Page 704 - The Toxicology of Fishes

684 The Toxicology of Fishes

Introductory and Historical Perspectives
Understanding basic mechanisms of toxicological processes is integral to assessment of risk. In aquatic
toxicology, however, the mechanistic evaluation of environmental chemicals is a much younger area of
investigation than in mammalian systems and only recently has received significant attention. Prior to
the 1970s and 1980s, the development of standardized bioassays and routine environmental monitoring
served as the primary means to assess and regulate chemical contaminants. Subsequent to this era,
investigators in aquatic toxicology turned their attention from whole organism responses to those at the
cellular or organ system levels of biological organization. This led to a proliferation of efforts and
information relating to clinical approaches utilizing hematology, histology, histochemistry, metabolism,
pharmacokinetics, and physiological or biochemical effects as measures of toxicity. In the l990s, these
approaches broadened with the increased use of molecular biology techniques that have greatly assisted
efforts to define and understand mechanisms of action. Since 2000, with the recent advances in -omic
methodologies, multiple endpoints have been identified as being modified by exposure to various stressors
with the potential development for stressor-specific responses. The results of mechanistic studies have
proven to be valuable to many applied areas of aquatic toxicology. In risk assessment, mechanistic data
have been useful in demonstrating that an adverse effect observed in the laboratory is directly related
to population-level effects. As costs associated with environmental compliance increase, regulators and
the regulated community, must make the most effective use of the funds and time allocated for reducing
environmental impacts. At present, assessment and regulatory decisions continue to be based primarily
on empirical measures that may be highly susceptible to change. Decisions based on a more complete
mechanistic understanding of chemical effects would be less likely to vary with technological trends.
Additionally, the more sensitive our assessment methodologies become, the earlier adverse effects of
environmental chemicals on aquatic ecosystems can be measured and, in turn, the more accurate the
evaluation of ecological risk.
Adverse effects to aquatic organisms begin with the release of a chemical into the environment. A
much relied upon means to evaluate ecological risk has been through environmental monitoring in which
chemical residues are assessed. This approach has provided useful information but with significant
limitations, not the least of which are the time and costs associated with chemical residue analysis.
Although time and cost restraints are arguable issues, a more significant challenge associated with such
an approach is the inability to quantitatively evaluate the availability of a chemical from the environmental
matrix to the aquatic organism. Furthermore, metabolism or limitations in available technology may
render a chemical difficult, if not impossible, to detect in environmental or biological samples. Appli-
cation of biomarkers in environmental monitoring may resolve many of these challenges by providing
a measure of availability of an environmental chemical to an aquatic organism by providing a direct
measure of the response of an organism to chemical exposure. Regarding biological response to sublethal
concentrations of environmental chemicals, Depledge et al. (1993) noted that an essential criterion of
the biomarker approach is the identification of early onset changes in otherwise healthy organisms that
predict increased risk of development of chemically induced pathologies. Other potential uses are listed
in Table 16.1.

TABLE 16.1
Potential Uses for Biomarkers in Field Studies.
To demonstrate residency or exposure for the purposes of interpreting responses as site specific
To demonstrate or define the characteristics of an unknown chemical or chemical mixture
To demonstrate bioavailability (or lack thereof)
To examine the time course of uptake
To provide in vitro opportunities for understanding mechanisms
To prioritize sites, stressors, or samples for further sampling or analyses
To direct testing during fractionation procedures to isolate unknowns
To evaluate the time course or success of remediation
To conduct surveillance