579 



toxicity at the individual, the population, and the community levels, 

 with emphasis on long-term exposure at sublethal concentrations. 



Food chain effects 



The outright poisoning of various organisms is easily recognized. 

 Perhaps an even more important problem is the effect of various toxi- 

 cants present in concentrations which reduce the populations of food 

 organisms to such a low level that they are inadequate for the forms 

 depending on them as a food supply. For example, fish such as salmon, 

 migrating downstream, will be feeding on their way to sea. If these 

 young fish are feeding on a subadequate food supply, then they must be 

 in a weakened condition for their subsequent journey in the ocean and, 

 thus, be more susceptible to attack by predators and disease than they 

 would be otherwise. Various kinds of pollution such as toxic materials 

 or heated effluents could cause such results. 



Another food chain effect is the phenomenon of biological magni- 

 fication. Biological magnification is an additional chronic effect of 

 toxic pollutants (such as heavy metals, pesticides, radionuclides, bac- 

 teria, and viruses) which must be recognized and studied. Many ani- 

 mals, and especially shellfish such as the oyster, have the ability to 

 remove from the environment and store in their tissues substances pres- 

 ent at nontoxic levels in the surrounding water. This process may con- 

 tinue until the body burden of the toxicant reaches such levels that the 

 animal's death would result if the pollutant were released into the 

 bloodstream by physiological activity. This may occur, as in the case of 

 chlorinated hydrocarbon pesticides (such as DDT and endrin) stored 

 in fat depots, when the animals's food supply is restricted and the body 

 fat is mobilized. The appearance of the toxicant in the bloodstream 

 causes the death of the animal. Equally disastrous is the mobilization 

 of body fat ito form sex products which may contain sufficiently 

 high levels of the pollutant so that the normal development of the 

 young is impossible. 



The biological magnification and storage of toxic residues of pollut- 

 ing substances and micro-organisms may have another after effect. 

 Herbivorous and carnivorous fish at lower trophic stages may grad- 

 ually build up DDT residues without apparent ill effect. Carnivorous 

 fish, mammals, and birds preying on these contaminated fish may be 

 killed immediately or suffer irreparable damage because of the pesti- 

 cide residue or infectious agent. 



A great deal of work must be done to determine the significance of 

 the phenomenon of biological magnification. We must develop tech- 

 niques for predicting potential hazards before toxic material is intro- 

 duced into the environment ; and poisons must be used so as to minimize 

 the possibility of biological magnification, 



BIOASSAY CRrrERIA 



Bioassay criteria are sorely needed to determine the effects of ther- 

 mal, domestic, and industrial wastes. Synergistic effects of pollutants 

 must also be known and understood. Once the tolerance limits of in- 

 dividual species for individual pollutants is known, the effects of com- 

 binations of various toxicants and physicochemical water character- 



