have not been shown, however, to be absolutely necessary for achieving the 

 not very ambitious objective of the practical investigations proposed here. 



The maximum concentration of a toxic pollutant that impairs neither the 

 appetite nor the food-conversion efficiency of the experimental fish so 

 much that their growth under natural conditions must be judged likely to be 

 seriously affected having been determined, what should be the next step? 

 May we now assume that this concentration can affect the growth of the fish 

 adversely only if it causes a reduction of the natural food supply? This 

 assumption is not usually justifiable, because an impairment of water 

 quality can reduce also the activity of animals and, consequently, their 

 success in seeking, pursuing, and capturing their prey, as well as in 

 eluding their enemies. Experiments have shown that exposures of salmonid 

 fishes to exceedingly low concentrations of sodium cyanide, for example, 

 has a very pronounced, rapidly produced, and lasting effect on their swim- 

 ming ability. Though not known to be otherwise demonstrably affected, they 

 become unable to resist currents of moderately high velocity nearly as long 

 as individuals not exposed to the cyanide (controls). Such effects may or 

 may not interfere with normal feeding activities. In experiments with 

 other fish that proved less susceptible, Cichlasoma bimaculatum , Leduc 

 (1966) found that the duration only of moderately rapid swimming, and not 

 that of very rapid swimming, was reduced by exposure of the fish to low 

 cyanide levels. A reduction of maximum swimming speeds sustainable for 

 long periods of time cannot be said obviously to impair the foraging 

 efficiency of fish; only short bursts of speed in the pursuit of prey are 

 commonly observed and are clearly essential to successful feeding of many 

 species. Still, any interference by toxic substances with the ability of 

 fish to exert themselves may, in some subtle way, cause food consumption 

 under natural conditions to decline, thus reducing growth. Some poisons 

 may interfere mainly with very rapid swimming. 



In experiments with artificial, concrete-lined ponds stocked with known 

 numbers of mosquitof ish, Gambusia af finis , and several largemouth bass, 

 Micropterus salmoides , which fed on the mosquitof ish, the food consumption 

 and growth rates of the bass were decidedly reduced when dissolved oxygen 

 concentrations were reduced (Brake, 1972; Warren, Doudoroff, and Shumway, 

 1973). The bass consumed fewer of the mosquitof ish and grew less in a pond 

 with a moderately reduced oxygen concentration than in a control pond, even 

 though laboratory tests had shown that they were capable of consuming much 

 more food and of growing much faster at the same oxygen concentrations and 

 temperatures when the food was more available. The mosquitof ish were pro- 

 vided with artificial cover (rolls of wire netting placed in the shallow 

 water near the periphery of each pond) and were not easily caught by the 

 bass. Consequently, the food-consumption and growth rates of the bass even 

 at high oxygen concentrations were dependent on the density of the prey, 

 that is, they increased when the number of mosquitofish placed in the ponds 

 was increased. Aquarium tests had shown that the appetite of the bass, or 

 the amount of food that they are able to consume when the supply of food is 

 unlimited and the prey easily captured (not protected) is reduced at 

 moderately low oxygen concentrations just as the food consumption in the 

 ponds was reduced (Lee, 1969; Warren et al . , 1973). The impairment of 

 appetite could have been somehow partly responsible for the reduction of 



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