Page 30 - The Toxicology of Fishes

10 The Toxicology of Fishes

FIGURE 2.1 Chemical inputs to and distribution within aquatic systems, emphasizing accumulation by fish. Pie charts
qualitatively represent chemical speciation and how it differs among system compartments.

Definitions of bioavailability vary markedly (National Research Council, 2003) and are often oriented
to specific situations; however, these various definitions all address how readily chemicals are accumu-
lated under different circumstances (e.g., for different chemicals, organisms, or environmental condi-
tions). Bioavailability is therefore defined here as the relative facility with which a chemical is transferred
from the environment to a specified location in an organism of interest. Although it is broad, this definition
is still useful because it identifies important factors that must be considered in further refining and
applying it for particular assessments:

1. Chemical uptake, and thus bioavailability, depend on certain morphological, physiological, and
biochemical attributes of an organism. As such, bioavailability must be defined in terms of a
particular type of organism and its physiological state. Caution is needed regarding how well
bioavailability relationships for one type of organism can be extrapolated to other types.
2. Bioavailability must be referenced to a specific chemical concentration in the organism of
interest. This could be the total chemical in the entire organism, the chemical within a particular
tissue, the chemical associated with a specific molecular receptor, or any other measure appro-
priate to the nature of the toxicity and the goals of the assessment.
3. Bioavailability must also be referenced to a specific environmental concentration. Often this
will be the total chemical concentration, with bioavailability being considered an aggregate
property of the combined chemical species; however, assessments might also focus on selected
subsets of the chemical species or address the comparative bioavailability of individual chemical
species. The spatial context of this concentration is also important; often the reference concen-
tration is that in immediate proximity to the organism, but sometimes it covers a large spatial
extent. In addition, an exposure time frame might be required because bioavailability relation-
ships can change with time.
4. Transfer pathways of interest must be specified. An assessment might be concerned with only
a single route of exposure (e.g., food) or with all possible routes. Depending on how reference
concentrations in the organism and the environment are defined, various transport pathways
and reactions inside and outside the organism also must be considered.

Whatever the definition and context of bioavailability assessments, their success depends on ade-
quately defining the processes that regulate chemical accumulation. Processes important for determining