THESIS APPROVAL

(LC50). However, death is not easily determined for some invertebrates, the median
effective concentration (EC50) is estimated rather than the LC50. The effect used for
estimating the EC50 is immobilization defined as lack of movement, loss of
equilibrium defined as inability to maintain normal posture (Parrish, 1985).
2. Sublethal Effects Tests
These tests are designed to determine the toxic effects of a chemical to
selected aquatic organisms exposed to low concentrations of the test material. In
general, organisms are not usually exposed to high toxic concentration of chemicals in
the aquatic environment unless they are restricted to the vicinity of a chemical release
site. Thereby, a greater biomass is exposed to sublethal concentrations of chemicals
than to acutely toxic lethal concentrations (Rand and Petrocelli, 1985). Sublethal
effects may be studied in the laboratory by a variety of procedures. The most
common effect criteria are behavioral changes, physiological changes, biochemical
changes, and histological changes. Some sublethal effects may indirectly result in
lethality, but they may have a profound effect on the future survival of the organisms.
Behavioral effects that have received considerable attention in aquatic organisms are
locomotion and swimming, attraction-avoidance, prey-predator relationship,
aggression and territoriality, and learning (Rand, 1985). Physiological and
biochemical tests include studies of enzyme inhibition, clinical chemistry,
hematology, and respiration (Mehrle and Mayer, 1985). Finally, histological studies
are also useful to determine changes in histological structure which may often
significantly modify the function of tissues and organs (Meyers and Hendricks, 1985).
3. Chronic Toxicity Tests
Sometimes a chemical does not have adverse effects on aquatic organisms in
acute toxicity tests. Chronic toxicity tests permit evaluation of the possible adverse
effects of the chemical under conditions of long-term exposure at sublethal
concentrations. In a chronic test, the test organism is exposed for an entire
reproductive life cycle. The exposure is generally initiated with an egg and continues
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