POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



343 



was identified; the eflFect of this factor on 

 population growth form was estimated; 

 and the physiological channels through 

 which the factor operates were assayed. 



Oxygen 



The minimal oxygen concentrations toler- 

 ated by natural populations of fresh-water 

 fishes have been investigated by Moore 

 (1942) in five Minnesota lakes under sum- 

 mer and winter conditions. During the 

 summer he found that, in general, when 

 oxygen concentrations reached 3.5 ppm 

 or lower at temperatures of 15° to 26° C, 

 most of the eight species" examined died 

 within twenty-four hours. On the other 

 hand, concentrations of 5.0 ppm or higher 

 were completely adequate under the con- 

 ditions of observation. This minimal thresh- 

 old is lower during the winter. Of twelve 

 species! tested at 0° to 4° C, the reduced 

 oxygen supplv must reach 2.0 ppm before 

 fatalities result, and the fish can withstand 

 this concentration for forty-eight hours. 

 Reduction of the oxygen to 1.0 ppm is 

 lethal for all the fishes except for an occa- 

 sional Ameiurtis melas. Concentrations 

 above 3.0 ppm are adequate for all species 

 during the winter season. 



In both summer and winter the small 

 forms disnlay a greater tolerance of reduced 

 oxygen than the larger forms. In this con- 

 nection Moore reports an interesting obser- 

 vation made at North Farms Reservoir, 

 VVallingford, Connecticut, where conditions 

 of high temperature, quiet water surface, 

 a large \\'ater-bloom in process of decay, 

 shallow water, and probably a high organic 

 content of bottom mud, all reacted together 

 to bring about almost complete oxygen de- 

 pletion. This resulted in a truly soectacular 

 mortality among the fish ponulations. It 

 was estimated that over 400.000 fishes died 

 in this small lake (less than 150 acres) be- 

 cause of oxygen starvation. G. E. Hutchin- 

 son± observed, however, that many individ- 



• The species studied were Esox hicitis, Huro 

 salmoides, Pomoxis sparoides, Aplodinotus 

 p,runniens. Eupomotis gihhosus, Perca ftaves- 

 ccn-f, Helioperra macrochira, and Amehirus 

 meJa.i. 



f The species studied were the same as those 

 above, excluding Aplodinotus drtinmens and 

 including five additional forms: Amhloplites 

 rnvesfris, Anomotis cyancUus, Allotia hnmilis, 

 Fundtdti!} diaphanua, and Nofemi<iontis cryso- 

 lettcas. 



t Unpublished material. 



uals of two of the smaller species (Eupo- 

 motis 0hhosus and Notropis bifrenatus) , 

 all of which were less than 32 mm. in 

 length, continued to swim actively at the 

 lake surface at the time when most of the 

 forms were dying at such a great rate. He 

 suggested that these individuals were able 

 to survive because a thin film of oxygen- 

 rich water at the surface was available and 

 the fish escaped death by making use of 

 this supply. Moore's study provides an 

 illustration of the density-independent ac- 

 tion of reduction of oxygen, which some- 

 times actually becomes a source of mor- 

 tality for natural aquatic populations. 



A number of the general ecological re- 

 lations of oxygen were discussed in Section 

 II (p. 192). 



Carbon Dioxide 



Davidson (1933) reports a most inter- 

 esting, though somewhat controversial, il- 

 lustration of excessive fish mortality prob- 

 ably caused by a sudden rise in carbon 

 dioxide pressure. He was working around 

 sunset along a salmon stream in Alaska in 

 early August when suddenly many salmon 

 (Onchorhynchus ^orbiischa), trout, and 

 freshwater bullheads turned belly up, as if 

 suffering from suffocation, and then died 

 "as though some immediate death dealing 

 substance had been thrown into the 

 stream." The total salmon population con- 

 sisted of 80,000 fishes, 5000 of which suc- 

 cumbed. 



When the fishes were dying there was 

 no wind at all and the water was still. As 

 the sun sank behind a mountain, the air 

 chilled perceptibly. Davidson and some 

 colleagues made certain environmental 

 measurements in the stream at the death 

 locality and downstream from it. The pH 

 readings were instructive: at the place 

 where the fishes were dying the fH was 

 5.6 at 65° F., while in th-^ wat^r below the 

 spot it was 6.1 at 65° F. The fixed carbon- 

 ates were the same in both areas, as was 

 the specific gravity. 



After about thirty minutes, durintr which 

 time the fishes were dvincf. a cool wind 

 came up, the salmon and other species re- 

 covered, and those remaininp' started to 

 mill about in characteristic fashion. David- 

 son believes that the lack of air movement 

 formed a temporary air blanket over the 

 stretch of the stream where the 80.000 sal- 

 mon were swimming. Since the pH of the 



