where metabolism can accelerate
elimination through the digestive tract.
The role of metabolism in limiting the bioavailability of BaP to fish, and thus risk to fish populations,
is uncertain because this has been investigated mainly for juvenile and adult fish, not for the embryo–
larval stages, which are likely more sensitive. The bioavailability of BaP to fish early life stages may
be particularly critical for determination of the phototoxicity risks of PAHs such as BaP because larvae
appear to be especially sensitive for such toxicity (Oris and Giesy, 1987). Some studies have indicated
that BaP is more bioaccumulative in the early life stages of fish than in older stages. Comparison of
BCFs for PCBs and PAHs provides an effective measure of the degree to which fish metabolize PAHs.
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The lipid-normalized, steady-state BCFs for BaP (measured as total [ C]-BaP) and PCB 105 in zebrafish
(Brachydanio rerio) larvae were found to be approximately equal to K , suggesting a lack of metabolism
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of BaP during the 7 days of exposure (Petersen and Kristensen, 1998). Similar results were found for
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other PAHs tested with larvae of three other fish species; however, the retention of [ C] from labeled
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BaP in fish embryos does not necessarily indicate the absence of metabolites, as clearance of [ C]-
labeled metabolites from an embryo may be slow. Exposure of trout embryos to BaP in water/DMSO
resulted in retention at hatch of 60 to 80% of the initial concentration in the egg, with indications that
some metabolites were being excreted (Kocan and Landolt, 1984). Confocal microscopy more recently
revealed that both BaP and its metabolites remained in water-exposed medaka embryos during very
early development (Hornung et al., 2005). A further complication is that PAH mixtures consist of both
inducers and inhibitors of CYP1A activity, and inhibition of CYP1A in fish embryos has been observed
to increase the embryotoxicity of BaP in mummichog (Fundulus heteroclitus) (Wassenburg and Di
Guilio, 2004).
This section has examined two factors that distinguish the bioavailability of BaP from TCDD and
should be considered in the assessment of the bioavailability of any hydrophobic organic chemical. First,
the strength of the association of a chemical with organic carbon can deviate from simple correlations
to hydrophobicity measures such as K . This can be true for reversible adsorption to or partitioning
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into organic phases but is especially important for systems contaminated with combustion products, in
which a portion of some chemicals might have low bioavailability due to occlusion within particles.
Second, biotransformation of PAHs both within the food chain and the receptor fish can greatly reduce
bioaccumulation within the fish compared to that expected in the absence of such metabolism. For
hydrophobic chemicals with slow rates of elimination, even slow rates of metabolism can contribute
substantially to overall elimination and thus to steady-state bioaccumulation. The impact of biotransfor-
mation will vary with the species and lifestage of the organism of concern. Both of these factors involve
various components in a food web, and the resultant bioavailability to a fish must be assessed in terms
of the entire web and its spatial extent.

Summary

The bioavailability of chemical contaminants to fish can vary greatly among chemicals, organisms, and
exposure conditions, so it is therefore of great importance to fish toxicology and risk assessments.
Although bioavailability is a conceptually simple concept—addressing the amount of chemical accumu-
lation by an organism relative to an environmental expousure—it is often difficult to assess and apply.
Bioavailability assessments must start with a clear definition of the risk situation of interest. Across what
exposure conditions are bioavailability comparisons being made? What organisms and chemicals are of
concern? What environmental concentrations and what concentrations within an organism are the ref-
erences for defining bioavailability? What are the spatial and time frameworks? What exposure path-
ways should be considered? Chemical speciation is a primary focus for understanding and describing