May, 1945 



Bennett: Overfishing, Onized Lake 



385 



taken from a random sample of smaller 

 fish measured individually, and individual 

 length measurements were taken on many 

 more fish. Finally, if a large number of a 

 given kind of fish still remained after 

 these samples were taken, they were sorted 

 to size, weighed collectively and counted. 

 Thus, the age-length frequency tables are 

 based on data derived from (1) 577 fish, 

 measured individually, the ages of which 

 were determined from scales; (2) an addi- 

 tional number of fish, also measured indi- 

 vidually, sorted into age groups on the 

 basis of measured lengths; and finally (3) 

 unaged fish, unmeasured individually but 

 of known length range, sorted into age 

 groups on the basis of age-length relation- 

 ships. Table 10 lists the numbers of the 

 various kinds of fish in each of the above 

 groups. 



The status of the population of each 

 species of fish present in Onized Lake at 

 the time of the final census was influenced 

 by fishing pressure and by the ability of 

 that species to reproduce successfully, as 

 well as to compete with other species of 

 fish. In the following discussion, individ- 

 ual species of fish are considered separately 

 in an attempt to integrate the information 

 furnished by the creel census, hoopnet 

 sampling, the final census, age determina- 

 tions and growth rates. 



Largemouth Bass 



Huro salmoides (Lacepede) 



The largemouth bass population' at 

 Onized Lake was, at the time of the final 

 census in 1941, one of the best observed in 

 22 artificial lakes in Illinois (Bennett 

 1943), not only from the standpoint of 

 number and weight per acre of lake surface, 

 but also with reference to rates of growth 

 and range of sizes. This population con- 

 tained individuals belonging to six annual 

 broods (one brood spawned each season, 

 1936-1941), and the lengths of fish recov- 

 ered ranged from li/^ to 22 inches. Forty- 

 seven bass, or 17.1 per cent of the 275 

 present, were of legal lengths, tables 7 and 

 8 ; Onized Lake contained over 23 legal- 

 sized bass per acre in spite of very heavy 

 fishing from 1939 through part of 1941. 

 The greatest catch of bass occurred in 

 1939 when 62 individuals of legal sizes 

 were taken, table 5 ; 29 were taken in the 

 season of 1940 and 8 in 1941 from May 



15 to June 24. These, and 13 others 

 taken in September and October of 1938, 

 make a total of 112 captured during the 

 period of the creel census. While the hook- 

 and-line catch of largemouth bass made 

 up only 2.8 per cent of the total number 

 of fish removed by anglers, their com- 

 bined weight was 179.1 pounds, or 15.5 

 per cent of the weight of all fish caught, 

 table 5. Possibly all of the legal-sized 

 bass in Onized Lake had become "edu- 

 cated" to avoid both artificial and natural 

 baits offered by fishermen. The lake is 

 so small that an experienced bait caster 

 may cover the entire water area from the 

 shore in about an hour. The fact that the 

 lake still contained 47 bass of legal sizes 

 in 1941, including 12 fish that ranged from 

 3 to 6 pounds in weight, suggests that fac- 

 tors other than angling may be responsi- 

 ble for low populations of bass in many 

 Illinois waters less intensively fished. 



The lengths and ages of the bass col- 

 lected from Onized Lake are shown in 

 table 11. The smallness of the number 

 of bass in the group (spawned in May, 

 1941) is due partially to predation at the 

 time of poisoning. Small fish are affected 

 by the treatment first, and all carnivorous 

 fish of larger sizes quickly feed upon them, 

 as they are readily captured at this time. 

 Also, many small fish sink to the bottom 

 of the lake as they die and are eaten or 

 torn to pieces by crayfish and turtles. A 

 fish census made by the poisoning method 

 is inaccurate in the numbers of small fish 

 of all kinds. 



The natural spawn and survival of 

 young bass in Onized Lake is considered 

 to have been adequate to maintain the pop- 

 ulation of this fish at a high level — much 

 higher than is usual in artificial lakes con- 

 taining several species of fish (Bennett 

 1943). As largemouth bass are known 

 to spawn successfully in mud-bottomed 

 ponds, it seems likely that low populations 

 in many lakes are the result of loss of eggs 

 through silting, through the roiling of 

 waters by fish (bullheads, carp and other 

 bottom feeders), or through the nest raid- 

 ing activities of sunfish and other species 

 when they are overlv abundant (Swingle 

 & Smith 1943). 



The growth rate of largemouth bass in 

 Onized Lake was faster than in most Illi- 

 nois waters. Average calculated lengths 

 by years of life of 81 fish measured and 



