BIOLOGICAL SURVEY OF THE UPPER MISSISSIPPI RIVER 



163 



respectively) that somewhere between station No. 7 (at Hastings, about 39 miles 

 below St. Paul) and station No. 9 (at Red Wing, about 50 miles below St. Paul) the 

 Mississippi River is recovering from its grossly polluted condition. 



It is precisely in that section of the Mississippi River (from stations 1 to 9) 

 where the oxygen content was decidedly low that fish were extremely scarce or absent 

 altogether (see p. 162). At the stations on the Mississippi River (Nos. 1 and 2), 

 where oxygen was plentiful, fish also were abundant. Fish were numerous also in 

 the relatively unpolluted tributaries — the Minnesota and the St. Croix Rivers. The 

 close correlation between the abundance of fish and the presence of oxygen is well 

 demonstrated by the results of the seining at a near station (No. 8, p. 162, fig. 1). 

 A seine haul made in the polluted waters of the Mississippi River proper took 

 one turtle only, whereas hauls made in the relatively unpolluted waters of the St. 

 Croix or in the partially polluted waters of the Mississippi, below the mouth of 

 the St. Croix, netted many fish. It is unfortunate that attempts to seine succeessfully 

 at station No. 9 met with failure, for it would be of great interest to know whether 

 the improvement in the condition of the river here was sufficient to permit fish to 

 live. The fact that one shiner was taken suggests that fish were present at this 

 station in August. 



Thompson (1925), who has made an extensive study of the oxygen requirements 

 of fishes in the Illinois River, writes: "It seems quite certain that dissolved-oxygen 

 concentration between zero and two parts per million will kill all kinds of fish. 

 Carp and buffalo have been found living in water showing as low as 2.5 parts per 

 million. As a rule, a variety of fishes was found only when there were four or more 

 parts per million, and the greatest variety of fishes was taken when there were nine 

 parts per million." 



If the findings of this author are applicable to the fish of the Upper Mississippi 

 River, an examination of the data on oxygen (Tables 3 and 4) shows (1) that no 

 fish whatsoever can live continuously in the waters at stations 3, 5, 6, and 7 during 

 August and the first week of September, or at station 9 during the first three weeks 

 of August; (2) that a limited variety of fishes (the more tolerant species) can live 

 at station No. 9 (Red Wing) after the third week in August; (3) that virtually any 

 fish can live, in so far as oxygen is concerned, at the stations not mentioned above 

 during August and September; and (4) at all stations, polluted and unpolluted, 

 with the possible exception of Nos. 7 and 9, during the high-water stage after the 

 heavy rains in September. 



These conclusions agree very well with the statements made on page 162, which 

 suggest that the commercial fish make their first appearance in the fall, during the 

 latter part of August, in the vicinity of Red Wing (station No. 9), and about the 

 middle of September farther up the river near St. Paul. 



The correlation between the abundance of the species and of the individuals of 

 species, and the characteristics of each station at which seine hauls were made, is 

 most striking. Table 13 shows the relationship between the amount of dissolved 

 oxygen, the character of the bottom fauna, the abundance of tolerant planktonts, 

 the estimated average number of fish per seine haul, and the approximate number 

 of species of fish. From this table it may be seen that the stations at which the 

 dissolved-oxygen content is high, the dominant bottom animals are clean-water 



