Peoria Lock and Dam were knotheads, whereas in 1973 and 1974, the in- 

 cidence had dropped to about 10 percent. Thompson (1928) theorized that 

 knothead was a rachitic disease, possibly caused by lack of vitamin D, 

 like rickets in mammals. 



At Sugar Creek Island (mile 94.3-95.2), carp with an abnormality in 

 coloration were frequently taken. The bronze color seemed to be missing 

 and the fish appeared predominantly pink, purple, and light yellow. The 

 pink muscles overlying the anterior and ventral edges of the operculum 

 could be seen through the skin and scales. The fish appeared to be in 

 good health. Hansen and Shoemaker (1943) found that color-deficient 

 carp comprised the following percentages of their total carp catch in 

 1942; 2 percent at Meredosia (mile 71.0), 5 percent at Browning (mile 

 97.0), and 2 percent at Havana (mile 120.0). Commercial fishermen re- 

 fer to these fish as "chicken carp." The cause of this color variation 

 is unknown. 

 Minnows, Family Cyprinidae 



The electrof ishing survey was not designed to sample minnow popula- 

 tions, although some minnows were taken sporadically. The emerald 

 shiner, Notropis atherinoides , was abundant throughout the river — 

 hundreds would often be driven ahead of the electrof ishing boat until 

 they reached shallow water, where they would be stunned and lie strewn 

 over the bottom. 

 Carpsuckers (Carpoides, spp.) 



Identification of the species is often difficult because they are 

 extremely variable in their morphological characters, and to compound 

 the difficulty, they probably hybridize (Trautman, 1957). The quillback 

 carpsucker, Carpiodes cyprinus , has no dentary nipple at the tip of the 

 lower jaw, and identifications of this species in the course of the 

 electrof ishing survey were probably accurate. Both the river carp- 

 sucker, Carpiodes carpio , and the highfin carpsucker, Carpiodes velifer , 

 possess dentary nipples. The anterior dorsal fin rays of the highfin 

 carpsucker are drawn out into a long point, in contrast to the shorter 

 dorsal fin of the river carpsucker, and if the long rays are present, 

 the fish can be positively identified as a highfin carpsucker. 



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