stream. The bluntnose minnow, fig. 10, which 

 showed a general increase in numbers up to 

 Division 6, dropped off in the last two divisions; 

 it had a distribution very similar to that of 

 the longear sunfish. 



Figs. 12-17 present an interesting picture of 

 the succession of fishes in Jordan Creek. The rock 

 bass, fig. 12, the stonecat, fig. 13, and the brindled 

 madtom were present in only the fast, rock-bottomed 

 pools of the lower area and showed a general de- 

 crease in the upstream stations of this area. 



The common shiner, fig. 14, was present in all 

 eight divisions but showed a general decrease of 

 weight and number in the upstream divisions. Al- 

 though we are more interested in the weight than in 

 the number of a species, an interesting correlation 

 (r =0.78) was found between the number of small- 

 mouth black bass and the number of common 

 shiners. In Illinois the common shiner has a 

 pattern of distribution similar to the patterns of the 

 smallmouth and the hog sucker; all are scarce in 

 the southern part of the state and increase 

 in abundance northward. 



The smallmouth black bass generally is 

 found in smaller streams than the largemouth, and 

 when these species occur together the smallmouth 

 is usually more abundant upstream than the large- 

 mouth. In Jordan Creek this situation was reversed; 

 the largemouth was taken in only the upper area, 

 fig. 15, and the smallmouth more abundantly in the 

 lower, fig. 6. As was mentioned previously, Jordan 

 Creek is unusual in that the upper area has more 

 characteristics typical of larger streams— slower 

 currents, larger pools, and softer bottoms-than has 

 the lower area. To some extent this situation has 

 caused for some of the fish a reversal of the dis- 

 tribution typical for a stream of the size of 

 Jordan Creek. 



The sand shiner, fig. 16, silverjaw minnow, 

 fig. 17, and spotfin shiner were much more abundant 

 in the upper area than in the lower. The 

 distributions of the numbers of these three species 

 were correlated. The silverjaw minnow and the 

 sand shiner had a correlation coefficient of 0.96, 

 which was the highest correlation calculated. The 

 spotfin had a correlation coefficient of 0.75 with 

 the silverjaw, 0.67 with the sand shiner. These 

 associations probably are explained by the fact 

 that these three species have a preference for the 

 soft bottoms of sand and silt in the upper area. 



The general pattern of distribution of the 

 white sucker, fig. 18, was bimodal, with a low in 

 Division 5 and peaks in Division 4 and Division 6. 



The general distribution of the bluegill, tig. 19, 

 was somewhat similar to that of the white sucker. 

 However, bluegills usually were collected near the 

 banks with cover, whereas white suckers were 

 collected in the open water of the pools. The green 

 sunfish, fig. 20, reached its peak of abundance in 

 Division 5, in which the bluegill and the 

 white sucker were low in abundance. The white 

 sucker had a higher correlation coefficient with 

 the smallmouth black bass than did the green 

 sunfish. 



In comparing the distribution of the stoneroUer 

 with that of the white sucker, figs. 21 and 18, it 

 was discovered that in the divisions of the stream 

 where the number and weight of the stoneroller were 

 high those of the white sucker were low, and vice 

 versa. However, these two bottom feeders probably 

 do not compete in their habitats, since the stone- 

 roller was found in faster and more shallow water 

 than the sucker. The yellow bullhead, fig. 22, had 

 an irregular distribution in Jordan Creek, with only 

 a general increase in abundance in the upstream 

 stations. The large adults of this species were 

 found at the edges of pools where the water was 

 deep and where the banks were covered with grass 

 or roots. The young of the golden redhorse, fig. 

 23, showed a definite preference for the softer 

 bottoms of the upper area. The correlation 

 coefficient for the weights of the golden red- 

 horse and the smallmouth black bass was 

 not significant. 



In relative abundance, the greenside and rain- 

 bow darters were replaced upstream by the fantail 

 and the orangethroat darter, fig. 24. Trautman 

 (1930) noted differences in habitat preferences of 

 the rainbow darter and the orangethroat darter. 



In the Jordan Creek study section, the rainbow 

 had a correlation coefficient of 0.76 (significant 

 at the 5 percent level) with the greenside, of -0.85 

 with the fantail, of -0.55 with the orangethroat, a 

 species very similar in appearance to the rainbow 

 and often confused with it. The greenside had a 

 correlation coefficient of -0.81 with the fantail, of 

 -0.60 with the orangethroat. The orangethroat had 

 a correlation coefficient of 0.88 with the fantail. 



The rainbow and the greenside showed a pref- 

 erence for the larger and faster rapids, whereas 

 the orangethroat and the fantail showed a pref- 

 erence for the smaller and slower rapids. 



The associations and succession of darters 

 are due to steep, swift rapids with large boulders 

 in the lower area and less turbulent, gravel riffles 

 in the upper area. 



■20 



