404 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 3 



illustrates the tendency of overfishing to 

 remove the larger and older fish. The 

 1941 census was made shortly after com- 

 pletion of the 1941 spawn, and, as the 

 1941 brood was composed of very small 

 fish, most of which were lost in the census, 

 this brood is represented as the pyramidal 

 base enclosed in dotted lines. When quad- 

 rilaterals representing the 1940 and 1939 

 broods are superimposed upon the theoreti- 

 cal pyramid, with the lines for the 1940 

 brood coinciding with the limits of the 

 theoretical quadrilateral for 1940, the lines 

 for the 1939 brood extend beyond the lim- 

 its of the theoretical quadrilateral for 

 the corresponding year. The figure 

 representing the 1938 brood shows that 

 this brood was only a small fraction of the 

 size of the 1939 brood and much smaller 

 than the theoretical quadrilateral for 

 1938; the effects of overfishing are thus 

 indicated. Older broods are represented 

 by a spearhead extending toward the apex 

 of the theoretical pyramid. 



Discussion 



All sources of evidence point to the 

 hypothesis that Onized Lake was severely 

 overfished. Yet every species of fish known 

 to be once numerous in the lake was pres- 

 ent in the final census, perhaps due to the 

 fact that fishing pressure drew its quota 

 from all kinds of fish present, rather than 

 from one or two of the more desirable 

 species. The many fishermen using the 

 lake represented a cross section of anglers, 

 their fishing gear running the gamut from 

 willow-pole to flyrod. The fish in this 

 lake were presented with a variety of baits 

 and hook sizes, attractive to all but the 

 smallest. 



It is possible that if the fishing intensity 

 had been directed toward a certain species, 

 to the exclusion of others, that species 

 might have been entirely eliminated. This 

 could be expected to occur, not directly 

 through intensive fishing, but indirectly 

 through the inability of reduced numbers 

 of spawn or young of the selected species 

 (because of fewer breeding adults) to com- 

 pete with the more numerous young and 

 adults of other species not affected by fish- 

 ipg. For all practical purposes, the lake 

 would have become "fished out" for the 

 selected species before all of the adults 

 had been caught, but it is rather unlikely 



that the few remaining could have pro- 

 duced enough young to compete with 

 young of the more numerous species. In 

 the 1941 census, the black bullheads and 

 yellow bass were only a few in number, 

 but in view of the fact that the hook-and- 

 line catch was made up of only small num- 

 bers of yellow bass and bullheads, and the 

 hoopnet collections of 1938 failed to indi- 

 cate abundant populations of these fish, 

 the conclusion must be reached that some 

 factors other than heavy fishing kept them 

 from becoming numerous. 



The black crappies of Onized Lake were 

 nearer extermination than any other spe- 

 cies known to be numerous in the past. 

 However, the creel census does not indi- 

 cate that particularly large catches of these 

 fish were made, fig. 5. Only during 1939 

 were appreciable numbers taken. In that 

 year the crappie catch ranked second to the 

 bluegill in numbers and well ahead of any 

 other species. In 1940, the total hook-and- 

 line catch of crappies was only 56; yet 

 hoopnets set during July of that year 

 caught 234 black crappies, more than twice 

 the number of bluegills and many times 

 the numbers of other fish caught, table 6. 

 The fact that only 56 crappies were taken 

 by anglers in 1940, and 31 in 1941, fol- 

 lowing a catch of 502 in 1939, suggests a 

 natural die-off of the older fish during the 

 latter half of the summer of 1940. The 

 only other evidence for the natural die-off 

 •assumption is that large numbers of crap- 

 pies were not present in the final census. 



All species of fish in Onized Lake from 

 which scales were studied showed poor 

 growth during the first season, and all but 

 one, the warmouth bass, grew at an excep- 

 tionally rapid rate throughout later years 

 of life. This situation can be explained 

 by assuming that the constant drain on the 

 adult fish population through heavy fishing 

 allowed increased success in spawning and 

 greater survival of young fish. Thus, 

 foods suitable for small fish were at a pre- 

 mium, and early growth was slow. How- 

 ever, once the young fish reached sizes 

 large enough to allow them to feed upon 

 larger food organisms, they grew rapidly, 

 because the constant take of adults eased 

 the food competition in this size range. 



The fishing intensity at Onized Lake 

 during the period of this study was so 

 great that most of the fish were caught be- 

 fore they reached the age of 3 years; a 



