Page 24 - The Toxicology of Fishes

4 The Toxicology of Fishes

Additionally, aquatic food chains are generally longer than terrestrial food chains; this also likely
contributes to observations that many persistent pollutants tend to achieve greater concentrations in
aquatic predators, including fishes, compared to terrestrial predators (Clements and Newman, 2002, pp.
300–302). Moreover, aquatic animals are often particularly vulnerable because of elevated exposures
arising from their living immersed in the exposure medium (surface water), having highly permeable
skin and gills, and other inherent sensitivities. Fish and amphibians, for example, are the only vertebrate
groups with anamniotic eggs (lacking a shell or amniotic membrane) and that undergo metamorphosis
in surface waters; hence, the embryo–larval stages of these animals are highly sensitive to chemical
pollutants (Kendall et al., 2001). These issues likely contribute to lower water quality guidelines to
protect aquatic life vs. human health, even though the calculations used to generate such guidelines are
more conservative (i.e., protective) in the context of human health (http://www.epa.gov/safewater/
mcl.html#mcls).
Fish, not surprisingly, play important roles in setting these water quality guidelines for freshwater and
marine systems. U.S. Environmental Protection Agency (EPA) guidelines provide specific recommen-
dations for freshwater and marine species to be used for acute and toxicity tests that are used in
establishing these guideline and other chemical safety assessments (U.S. EPA, 1996; see Chapter 15 in
this volume). In addition to their role in establishing surface water quality criteria, toxicity tests provide
information concerning, for example, relative acute toxicities among various chemicals, relative sensi-
tivities of different species to selected chemicals, and the sublethal effects of chemicals and chemical
mixtures. Also, data generated from toxicity tests play important roles in ecological risk assessments
(see Chapter 18 in this volume).
In addition to their central role in toxicity testing for ecological effects, fish are perhaps the most
employed organisms for biomonitoring. This is likely due in part to the aforementioned propensities of
aquatic systems to receive and accumulate environmental contaminants and the diversity and importance
of fishes in these systems. Fish-targeted biomonitoring includes a wide variety of approaches for detecting
impacts of aquatic contamination, from direct measures of mortality, to broad analyses of population
dynamics and community structure, to detection of measures of subcellular change. This last approach,
embedded in the term biomarkers (see Chapter 16 and the case studies in Unit IV in this volume), has
perhaps benefited the most from modern toxicological research with fish models wherein elucidation of
the mechanisms of toxic action has received serious consideration and great strides have been made.
In a relatively short time, several decades, aquatic toxicology has moved from a descriptive approach,
which was necessary to explore those concentrations of single toxicants within water that were not
compatible with the life of individual fishes, to considerations of sublethal concentrations that do not
cause death over the short term but do harm the individual, thus making it expend resources to survive
in a state of altered equilibrium. Biomarkers of exposure, response, and genetic susceptibility were
derived from research in this area, and these helped to cut across questions of bioavailability as the
emphasis shifted to host response. Biomarkers illustrate the multiple organ, tissue, and cellular sites of
action and the spectrum of responses that were possible. The resultant toolkit, amply illustrated in this
volume, is a suite of biomarkers and validated methods that are now used to assess chemical exposures
and effects or responses arising from various forms of chronic toxicity.
Again, recent toxicological research with fishes has pursued the study of mechanisms of action. This
approach has provided potential tools for ecotoxicologic investigations; however, problems of biocom-
plexity and issues at higher levels of biological organization remain a challenge. In the 1980s and 1990s
and continuing to a lesser extent today, organisms residing in highly contaminated field sites or exposed
in the laboratory to calibrated concentrations of individual compounds were carefully analyzed for their
responses to priority pollutants. Correlation of biochemical and structural analyses in cultured cells and
tissues, as well as in vivo exposures, led to the production and application of biomarkers of exposure
and effect and to our awareness of important effects such as cancer and endocrine disruption in wild
fishes. To gain acceptance of these findings in the greater environmental toxicology community, validation
of the model vs. other, better established (often rodent) models was necessary and became a major focus.
Resultant biomarkers were applied to heavily contaminated and reference field sites as part of effects
assessment and in investigations following large-scale disasters such as oil spills or industrial accidents.
It should be noted that the total number of fish species used in mechanistic research and for biomonitoring