etc.). The solution is well-known but too often neg- 

 lected — roadside plantings to stabilize the soil and, 

 in agricultural areas, contour plowing, and other soil 

 conservation methods. 



Effects of chemical wastes. — Chemicals usually 

 affect aquatic organisms by their direct toxic action, 

 though there may be secondary effects such as ex- 

 tremes of pH or changes in osmotic pressure. Attempts 

 have been made to establish precise tolerances for 

 various chemical substances, based largely on their 

 toxicity to fish. However, such figures have been 

 shown to be extremely misleading (Doudoroff and 

 Katz, 1950, 1953) because "the minimal harmful 

 concentration of a toxic substance may vary greatly, 

 depending on the duration of the test, the species 

 and age of the test animals, the dissolved mineral 

 content of the water used as a solvent or diluent, 

 the concentration of other waste components having 

 a pronounced synergetic or antagonistic effect, the 

 temperature, and other factors." Under the circum- 

 stances, safe concentration limits of toxic wastes 

 were not definitely prescribed. 



Doudoroff and Katz did conduct extensive tests, 

 however, using the method of "bioassay" or exposure 

 of known concentrations to fish. Their conclusions 

 are too voluminous to repeat here but a few points 

 may be listed: (1) pH values above 5.0 and ranging 

 upward to 9.0, at least, are not lethal for most fresh- 

 water fishes; (2) none of the strong alkalies which 

 are important as industrial wastes (NaOH, CA(OH) 2 , 

 and KOH) has been clearly shown to be lethal to 

 fully developed fish when its concentration is insuf- 

 ficient to raise the pH well above 9.0; (3) solutions 

 of ammonia, ammonium hydroxide, or ammonium salts 

 can be very toxic to fish even when the pH is not 

 very high (that is, below pH 9.0); (4) the common 

 strong mineral acids (that is, H 2 S0 4 , HC1, and HNO } ) 

 and also phosphoric acid (H 3 POJ and some moderately 

 weak organic acids apparently can be directly lethal 

 to fully developed fish only when the pH is reduced 

 thereby to about 5.0 or lower; (5) a number of weak 

 inorganic acids (such as carbonic, tannic, etc.) can 

 cause pronounced toxicity without lowering the pH 

 as low as 5.0; (6) the susceptibility to free carbon 

 dioxide varies greatly; for example, sensitive species 

 may succumb rapidly at free C0 2 concentrations 

 between 100 and 200 ppm in the presence of much 

 dissolved oxygen; (7) solutions of hydrogen sulfide, 

 free chlorine, cyanogen chloride, carbon monoxide, 

 and ozone all are extremely toxic and have been 

 reported as lethal to sensitive fish in concentrations 

 near 1 ppm or less; (8) all metal cations can be toxic 

 in rather dilute (less than 0.05 m) physiologically 

 unbalanced solutions of single metal salts; (9) sodium, 

 calcium, strontium, and magnesium ions are among 

 the least harmful of the metallic cations; (10) silver, 

 mercury, copper, lead, cadmium, aluminum, zinc, 

 nickel, and univalent chromium, and perhaps also tin 

 and iron, can be classed as metals of high toxicity; 

 (11) cupric, mercuric, and silver salts are extremely 

 toxic. 



These are, of course, generalizations and would 

 differ not only for each species of fish but also for 



35 



Usinger: Introduction 



other kinds of aquatic orgai including 



In some cases insecticides are regarded as pollu* 

 tants or U)xic- agent iron, the viewpoint of fish 

 production. Either qui to larvicidee applied 



directly to tho water or as application- on agricultural 

 crops that run off or arc washed into Btreamfl or I 

 these materials are a threat to aquatic re OUI 



Fortunately, the conflicting interests of agriculturi i-, 

 mosquito control agencies, and sport fishermen can 



be resolved in most cases l>\ judicious choice of 

 materials, careful timing of applications, and adjust- 

 ment of concentrations to fit the tolerance-- of fish 

 and fish-food organisms. (See discussion under mos- 

 quito control.) 



Tolerances to chemicals (except insecticides) have 

 not been studied for many insects but it has been 

 observed that the copper sulfate treatment for reduc- 

 tion of algae in swimming pools has no apparent 

 effect on insects. On the other hand, chlorine at the 

 concentrations used for purification of drinking water 

 may produce a residual that is toxic to aquatic- 

 insects and fish; hence treated water cannot be used 

 safely for rearing insects in the laboratory unless it 

 is detoxified by boiling, filtering, or by holding in 

 a container for twenty-four hours. 



Effects of organic wastes* — The effects of organic 

 pollutants are more complex than those of mechanical 

 or chemical agents. In general, the action of bacteria 

 on organic material causes a deficiency of oxygen. 

 Patrick (1953) has described the process as follows: 

 "These organisms use the complex wastes ... as a 

 source of energy in their metabolism. In so doing they 

 break down the wastes into substances that can be 

 used as a source of food by other organisms. These 

 processes, which are often referred to as decay or 

 decomposition, occur most rapidly when the bacterial 

 population is of optimum size. When the bacteria 

 become too numerous the processes are slowed down. 

 The protozoa and other small invertebrates which 

 feed on bacteria are instrumental in keeping the 

 bacterial populations in check. The algae are also 

 at the base of the food chain. They are able to utilize 

 inorganic substances to make proteins and carbo- 

 hydrates, which are used as a source of food by 

 other organisms. Indeed, algae have often been 

 referred to as the grasses of the sea. Upon them not 

 only the many different invertebrates, but also some 

 fish and other vertebrates, feed directly. Besides 

 their value as a source of food they also replenish 

 the oxygen supply ... by photosynthesis." 



Insects fit into this complex picture in diverse 

 ways. When exposed to organic pollution in a stream 

 they follow a typical pattern (intro. fig. 51). Imme- 

 diately below a sewage outfall a septic zone develops 

 with turbid water and noxious odors. The bottom is 

 coated with zoogloea and inhabited by tubificid worms. 

 Oxygen is low or absent. The bacterial count is high 

 and plankton organisms include Oscillatoria and 

 Sphaerotilus. Fish are absent. The insect fauna is 

 limited to larvae which breathe surface air through 

 tubes — Tubifera (Eristalis), Culex. Below the septic 

 zone is the zone of recovery with clearer water, 

 cleaner bottom, and more dissolved oxygen. Here 



