Page 681 - The Toxicology of Fishes

Fish Toxicity Studies 661

• To meet regulatory requirements to regulate development, manufacture, or release
• To determine chemical classifications (and rankings)
• To develop numerical water and sediment quality criteria

The toxicity test method and design selected depend on the specific questions to be addressed. Toxicity
test programs usually focus on one or more of the following questions:

• Is the substance lethal to test organisms and at what concentration?
• What are the effects on organisms exposed to low, sublethal concentrations during part or all
of its life cycle?
• What are the effects of short-term “slug” or “spike” exposures (or episodic events)?
• Which chemical is most toxic?
• What indigenous organism is most sensitive, and what is the distribution of sensitivities?
• What are the environmental conditions that make the chemical most toxic? Interaction with
environmental variables?
• Is the toxicity of the chemical similar in laboratory and field?
• What are realistic environmental exposure concentrations, and what is the safe concentration
of the chemical?
• What are the effects of mixtures of chemicals?

Laboratory toxicity tests with juvenile fish are typically conducted with single-species exposures, and
most often only a few standard fish species are studied for a single chemical in an aquatic toxicity
program. The results of a limited number of standard test species are often used to extrapolate to a vast
number of indigenous species making up an aquatic community. Indigenous species are most often used
in toxicity testing on a site-specific basis when testing effluent discharges, when potential hazards are
predicted, or in monitoring programs. Indigenous species are typically not used in testing industrial
chemicals or pesticides. Aquatic toxicity programs, based on their objectives, should incorporate the use
of multiple, indigenous fish species from a range of behavioral strategies.
Identifying the toxicological effects of a chemical on individuals in the laboratory are most often
easier to determine than the effects on communities because in nature effects are regulated by the
physicochemical conditions of systems that can enhance or inhibit toxic impacts on organisms. Chemicals
can act in concert with other chemical and nonchemical stressors. Organisms in natural systems may
also be physiologically or immunologically compromised prior to chemical exposure. This is common
today in many freshwater and saltwater systems throughout the world because of prior widescale
contamination of organochlorine compounds, such as dichlorodiphenyltrichloroethane (DDT), dichlo-
rodiphenyldichloroethylene (DDE), and dichlorodiphenyldichloroethane (DDD). Furthermore, species
may differ in the uptake, accumulation, distribution, metabolism, and excretion of chemicals (see
Chapters 2 to 4). Species differences are another reason why extrapolation to communities is difficult.
Our ability to predict ecotoxicological effects from single species tests is rudimentary; however, our
potential to predict ecotoxicological effects on communities will be expanded when studies evaluate
realistic interactive effects. Another approach in testing is to study the effects of realistic environmental
exposure concentrations and then define the range of the lowest exposure concentrations that produce
adverse responses for indigenous species and examine the extent to which environmental conditions
alter minimum exposures and different responses.
Single-species fish tests are technically an initial step to study the direct effects of single chemicals,
effluents, and mixtures of chemicals. Direct toxic effects on individual fish refer to lethality and to
biochemical, physiological, and behavioral changes that may result from exposure and affect function.
Indirect (or secondary) effects are more complex to assess. Indirect effects are those effects that occur
as a result of a direct effect on the reduction or elimination of susceptible species (prey or predator) and
may include disturbances in biological interactions and processes. Indirect effects may also occur as a
result of a direct effect on the physical or chemical environment; for example, a chemical that decreases
predator abundances may increase prey abundance and competition. This result may be classified as a