384 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 3 



Table 9. — Number of fish used in a study of the growth of Onized Lake fish. 



Kind 



Largemouth bass 

 Black crappie. . . . 



Bluegill 



Warmouth bass . . 

 Green sunfish . . . . 



Yellow bass 



Carp 



Golden shiner. . . . 

 Total 



mutilated or decayed were discarded. Most 

 of the larger fish were measured and 

 "scaled," and random samples were taken 

 from among the other, more numerous 

 individuals of all species, so that the scale 

 collections are believed to represent fairly 

 all broods of all species then present. In 

 this, as in other scale collections, the scales 

 from a small number of fish had to be dis- 

 carded, because all these scales were regen- 

 erate, or the age determinations were un- 

 certain. The fish that were used in this 

 study are listed in table 9. 



A monograph on the growth of an indi- 

 vidual species of fish should show the rela- 

 tionship between the growth increments 

 of body (length) and the growth incre- 

 ments of a selected scale measured in a 

 designated manner, in order to expose any 

 disproportionate growth increments of 

 scales that may require correction of 

 body lengths calculated from scale meas- 

 urements. This time-consuming process is 

 hardly applicable to gross population 

 studies involving many kinds of fish, par- 

 ticularly where the fish are killed with poi- 



son and must be handled in the least pos- 

 sible time to prevent loss from spoilage. 

 All calculated lengths given in this study 

 are direct proportions based upon measure- 

 ment of the anterior radius of the scale 

 and the total length of fish. Even though 

 errors are introduced through this method, 

 they are probably of small magnitude, and 

 calculated lengths of fish from various 

 waters, when handled in this way, are be- 

 lieved to be comparable. 



Tables showing the age-length frequen- 

 cies for each of the more abundant kinds 

 of fish in Onized Lake at the time of the 

 poison census are included with the fol- 

 lowing discussion. As the actual ages 

 were determined on random samples, 

 rather than on all fish, a description of the 

 procedure is given below. When the poi- 

 soned population was handled, fish of legal 

 lengths (bass) or of desirable sizes (other 

 species) were carefully separated from the 

 more numerous smaller individuals. Be- 

 cause of the relatively small numbers of 

 large individuals, scales were taken from 

 a high percentage of them. Scales were 



Table 10. — Fish recovered from Onized Lake in the final census, June 24-28, 1941, and group- 

 ings used in the construction of age-length frequency tables. 



