164 



BULLETIN OF THE BUREAU OF FISHERIES 



forms, or the tolerant bottom forms are relatively scarce, and the presumably 

 tolerant planktonts (Pleurosigma spencerii, Lyngbya sp.? and Rotifer sp.?) are 

 absent or sparse, have many fish and many species of fish; and, vice versa, those 

 stations at which the oxygen concentration is low, the dominant bottom forms are 

 tolerant, clean-water forms are absent, and the tolerant planktons are relatively 

 very abundant, have practically no fish. Table 13 shows, beyond any doubt what- 

 soever, that the absence or scarcity of fish in August in that section of the Mississippi 

 River that extends from the beginning of the metropolitan area of the Twin Cities 

 to Prescott, Wis. (a distance of approximately 39 miles), is due to the pollution from 

 the former cities. 



Table 13. — Relationship between the amount of dissolved oxygen, the character of the bottom fauna, 

 and the abundance of the tolerant planktonts Pleurosigma spencerii, Lyngbya sp. ?, and Rotifer, 

 and the estimated number of fish per seine haul and the approximate number of species at stations 

 1 to 8, No. 3 excepted 



Station 



1 



2 



4 



5 



6 



7 



8 



Parts per million dissolved oxygen, average 

 for August 



6.59 



6.08 



5.70 



0.87 



0.51 



0.39 



7.10 



Character of bottom fauna 



Abundance of Pleurosigma spencerii, aver- 

 age number per liter 



Clean- water 

 forms dom- 

 inate; tubi- 

 ficids few. 



0 



31 

 .5 



i 1, 000? 

 > 14? 



Few midge 

 larvae only. 



Few tubifi- 

 cids only. 



1,600 



0 



1.2 

 330? 

 24? 



Tolerant forms 

 abundant; 

 no clean- 

 water forms. 



17,070 



1,276 



54. 1 

 1/3 

 1 



As in 5. 



16, 045 



8,535 



169 

 0 



0 



As in 5. 



16, 150 



2, 120 



111.5 

 0 

 0 



As in 1. 



0 

 0 

 0 



330? 

 17? 



Abundance of Lyngbya sp. 1, average num- 

 ber per liter 



Abundance oi Rotifer sp. ?, average number 

 per liter.- .- 



Average number of fish per seine haul 



Number of fish species per seine haul 



430 

 0 



750? 

 19? 



1 Questionable values were estimated. 



• Questionable values indicated that the species thrown back and not identified are not included. 



It may be emphasized here that, so far as our data show, the distribution of fish 

 in the polluted upper Mississippi River is primarily — probably entirely — limited by 

 the amount of oxygen present in the waters. If toxic chemical ingredients are 

 present in sufficient quantities to act as poisons, they may be the primary controlling 

 factors; but we have no direct information concerning this subject. The abundance 

 of plankton and tolerant bottom forms in the grossly polluted waters suggests that 

 chemical poisons, if present, are not sufficiently concentrated to destroy life outright. 

 The paucity in the variety of bottom foods in the polluted areas may be a minor 

 factor in the distribution of fish that subsist mainly on bottom fauna; but plankton 

 as food is not a factor in the scarcity of fish, for we have shown (p. 154) that the plank- 

 tonts are abundant, in individuals and in variety of species, in both polluted and 

 unpolluted waters. 



In conclusion, it should be stated that the data presented in this paper suggest 

 that the pollution of the upper Mississippi River is severe only (in so far as fish are 

 concerned) during the periods of low water — that is, sometime during midwinter 

 (January and February) and during midsummer (July and August). (See p. 142.) 

 From the point of view of conservation this is highly significant; it should be investi- 

 gated by continuous observations throughout a period of at least one year. 



