In the past 15 years the commercial fishery of the 

 middle river, from Hennepin to Beardstown, has shown 

 a sharp decline. The commercial yield in this stretch 

 has dropped from 5.07 million poimds in 1950 (Starrett 

 & Parr 1951:18) to 0.91 million pounds in 1964 (Star- 

 rett, Lopinot, & Harth, unpublished), a reduction of 

 4.16 million pounds. Because of the condition of the 

 water in the river, the commercial fishery above Hen- 

 nepin has been limited to the activities of one part-time 

 fisherman who fishes near the mouth of the Fo.x River 

 near Ottawa. 



No attempt will be made here to discuss the condi- 

 tion of the populations of all fish species living in the 

 river, but certain important species are included for 

 closer scrutiny. 



Carp 



Since carp is the only species of fish that occurs 

 abundantly in all sections of the river it has been used 

 in our studies as an indicator of the effects of pollution. 

 Much of the decline in the commercial catch since 1950 

 has resulted from the scarcity of carp of commercial 

 size (17 inches or more in total length) in the middle 



section of the river. Small carp are often abundant in 

 this section but most of them disappear before attaining 

 commercial size. The commercial catch of carp in the 

 Alton Pool has changed little since 1950. 



1 here are two noticeable effects of pollution on this 

 species. First, the length-depth ratio of individuals goes 

 up with increasing pollution. By dividing the depth into 

 the standard length, an index is obtained which, if 3 or 

 greater, indicates that the fish is too thin for commercial 

 uses. Any index under 3 would indicate a satisfactory- 

 commercial fish. Second, carp exhibit a rachitic bone 

 malformation (an abnormality characterized bv mal- 

 formed heads and gill covers) known as a '"knothead" 

 condition. This becomes more conspicuous (Fig. 8) 

 with increased pollution. 



Thompson (1928) found that carp developed the 

 knothead condition in association with the polluted 

 condition of the river during and following \Vorld War 

 I. Fig. 9 shows the percentages of the carp population 

 having the knothead condition in various parts of the 

 river, as reported by Thompson ( 1928:302) and in our 

 checks in 1963. Upstream from Peoria, conditions were 

 similar in 1926 and 1927 to those in 1963. The absence 



Fig. 8. — Knothead condition 

 in carp. Left, normal: center, 

 moderate knothead condition ; 

 right, extreme knothead condi- 

 tion. (From Thompson, 1928.) 



JOUET. 



Fig. 

 heads i 

 Illinois 

 (Thoin 

 (lllinoi 

 data 



9. — Percentage of knot- 

 n carp populations in the 



River in 1926 and 1927 



pson, I92fi) and in 1963. 



Natural History Survey 



BEARDSTOWN 



100 120 140 160 ISO 200 



MILES FROM MISSISSIPPI RIVER 



16 



