342 



Illinois N.atur.al History Survey Bulletin 



Vol. 28, Art. 2 



Table 15. — Correlation between size of drainage area (square miles) and the number of 

 species per collection and the number and wei^iht of fish taken per 100 square yards at 70* 

 stations in 1959. 



■One station, of the 71 used In related analyses, was omitted from the calculations in this table. 



collections from all of the drainage sys- 

 tems in the county showed an increase in 

 the number of species in a downstream 

 direction. When all of our collections for 

 the county were combined, table 14 and 

 fig. 13, we found a significant positive re- 

 lationship (r = 0.64. table 15) between 

 number of species and downstream direc- 

 tion. The average numbers uf species 

 taken in the areas farthest downstream in 

 both 1928 and 1959 were slightly below 

 the projected average numbers, possibly 

 because the sampling methods were not so 

 well adapted to the largest water areas as 

 to the smaller areas upstream. 



An increase in the number of species 

 in a downstream direction probably re- 

 sulted from the greater variety of habitats 

 associated with increasing stream size: 

 many stations in large streams included 

 units of small stream habitats. Thompson 

 & Hunt pointed out that only unspe- 

 cialized species can live under the widely 

 varying conditions of the small streams. 



Number of Fish and Stream Size. 

 — The nctiuil number of fishes per unit 

 area decreases downstream. Most of our 

 collections supported this hypothesis of 

 Thompson 5: Hunt. Among our collec- 

 tions, only those from the Kaskaskia 

 showed no definite inverse relationship be- 

 tween numbers of fish and stream size, 

 table 15. Our collections from all of the 

 streams in the county averaged together 

 revealed a definite decrease in the number 

 of individuals per unit area downstream, 

 fig. 13 and table 15. 



Fish Weight and Stream Size. — 



U ith this decrease of number of fishes 

 downstream there is a corresponding in- 

 crease in the average size of the individ- 

 uals, so that, other factors being equal, the 

 total amount of fish flesh per unit area 

 is probably almost constant. The average 

 size of the fish we collected fluctuated 

 greatly from station to station, even in 

 streams of similar size; however, the 

 average size of individual fish increased 

 generally in the downstream direction. 

 The downstream increase in average size 

 of individuals was influenced by the oc- 

 currence of a greater number of large 

 adults of large species (the suckers, cat- 

 fishes, basses) than was found upstream. 



In our collections, the weight of fish 

 flesh per unit area, as well as the average 

 size of fish, fluctuated greatly from sta- 

 tion to station, even in streams of similar 

 size. The correlation between fish weight 

 and stream size was low in each stream 

 and for the county as a whole, table 15. 

 However, the average numbers of pounds 

 of fish per 100 square yards were similar 

 enouiih in streams of different sizes, table 

 14, that when plotted, fig. 13, they lend 

 some support to the idea that total weight 

 of fish flesh per unit area is similar in 

 streams of different sizes. 



Frequenc>' Distribution and 

 Stream Size. — Fishes . . . exhibit fre- 

 quencies which vary u'ith stream size in a 

 very consistent and definite manner for 

 each species. The frequency distribution 

 of our fishes in relation to stream size is 



