Page 645 - The Toxicology of Fishes

Toxicity Resistance 625

Gloucester
Hot Spot, NBH

FIGURE 13.17 MHC class IIB structural diagram with inferred locations of population-specific amino acid changes
superimposed for a reference population (Gloucester, black) and New Bedford Harbor (gray). Each colored dot represents
a single population-specific amino acid change. (Adapted from Cohen, S., Mol. Biol. Evol., 19, 1870–1880, 2002.)

sites may be one of the most informative observations regarding the contribution of contaminant exposure
to altered community structure.
Chesser and Sugg (1996) developed an ecosystem dynamics model to evaluate the impacts of selective
agents on populations and community dynamics. These authors considered the roles that species redun-
dancy, adaptation, and immigration play in the ability of an ecosystem to cope with toxic insults. Toxic
exposures that threaten sensitive genotypes and species can change both the structure (number of species
and population sizes) and the function (energy flow) of an ecosystem. The loss of sensitive species and
genotypes results in simplified ecosystems that are less predictable and demonstrate lowered resiliency
and lowered ability to rebound from subsequent perturbations.
Other disturbances at the ecosystem scale and beyond may be associated with the occurrence of
chemically adapted fish. Fish inhabiting contaminated sites can become vehicles for the transfer of toxic
chemicals through the food web. This may be particularly important with persistent, lipophilic contam-
inants (e.g., organochlorine pesticides and DLCs) because of their tendency to accumulate in fish.
Although trophic transfer is not restricted to adapted populations of fish, adapted individuals may be
capable of accumulating and transferring higher levels of toxicants than nonadapted individuals (Figure
13.18). Transfer of chemicals from adapted individuals to nonadapted individuals can be lethal. In one
study, for example, insecticide-resistant sunfish tolerated a diet of insecticide-resistant mosquitofish
containing concentrations of endrin that killed control sunfish fed the same diet (Ferguson et al., 1966).
In another study, a single resistant mosquitofish eliminated enough endrin into 10 liters of water to kill
all control mosquitofish maintained in that water (Culley and Fergusen, 1969). Trophic transfer can
result in the bioaccumulation of contaminants in top predators, fish-eating birds, and other wildlife that
use the ecosystem as a feeding ground.
Recent concern has arisen over the acceleration in the evolution of drug-resistant disease organisms
and pesticide-resistant insects (Palumbi, 2001). Documented cases of toxicity resistance in fish appear to
primarily involve populations of nonmigratory fish species limited to specific highly contaminated sites;
however, considerable movement and breeding between local populations of fish is expected. Increases
in the frequencies of resistance genes could therefore increase the frequency of deleterious alleles in the
gene pool of nonadapted populations. In addition, it is likely that fish inhabiting moderately contaminated
sites or migratory species that pass through contaminated areas seasonally may experience contaminant-