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BULLETIN OF THE BUREAU OF FISHERIES 



and (4) that at station No. 14, above Winona (situated about 110 miles below St. 

 Paul) very little, if any, pollution was present, for conditions here compared favorably 

 with those at station No. 1, situated above the polluted areas at Minneapolis. 



7. Fewer species of bottom forms were taken in the more polluted waters (about 

 6 species) than in the less polluted or unpolluted waters (about 20 species). 



8. I found that no correlation existed between the total number of plankton 

 individuals and the degree of pollution in the upper Mississippi River system, and 

 therefore the abundance of plankton can not be employed as a criterion of the 

 degree of pollution. 



9. My samples show that, on the whole, the character of the phytoplankton 

 and zooplankton changes very little with the degree of pollution in the river; the 

 plankton organisms that are most abundant in the unpolluted waters are, in general, 

 also most abundant in the grossly polluted waters. 



10. The hydrometric data show that the discharge of the Mississippi River 

 and of its tributaries varies considerably during the year, the rate alternating in 

 cycles of minimum and maximum flow. The periods of low water occur sometime 

 during midsummer (July and August) and midwinter (January and February), the 

 periods of high water sometime during March, April, and May and during September 

 and October. 



11. The data on seine hauls show (a) that fish were abundant at stations 1 

 and 2, on the Mississippi River at the beginning of the polluted metropolitan area; 

 at stations 4 and 8, on the Minnesota and St. Croix Rivers, respectively; and in 

 the Mississippi River about a mile below the mouth of the St. Croix, but (6) very 

 scarce, if present at all, in the section of the Mississippi River that extends from 

 station No. 5 (just below St. Paul) to the St. Croix River, a section about 39 

 miles long. In 1926, commercial fishing in the Mississippi River commenced about 

 the latter part of August in the vicinity of Red Wing and about the middle of Sep- 

 tember in the vicinity of St. Paul. 



12. From a study of dissolved-oxygen concentration, it was concluded (a) that 

 no species of fish could live continuously in the Mississippi River between stations 

 3 (in the metropolitan area) and 7 (Hastings) during August and the first week of 

 September, or at station 9 (Red Wing) during the first three weeks in August; (6) 

 that the more tolerant fish could live at station No. 9 after the third week in August; 

 and (c) that virtually any fish could five, in so far as oxygen is concerned, at the stations 

 not included in the above during August and September, and (d) at all stations, 

 polluted and unpolluted (with the possible exception of 7 and 9), during the high- 

 water stage after the heavy rains in September. 



13. At stations where the dissolved-oxygen content is high, it was shown that 

 the dominant bottom animals are clean-water forms, the tolerant bottom forms are 

 relatively scarce, the tolerant planktons are absent or sparse, and fish are numerous 

 and of many species; and, vice versa, at stations where the oxygen concentration 

 is low the dominant bottom forms are tolerant, clean-water forms are absent, the 

 tolerant planktons are relatively very abundant, and there are virtually no fish. 

 From these facts it was concluded that the absence or scarcity of fish in the upper 

 Mississippi River during August, 1926, was due primarily to the pollution from 

 Minneapolis and St. Paul. 



